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INTRODUCTION TO JAVA CENG 104 FUNDAMENTALS OF COMPUTER PROGRAMMING II Melek OKTAY [email protected] Referenced Book The metarils of this course are taken from two different books “Java Software Solutions: Foundations of Program Design, 6th Edition” “Java How to Program, 6/e-7/e Deitel” JAVA TRANSLATION Java compiler Java bytecode Bytecode interpreter Bytecode compiler © 2007 Pearson Addison-Wesley. All rights reserved Java source code Machine code 1-3 “WRITE ONCE, RUN EVERYWHERE” FOCUS OF THE COURSE Object-Oriented Software Development problem solving program design, implementation, and testing object-oriented concepts classes objects encapsulation inheritance polymorphism graphical user interfaces not included (out of scope) © 2007 Pearson Addison-Wesley. All rights reserved 1-5 JAVA A programming language employs a set of rules that dictate how the words and symbols can be put together to form valid program statements The Java programming language was created by Sun Microsystems, Inc. It was introduced in 1995 and it's popularity has grown quickly since © 2007 Pearson Addison-Wesley. All rights reserved A programming language specifies the words and symbols that we can use to write a program 1-6 JAVA PROGRAM STRUCTURE In the Java programming language: A program is made up of one or more classes A class contains one or more methods A method contains program statements These terms will be explored in detail throughout the course A Java application always contains a method called main © 2007 Pearson Addison-Wesley. All rights reserved See Lincoln.java (page 27) 1-7 JAVA PROGRAM STRUCTURE // comments about the class public class MyProgram { class body Comments can be placed almost anywhere © 2007 Pearson Addison-Wesley. All rights reserved class header } 1-8 JAVA PROGRAM STRUCTURE // comments about the class public class MyProgram { comments about the method public static void main (String[] args) { method body } method header © 2007 Pearson Addison-Wesley. All rights reserved // } 1-9 COMMENTS They should be included to explain the purpose of the program and describe processing steps They do not affect how a program works Java comments can take three forms: © 2007 Pearson Addison-Wesley. All rights reserved Comments in a program are called inline documentation // this comment runs to the end of the line /* this comment runs to the terminating symbol, even across line breaks /** this is a javadoc comment */ */ 1-10 IDENTIFIERS An identifier can be made up of letters, digits, the underscore character ( _ ), and the dollar sign Identifiers cannot begin with a digit Java is case sensitive - Total, total, and TOTAL are different identifiers By convention, programmers use different case styles for different types of identifiers, such as title case for class names - Lincoln upper case for constants - MAXIMUM © 2007 Pearson Addison-Wesley. All rights reserved Identifiers are the words a programmer uses in a program 1-11 IDENTIFIERS Sometimes we are using another programmer's code, so we use the identifiers that he or she chose (such as println) Often we use special identifiers called reserved words that already have a predefined meaning in the language © 2007 Pearson Addison-Wesley. All rights reserved Sometimes we choose identifiers ourselves when writing a program (such as Lincoln) A reserved word cannot be used in any other way 1-12 RESERVED WORDS The Java reserved words: else enum extends false final finally float for goto if implements import instanceof int interface long native new null package private protected public return short static strictfp super switch synchronized this throw throws transient true try void volatile while © 2007 Pearson Addison-Wesley. All rights reserved abstract assert boolean break byte case catch char class const continue default do double 1-13 WHITE SPACE Spaces, blank lines, and tabs are called white space © 2007 Pearson Addison-Wesley. All rights reserved White space is used to separate words and symbols in a program Extra white space is ignored A valid Java program can be formatted many ways Programs should be formatted to enhance readability, using consistent indentation See Lincoln2.java (page 33) See Lincoln3.java (page 34) 1-14 2.3 MODIFYING OUR FIRST JAVA PROGRAM (CONT.) Modifying programs Welcome2.java (Fig. 2.3) produces same output as Welcome1.java (Fig. 2.1) Using different code 9 10 System.out.print( "Welcome to " ); System.out.println( "Java Programming!" ); Line 9 displays “Welcome to ” with cursor remaining on printed line Line 10 displays “Java Programming! ” on same line with cursor on next line 16 1 // Fig. 2.3: Welcome2.java 2 // Printing a line of text with multiple statements. 17 OUTLINE 3 4 public class Welcome2 5 { Welcome2.java 1. Comments 2. Blank line 6 // main method begins execