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Chapter 5 Methods 郝聚涛 [email protected] Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 1 Motivations A method is a construct for grouping statements together to perform a function. Using a method, you can write the code once for performing the function in a program and reuse it by many other programs. For example, often you need to find the maximum between two numbers. Whenever you need this function, you would have to write the following code: int result; if (num1 > num2) result = num1; else result = num2; Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 2 Objectives To define methods, invoke methods, and pass arguments to a method (§5.2-5.5). To develop reusable code that is modular, easy-to-read, easy-todebug, and easy-to-maintain. (§5.6). To use method overloading and understand ambiguous overloading (§5.7). To design and implement overloaded methods (§5.8). To determine the scope of variables (§5.9). To know how to use the methods in the Math class (§§5.10-5.11). To learn the concept of method abstraction (§5.12). To design and implement methods using stepwise refinement (§5.12). Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 3 Defining Methods A method is a collection of statements that are grouped together to perform an operation. Define a method modifier method header return value type Invoke a method method name formal parameters int z = max(x, y); public static int max(int num1, int num2) { actual parameters (arguments) int result; method body if (num1 > num2) result = num1; else result = num2; return result; parameter list method signature return value } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 4 Method Signature(方法签名) Method signature is the combination of the method name and the parameter list. Define a method modifier method header return value type Invoke a method method name formal parameters int z = max(x, y); public static int max(int num1, int num2) { actual parameters (arguments) int result; method body if (num1 > num2) result = num1; else result = num2; return result; parameter list method signature return value } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 5 Formal Parameters(形式参数) The variables defined in the method header are known as formal parameters. Define a method modifier method header return value type Invoke a method method name formal parameters int z = max(x, y); public static int max(int num1, int num2) { actual parameters (arguments) int result; method body if (num1 > num2) result = num1; else result = num2; return result; parameter list method signature return value } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 6 Actual Parameters(实际参数,实参) When a method is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. Define a method modifier method header return value type Invoke a method method name formal parameters int z = max(x, y); public static int max(int num1, int num2) { actual parameters (arguments) int result; method body if (num1 > num2) result = num1; else result = num2; return result; parameter list method signature return value } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 7 Actual Parameters and Formal Parameters Location of actual parameter or argument to formal parameter is important. •Number of actual parameters must equal number of formal parameters. •Type of actual parameter must match type of formal parameter. Define a method modifier method header return value type Invoke a method method name formal parameters int z = max(x, y); public static int max(int num1, int num2) { actual parameters (arguments) int result; method body if (num1 > num2) result = num1; else result = num2; return result; parameter list method signature return value } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 8 Return Value Type A method may return a value. The returnValueType is the data type of the value the method returns. If the method does not return a value, the returnValueType is the keyword void. For example, the returnValueType in the main method is void. Define a method modifier method header return value type Invoke a method method name formal parameters int z = max(x, y); public static int max(int num1, int num2) { actual parameters (arguments) int result; method body if (num1 > num2) result = num1; else result = num2; return result; parameter list method signature return value } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 9 Example Return Value Types Return type int public static int max(int a, int b) { public static double max(int a, int b) { Returns type integer Returns type double public static void printn(String c) { Does not return a value, so no return statement in this method. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 10 Method body The method body is the code executed to produce the results. Define a method modifier method header return value type Invoke a method method name formal parameters int z = max(x, y); public static int max(int num1, int num2) { actual parameters (arguments) int result; method body if (num1 > num2) result = num1; else result = num2; return result; parameter list method signature return value } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 11 Modifiers Identifies how the method is used: •public: any other program has access to this method •static: static methods can be called without creating an instance of the class Define a method modifier method header return value type Invoke a method method name formal parameters int z = max(x, y); public static int max(int num1, int num2) { actual parameters (arguments) int result; method body if (num1 > num2) result = num1; else result = num2; return result; parameter list method signature return value } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 12 Modifiers Java modifiers public: all methods have access to the method protected: only methods in the same package have access to the method private: only methods in the same class has access to the method Java structure Package: contains one or more classes Class: contains one or more methods Subclass: derived from a class Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 13 Calling Methods Listing 5.1 Testing the max method This program demonstrates calling a method max to return the largest of the int values Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 14 TestMax public class TestMax { /** Main method */ public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println("The maximum between " + i + " and " + j + " is " + k); } /** Return the max between two numbers */ public static int max(int num1, int num2) { int result; if (num1 > num2) return (x>y)?x:y; result = num1; else result = num2; return result; Run TestMax } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 15 animation Calling Methods, cont. pass the value of i pass the value of j Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 16 animation Trace Method Invocation i is now 5 i 5 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 17 animation Trace Method Invocation j is now 2 i 5 j 2 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 18 animation Trace Method Invocation invoke max(i, j) i 5 j 2 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 19 animation Trace Method Invocation invoke max(i, j) Pass the value of i to num1 Pass the value of j to num2 i 5 j 2 num1 num2 5 2 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 20 animation Trace Method Invocation declare variable result i 5 j 2 num1 num2 result 5 2 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 21 animation Trace Method Invocation (num1 > num2) is true since num1 is 5 and num2 is 2 i 5 j 2 num1 num2 result 5 2 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 22 animation Trace Method Invocation result is now 5 i 5 j 2 num1 num2 result 5 2 5 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 23 animation Trace Method Invocation return result, which is 5 i 5 j 2 num1 num2 result 5 2 5 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 24 animation Trace Method Invocation return max(i, j) and assign the return value to k i 5 j 2 k 5 num1 num2 result 5 2 5 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 25 animation Trace Method Invocation Execute the print statement i 5 j 2 k 5 num1, num2, and result no longer exist Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 26 CAUTION A return statement is required for a value-returning method. The method shown below in (a) is logically correct, but it has a compilation error because the Java compiler thinks it possible that this method does not return any value. public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return –1; } (a) Should be public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else return –1; } (b) To fix this problem, delete if (n < 0) in (a), so that the compiler will see a return statement to be reached regardless of how the if statement is evaluated. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 27 Reuse Methods from Other Classes NOTE: One of the benefits of methods is for reuse. The max method can be invoked from any class besides TestMax. If you create a new class Test, you can invoke the max method using ClassName.methodName (e.g., TestMax.max). public class Test { public static void main(String[] args) { int a = 4; int b = 8; int c = TestMax.max(a,b); System.out.println("The largest of " + a + " and " + b + " is " + c); } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 28 Call Stacks pass the value of i pass the value of j Each time a method is invoked, the system stores parameters and variables in an area of memory, known as a stack, which stores elements in last-in first-out fashion. When a method calls another method, the caller's stack space is kept intact, and new space is created to handle the new method call. When a method finishes its work and returns to its caller, its associated space is released. Space required for the max method num2: 2 num1: 5 Space required for the max method result: 5 num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5 Space required for the main method k: j: 2 i: 5 Space required for the main method k: j: 2 i: 5 Space required for the main method k: 5 j: 2 i: 5 (a) The main method is invoked. (b) The max method is invoked. (c) The max method is being executed. (d) The max method is finished and the return value is sent to k. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 Stack is empty (e) The main method is finished. 29 animation Trace Call Stack i is declared and initialized i: 5 The main method is invoked. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 30 animation Trace Call Stack j is declared and initialized j: 2 i: 5 The main method is invoked. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 31 animation Trace Call Stack Declare k Space required for the main method k: j: 2 i: 5 The main method is invoked. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 32 animation Trace Call Stack Invoke max(i, j) Space required for the main method k: j: 2 i: 5 The main method is invoked. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 33 animation Trace Call Stack pass the values of i and j to num1 and num2 num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5 The max method is invoked. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 34 animation Trace Call Stack pass the values of i and j to num1 and num2 result: num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5 The max method is invoked. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 35 animation Trace Call Stack (num1 > num2) is true result: num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5 The max method is invoked. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 36 animation Trace Call Stack Assign num1 to result Space required for the max method result: 5 num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5 The max method is invoked. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 37 animation Trace Call Stack Return result and assign it to k Space required for the max method result: 5 num2: 2 num1: 5 Space required for the main method k:5 j: 2 i: 5 The max method is invoked. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 38 animation Trace Call Stack Execute print statement Space required for the main method k:5 j: 2 i: 5 The main method is invoked. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 39 void Method Example This type of method does not return a value. The method performs some actions. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 40 TestVoidMethod public class TestVoidMethod { public static void main(String[] args) { System.out.print("The grade is "); printGrade(78.5); } public static void printGrade(double score) { if (score >= 90.0) { System.out.println('A'); } else if (score >= 80.0) { System.out.println('B'); } else if (score >= 70.0) { System.out.println('C'); } else if (score >= 60.0) { System.out.println('D'); } else { System.out.println('F'); } } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 41 Passing Parameters public static void nPrintln(String message, int n) { for (int i = 0; i < n; i++) System.out.println(message); } Suppose you invoke the method using nPrintln(“Welcome to Java”, 5); What is the output? Welcome to Java Welcome to Java Welcome to Java Welcome to Java Welcome to Java Suppose you invoke the method using nPrintln(“Computer Science”, 15); What is the output? Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 42 Pass by Value(值传递) • Pass the value of the variable, not the memory location • Make changes to value will not change the value in the original variable Listing 5.2 Testing Pass by value This program demonstrates passing values to the methods. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 43 TestPassByValue public class TestPassByValue { /** Main method */ public static void main(String[] args) { // Declare and initialize variables int num1 = 1; int num2 = 2; System.out.println("Before invoking the swap method, num1 is " + num1 + " and num2 is " + num2); // Invoke the swap method to attempt to swap two variables swap(num1, num2); System.out.println("After invoking the swap method, num1 is " + num1 + " and num2 is " + num2); } /** Swap two variables */ public static void swap(int n1, int n2) { System.out.println("\tInside the swap method"); System.out.println("\t\tBefore swapping n1 is " + n1 + " n2 is " + n2); // Swap n1 with n2 int temp = n1; n1 = n2; html n2 = temp; System.out.println("\t\tAfter swapping n1 is " + n1 + " n2 is " + n2); } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 run 44 Pass by Value, cont. The values of num1 and num2 are passed to n1 and n2. Executing swap does not affect num1 and num2. Space required for the swap method temp: n2: 2 n1: 1 Space required for the main method num2: 2 num1: 1 The main method is invoked Space required for the main method num2: 2 num1: 1 The swap method is invoked Space required for the main method num2: 2 num1: 1 The swap method is finished Stack is empty The main method is finished After the swap, n2 = 1 and n1 = 2 But the values in num1 and num2 do not change Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 45 Modularizing Code Methods can be used to reduce redundant coding and enable code reuse. Methods can also be used to modularize code and improve the quality of the program. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 46 GreatestCommonDivisorMethod import java.util.Scanner; public class GreatestCommonDivisorMethod { /** Main method */ public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter first integer: "); int n1 = input.nextInt(); System.out.print("Enter second integer: "); int n2 = input.nextInt(); System.out.println("The greatest common divisor for " + n1 + " and " + n2 + " is " + gcd(n1, n2)); } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 47 GreatestCommonDivisorMethod (cont.) /** Return the gcd of two integers */ public static int gcd(int n1, int n2) { int gcd = 1; // Initial gcd is 1 int k = 1; // Possible gcd while (k <= n1 && k <= n2) { if (n1 % k == 0 && n2 % k == 0) gcd = k; // Update gcd k++; } return gcd; // Return gcd } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 48 PrimeNumberMethod public class PrimeNumberMethod { public static void main(String[] args) { final int NUMBER_OF_PRIMES = 50; // Number of primes to display final int NUMBER_OF_PRIMES_PER_LINE = 10; // Display 10 per line int count = 0; // Count the number of prime numbers int number = 2; // A number to be tested for primeness System.out.println("The first 50 prime numbers are \n"); // Repeatedly find prime numbers Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 49 PrimeNumberMethod (cont) while (count < NUMBER_OF_PRIMES) { // Print the prime number and increase the count if (isPrime(number)) { count++; // Increase the count if (count % NUMBER_OF_PRIMES_PER_LINE == 0) { // Print the number and advance to the new line System.out.printf("%-5s\n", number); } else System.out.printf("%-5s", number); } // Check if the next number is prime number++; } } /** Check whether number is prime */ number++; Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 50 PrimeNumberMethod (cont) public static boolean isPrime(int number) { for (int divisor = 2; divisor <= number / 2; divisor++) { if (number % divisor == 0) { // If true, number is not prime return false; // number is not a prime } } return true; // number is prime } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 51 Overloading Methods(重载) Listing 5.3 Overloading the max Method public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 52 TestMethodOverload public class TestMethodOverloading { /** Main method */ public static void main(String[] args) { // Invoke the max method with int parameters System.out.println("The maximum between 3 and 4 is " + max(3, 4)); // Invoke the max method with the double parameters System.out.println("The maximum between 3.0 and 5.4 is " + max(3.0, 5.4)); // Invoke the max method with three double parameters System.out.println("The maximum between 3.0, 5.4, and 10.14 is " + max(3.0, 5.4, 10.14)); } /** Return the max between two int values */ public static int max(int num1, int num2) { if (num1 > num2) return num1; else return num2; } /** Find the max between two double values */ TestMethodOverload Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 run 53 TestMethodOverload (cont) public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; } /** Return the max among three double values */ public static double max(double num1, double num2, double num3) { return max(max(num1, num2), num3); } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 54 If you call max with int parameters, the max method that expects int parameters will be invoked; max(3, 4); public static int max(int num1, int num2) if you call max with double parameters, the max method that expects double parameters will be invoked. max(3.0, 5.4)); public static double max(double num1, double num2) This is referred to as method overloading; that is, two methods have the same name but different parameter lists within one class. The Java compiler determines which method is used based on the method signature. Overloaded methods must have different parameter lists. You cannot overload methods based on different modifiers or return types. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 55 max(3, 4); -----public static int max(int num1, int num2) max(3.0, 5.4)); -----public static double max(double num1, double num2) ------------------------------------------------------------------- max(3, 4); -----public static double max(double num1, double num2)(自动转型) max((int)3.0,(int) 5.4)); -----public static int max(int num1, int num2) Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 56 Ambiguous Invocation Sometimes there may be two or more possible matches for an invocation of a method, but the compiler cannot determine the most specific match. This is referred to as ambiguous invocation. Ambiguous invocation is a compilation error. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 57 Ambiguous Invocation(歧义调用) public class AmbiguousOverloading { public static void main(String[] args) { System.out.println(max(1, 2)); } public static double max(int num1, double num2) { if (num1 > num2) return num1; else return num2; } Compiler cannot determine if 1 is an integer or a double value and if 2 is an int or double value. This will produce a compile error. public static double max(double num1, int num2) { if (num1 > num2) return num1; else return num2; } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 58 Scope of Local Variables A local variable(局部变量): a variable defined inside a method. Scope(作用域): the part of the program where the variable can be referenced. The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared before it can be used. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 59 Scope of Local Variables, cont. You can declare a local variable with the same name multiple times in different non-nesting blocks in a method, but you cannot declare a local variable twice in nested blocks. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 60 Scope of Local Variables, cont. A variable declared in the initial action part of a for loop header has its scope in the entire loop. a variable declared inside a for loop body has its scope limited in the loop body from its declaration and to the end of the block that contains the variable. The scope of i The scope of j public static void method1() { . . for (int i = 1; i < 10; i++) { . . int j; . . . } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 61 Scope of Local Variables, cont. It is fine to declare i in two non-nesting blocks public static void method1() { int x = 1; int y = 1; It is wrong to declare iin two nesting blocks public static void method2() { int i = 1; int sum = 0; for (int i = 1; i < 10; i++) { x += i; } } for (int i = 1; i < 10; i++) { y += i; } for (int i = 1; i < 10; i++) { sum += i; } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 62 Scope of Local Variables, cont. // Fine with no errors public static void correctMethod() { int x = 1; int y = 1; // i is declared for (int i = 1; i < 10; i++) { x += i; } // i is declared again for (int i = 1; i < 10; i++) { y += i; } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 63 Scope of Local Variables, cont. // With no errors public static void incorrectMethod() { int x = 1; int y = 1; for (int i = 1; i < 10; i++) { int x = 0; //this x will go away when for ends x += i; } System.out.println(“X = “ + x); } X=1 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 64 局部变量声明 局部变量的一般形式如下 [变量修饰符] 变量类型 变量名; 1. 变量修饰符可以是final,表示这是常量。 2. 变量类型可以是Java中任意合法的基本类型或复 合类型。 3. 变量名是用户自定义标识符,遵循标识符的一般 规则。 4. 可以在一行中定义多个局部变量,以逗号分隔。 5. 定义变量时可以同时赋初值。 6. 局部变量必须要先定义后使用。 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 65 局部变量与成员变量区别 1. 局部变量没有访问权限修饰符,不能用public、private和protected来 2. 3. 4. 5. 6. 7. 修饰。这是因为它只能在定义它的方法内部使用。 局部变量不能用static修饰,没有"静态局部变量",这是Java和C/C++ 的一个细微差别。 系统不会自动为局部变量赋初值,但对于成员变量,系统会自动赋初 值。基本类型的值为0,复合类型的值为null。 局部变量的作用域仅限于定义它的方法,在该方法的外部无法访问它。 成员变量的作用域在整个类内部都是可见的,所有成员方法都可以使 用它。如果访问权限允许,还可以在类的外部使用成员变量。 局部变量的生存周期与方法的执行期相同。当方法执行到定义局部变 量的语句时,局部变量被创建;执行到它所在的作用域的最后一条语 句时,局部变量被销毁。类的成员变量,如果是实例成员变量,它和 对象的生存期相同。而静态成员变量的生存期是整个程序运行期。 在同一个方法中,不允许有同名的局部变量。在不同的方法中,可以 有同名的局部变量,它们互不干涉。 局部变量可以和成员变量同名,且在使用时,局部变量具有更高的优 先级。 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 66 public class localVariable{ public void method_1(){ int va = 0; //正确 public int pva; //错误,不能有访问权限 static int sa; //错误,不能是静态的 final int CONST = 10; //正确,可以是常量 double va =0.0; //错误,与前面的va同名 vb = 100.0; //错误,vb还未定义 double vb; vb = 100.0; //正确,现在可以使用了 } public void method_2(){ va = 0; //错误,method_1()中的变量va在此不可用 int CONST = 100; //正确,它与method_1()中的CONST 不同 } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 67 public class localVSmember{ private int iVar = 100; public void method_1(){ int iVar; //正确可以与成员变量同名 iVar = 200; //这里访问的是局部变量 this.iVar = 300; //这里访问的是成员变量 } public void method_2(){ iVar = 400; //这里访问的是成员变量 } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 68 Method Abstraction You can think of the method body as a black box that contains the detailed implementation for the method. Optional arguments for Input Optional return value Method Header Black Box Method body Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 69 Benefits of Methods • Write a method once and reuse it anywhere. • Information hiding. Hide the implementation from the user. • Reduce complexity. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 70 The Math Class Java has predefined classes and methods Java API Definition http://java.sun.com/j2se/1.5.0/docs/api/index.html Example http://www.java2s.com/Code/JavaAPI/CatalogJavaAPI.htm java.lang contains Math class import java.lang.Math; Class constants: PI E 自然对数的底 Class methods: Trigonometric Methods 三角函数方法 Exponent Methods 指数函数方法 min, max, abs, and random Methods Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 71 Trigonometric Methods sin(double a) cos(double a) tan(double a) acos(double a) asin(double a) atan(double a) a is in Radians To convert degrees to Radians (弧 度) use Examples: Math.sin(0) returns 0.0 Math.sin(Math.PI / 6) returns 0.5 Math.sin(Math.PI / 2) returns 1.0 Math.cos(0) returns 1.0 Math.cos(Math.PI / 6) returns 0.866 Math.cos(Math.PI / 2) returns 0 a = Math.toRadians(degrees) Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 72 Exponent Methods exp(double a) Returns e raised to the power of a. log(double a) Returns the natural logarithm of a. log10(double a) Returns the 10-based logarithm of a. Examples: Math.exp(1) returns 2.71 Math.log(2.71) returns 1.0 Math.pow(2, 3) returns 8.0 Math.pow(3, 2) returns 9.0 Math.pow(3.5, 2.5) returns 22.91765 Math.sqrt(4) returns 2.0 Math.sqrt(10.5) returns 3.24 pow(double a, double b) Returns a raised to the power of b. sqrt(double a) Returns the square root of a. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 73 Rounding Methods(取整) double ceil(double x) x rounded up to its nearest integer. This integer is returned as a double value. double floor(double x) x is rounded down to its nearest integer. This integer is returned as a double value. double rint(double x) x is rounded to its nearest integer. If x is equally close to two integers, the even one is returned as a double. int round(float x) Return (int)Math.floor(x+0.5). long round(double x) Return (long)Math.floor(x+0.5). Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 74 Rounding Methods Examples Math.ceil(2.1) returns 3.0 Math.ceil(2.0) returns 2.0 Math.ceil(-2.0) returns –2.0 Math.ceil(-2.1) returns -2.0 Math.floor(2.1) returns 2.0 Math.floor(2.0) returns 2.0 Math.floor(-2.0) returns –2.0 Math.floor(-2.1) returns -3.0 Math.rint(2.1) returns 2.0 Math.rint(2.0) returns 2.0 Math.rint(-2.0) returns –2.0 Math.rint(-2.1) returns -2.0 Math.rint(2.5) returns 2.0 Math.rint(-2.5) returns -2.0 Math.round(2.6f) returns 3 Math.round(2.0) returns 2 Math.round(-2.0f) returns -2 Math.round(-2.6) returns -3 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 75 min, max, and abs max(a, b)and min(a, b) Returns the maximum or minimum of two parameters. abs(a) Returns the absolute value of the parameter. random() Returns a random double value in the range [0.0, 1.0). Examples: Math.max(2, 3) returns 3 Math.max(2.5, 3) returns 3.0 Math.min(2.5, 3.6) returns 2.5 Math.abs(-2) returns 2 Math.abs(-2.1) returns 2.1 Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 76 The random Method Generates a random double value greater than or equal to 0.0 and less than 1.0 (0 <= Math.random() < 1.0). Examples: (int)(Math.random() * 10) Returns a random integer between 0 and 9. 50 + (int)(Math.random() * 50) Returns a random integer between 50 and 99. In general, a + Math.random() * b Returns a random number between a and a + b, excluding a + b. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 77 Case Study: Generating Random Characters As introduced in Section 2.9, each character has a unique Unicode between 0 and FFFF in hexadecimal (65535 in decimal). To generate a random character is to generate a random integer between 0 and 65535 using the following expression: (note that since 0 <= Math.random() < 1.0, you have to add 1 to 65535.) (int)(Math.random() * (65535 + 1)) Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 78 Case Study: Generating Random Characters, cont. Now let us consider how to generate a random lowercase letter. The Unicode for lowercase letters are consecutive integers starting from the Unicode for 'a', then for 'b', 'c', ..., and 'z'. The Unicode for 'a' is (int)'a' So, a random integer between (int)'a' and (int)'z' is (int)((int)'a' + Math.random() * ((int)'z' - (int)'a' + 1) Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 79 Case Study: Generating Random Characters, cont. Now let us consider how to generate a random lowercase letter. The Unicode for lowercase letters are consecutive integers starting from the Unicode for 'a', then for 'b', 'c', ..., and 'z'. The Unicode for 'a' is (int)'a' So, a random integer between (int)'a' and (int)'z' is (int)((int)'a' + Math.random() * ((int)'z' - (int)'a' + 1) Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 80 Case Study: Generating Random Characters, cont. As discussed in Section 2.9.4, all numeric operators can be applied to the char operands. The char operand is cast into a number if the other operand is a number or a character. So, the preceding expression can be simplified as follows: 'a' + Math.random() * ('z' - 'a' + 1) So a random lowercase letter is (char)('a' + Math.random() * ('z' - 'a' + 1)) Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 81 Case Study: Generating Random Characters, cont. To generalize the foregoing discussion, a random character between any two characters ch1 and ch2 with ch1 < ch2 can be generated as follows: (char)(ch1 + Math.random() * (ch2 – ch1 + 1)) Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 82 The RandomCharacter Class // RandomCharacter.java: Generate random characters public class RandomCharacter { /** Generate a random character between ch1 and ch2 */ public static char getRandomCharacter(char ch1, char ch2) { return (char)(ch1 + Math.random() * (ch2 - ch1 + 1)); } /** Generate a random lowercase letter */ public static char getRandomLowerCaseLetter() { return getRandomCharacter('a', 'z'); } html Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 83 /** Generate a random uppercase letter */ public static char getRandomUpperCaseLetter() { return getRandomCharacter('A', 'Z'); } /** Generate a random digit character */ public static char getRandomDigitCharacter() { return getRandomCharacter('0', '9'); } /** Generate a random character */ public static char getRandomCharacter() { return getRandomCharacter('\u0000', '\uFFFF'); } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 84 TestRandomCharacter.java public class TestRandomCharacter { /** Main method */ public static void main(String args[]) { final int NUMBER_OF_CHARS = 175; final int CHARS_PER_LINE = 25; // Print random characters between 'a' and 'z', 25 chars per line for (int i = 0; i < NUMBER_OF_CHARS; i++) { char ch = RandomCharacter.getRandomLowerCaseLetter(); if ((i + 1) % CHARS_PER_LINE == 0) System.out.println(ch); else System.out.print(ch); } } } html run Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 85 Stepwise Refinement (逐步求精) The concept of method abstraction can be applied to the process of developing programs. When writing a large program, you can use the “divide and conquer” (分治)strategy, also known as stepwise refinement, to decompose it into subproblems. The subproblems can be further decomposed into smaller, more manageable problems. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 86 PrintCalender Case Study Let us use the PrintCalendar example to demonstrate the stepwise refinement approach. run Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 87 Design Diagram printCalendar (main) printMonth readInput printMonthTitle printMonthBody /** Main method */ public static void main(String[] args) { //readInput box getStartDay getMonthName // Prompt the user to enter year String yearString = JOptionPane.showInputDialog( "Enter full year (e.g., 2001):"); // Convert string into integer int year = Integer.parseInt(yearString); getTotalNumOfDays // Prompt the user to enter month String monthString = JOptionPane.showInputDialog( "Enter month in number between 1 and 12:"); // Convert string into integer getNumOfDaysInMonth int month = Integer.parseInt(monthString); //end of readInput box // Print calendar for the month of the year printMonth(year, month); //this is a call to printMonth method isLeapYear } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 88 Design Diagram printCalendar (main) /** A stub for printMonth may look like this */ public static void printMonth(int year, int month) { System.out.print(month + " " + year); } printMonth readInput printMonthTitle printMonthBody /** A stub for printMonthTitle may look like this */ getMonthName public static void printMonthTitle(int year, int month) { } getStartDay getTotalNumOfDays getNumOfDaysInMonth isLeapYear Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 89 Design Diagram printCalendar (main) printMonth readInput printMonthTitle getMonthName /** A stub for getMonthName may look like this */ public static String getMonthName(int month) { printMonthBody getStartDay getTotalNumOfDays return "January"; // a dummy value getNumOfDaysInMonth } isLeapYear Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 90 Design Diagram printCalendar (main) printMonth readInput printMonthTitle printMonthBody getStartDay getMonthName /** A stub for getStartDay may getTotalNumOfDays look like this */ public static int getStartDay(int year, int month) { return 1; // a dummy value getNumOfDaysInMonth } /** A stub for getNumberOfDaysInMonth may look like this */ public static int getNumberOfDaysInMonth(int year, int month) { isLeapYear return 31; // a dummy value } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 91 Design Diagram printCalendar (main) printMonth readInput printMonthTitle getMonthName printMonthBody getStartDay getTotalNumOfDays /** A stub for getTotalNumberOfDays may look like this */ public static int getTotalNumberOfDays(int year, int month) { getNumOfDaysInMonth return 10000; // a dummy value } isLeapYear Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 92 Design Diagram printCalendar (main) printMonth readInput printMonthTitle getMonthName printMonthBody getStartDay getTotalNumOfDays getNumOfDaysInMonth /** A stub for isLeapYear may look like this */ public static boolean isLeapYear(int year) { return true; // a dummy value } isLeapYear Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 93 Implementation: Top-Down •Top-down approach is to implement one method in the structure chart at a time from the top to the bottom. Stubs can be used for the methods waiting to be implemented.. •Implement the main method first and then use a stub for the printMonth method. For example, let printMonth display the year and the month in the stub. Thus, your program may begin like this: Stub 存根,未完善方法 A stub is a simple but incomplete version of a method. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 94 Skeleton for PrintCalendar // PrintCalendar.java: Print a calendar for a given month in a year import javax.swing.*; public class PrintCalendar { /** Main method */ public static void main(String[] args) { //readInput box // Prompt the user to enter year String yearString = JOptionPane.showInputDialog( "Enter full year (e.g., 2001):"); // Convert string into integer int year = Integer.parseInt(yearString); // Prompt the user to enter month String monthString = JOptionPane.showInputDialog( "Enter month in number between 1 and 12:"); // Convert string into integer int month = Integer.parseInt(monthString); //end of readInput box // Print calendar for the month of the year printMonth(year, month); //this is a call to printMonth method } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 95 Skeleton for PrintCalendar cont. /** A stub for printMonth may look like this */ public static void printMonth(int year, int month) { System.out.print(month + " " + year); } /** A stub for printMonthTitle may look like this */ public static void printMonthTitle(int year, int month) { } /** A stub for getMonthName may look like this */ public static String getMonthName(int month) { return "January"; // a dummy value } /** A stub for getStartDay may look like this */ public static int getStartDay(int year, int month) { return 1; // a dummy value } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 96 Skeleton for PrintCalendar cont. /** A stub for getNumberOfDaysInMonth may look like this */ public static int getNumberOfDaysInMonth(int year, int month) { return 31; // a dummy value } /** A stub for getTotalNumberOfDays may look like this */ public static int getTotalNumberOfDays(int year, int month) { return 10000; // a dummy value } /** A stub for isLeapYear may look like this */ public static boolean isLeapYear(int year) { return true; // a dummy value } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 97 Implementation: Bottom-Up Bottom-up approach is to implement one method in the structure chart at a time from the bottom to the top. For each method implemented, write a test program to test it. Both top-down and bottom-up methods are fine. Both approaches implement the methods incrementally and help to isolate programming errors and makes debugging easy. Sometimes, they can be used together. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 98 PrintCalendar.java import java.util.Scanner; public class PrintCalendar { /** Main method */ public static void main(String[] args) { // Prompt the user to enter year Scanner input = new Scanner(System.in); // Prompt the user to enter year System.out.print("Enter full year (e.g., 2001): "); int year = input.nextInt(); // Prompt the user to enter month System.out.print("Enter month in number between 1 and 12: "); int month = input.nextInt(); // Print calendar for the month of the year printMonth(year, month); } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 99 PrintCalendar.java /** Print the calendar for a month in a year */ static void printMonth(int year, int month) { // Print the headings of the calendar printMonthTitle(year, month); // Print the body of the calendar printMonthBody(year, month); } /** Print the month title, e.g., May, 1999 */ static void printMonthTitle(int year, int month) { System.out.println(" " + getMonthName(month) + " " + year); System.out.println("-----------------------------"); System.out.println(" Sun Mon Tue Wed Thu Fri Sat"); } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 100 PrintCalendar.java /** Get the English name for the month */ static String getMonthName(int month) { String monthName = null; switch (month) { case 1: monthName = "January"; break; case 2: monthName = "February"; break; case 3: monthName = "March"; break; case 4: monthName = "April"; break; case 5: monthName = "May"; break; case 6: monthName = "June"; break; case 7: monthName = "July"; break; case 8: monthName = "August"; break; case 9: monthName = "September"; break; case 10: monthName = "October"; break; case 11: monthName = "November"; break; case 12: monthName = "December"; } return monthName; } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 101 PrintCalendar.java /** Print month body */ static void printMonthBody(int year, int month) { // Get start day of the week for the first date in the month int startDay = getStartDay(year, month); // Get number of days in the month int numberOfDaysInMonth = getNumberOfDaysInMonth(year, month); // Pad space before the first day of the month int i = 0; for (i = 0; i < startDay; i++) System.out.print(“ “); for (i = 1; i <= numberOfDaysInMonth; i++) { if (i < 10) System.out.print(“ “ + i); else System.out.print(“ “ + i); if ((i + startDay) % 7 == 0) System.out.println(); } System.out.println(); } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 102 PrintCalendar.java /** Get the start day of month/1/year */ static int getStartDay(int year, int month) { final int START_DAY_FOR_JAN_1_1800 = 3; // Get total number of days from 1/1/1800 to month/1/year int totalNumberOfDays = getTotalNumberOfDays(year, month); // Return the start day for month/1/year return (totalNumberOfDays + START_DAY_FOR_JAN_1_1800) % 7; } /** Get the total number of days since January 1, 1800 */ static int getTotalNumberOfDays(int year, int month) { int total = 0; // Get the total days from 1800 to 1/1/year for (int i = 1800; i < year; i++) if (isLeapYear(i)) total = total + 366; else total = total + 365; // Add days from Jan to the month prior to the calendar month for (int i = 1; i < month; i++) total = total + getNumberOfDaysInMonth(year, i); return total; } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 103 PrintCalendar.java /** Get the number of days in a month */ static int getNumberOfDaysInMonth(int year, int month) { if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) return 31; if (month == 4 || month == 6 || month == 9 || month == 11) return 30; if (month == 2) return isLeapYear(year) ? 29 : 28; return 0; // If month is incorrect } /** Determine if it is a leap year */ static boolean isLeapYear(int year) { return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0); } } Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 104 Packages There are four reasons for using packages 1. To locate classes. Classes with similar functions can be placed in the same package to make them easy to locate. 2. To avoid naming conflicts. two classes with the same name. To prevent this, put your classes into packages so that they can be referenced through package names. 3. To distribute software conveniently. Packages group related classes so that they can be easily distributed. 4. To protect classes. Packages provide protection so that the protected members of the classes are accessible to the classes in the same package, but not to the external classes. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 105 Package-Naming Conventions Packages are hierarchical, and you can have packages within packages. For example, java.lang.Math Java designers recommend that you use your Internet domain name in reverse order as a package prefix create a package named mypackage on a host machine with the Internet domain name prenhall.com. To follow the naming convention, you would name the entire package com.prenhall.mypackage. By convention, package names are all in lowercase. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 106 Package Directories Java expects one-to-one mapping of the package name and the file system directory structure set classpath=.;c:\book; You can add as many directories as necessary in classpath. The order in which the directories are specified is the order in which the classes are searched. If you have two classes of the same name in different directories, Java uses the first one it finds. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 107 Putting Classes into Packages package packagename; the first noncomment and nonblank statement in the program 1. Create Format.java and save it into c:\book\com\prenhall\mypackage. package com.prenhall.mypackage; public class Format { public static double format( double number, int numberOfDecimalDigits) { return Math.round(number * Math.pow(10, numberOfDecimalDigits)) / Math.pow(10, numberOfDecimalDigits); } } 2. Compile Format.java and place it in c:\book\com\prenhall\mypackage. Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 108 Using Classes from Packages There are two ways to use classes from a package 1. One way is to use the fully qualified name of the class com.prenhall.mypackage.Format 2. use the import statement import javax.swing.*; called an import on demand import javax.swing.JOptionPane; The program uses an import statement to get the class. You cannot import entire packages, such as com.prenhall.*.* Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved. 0136012671 109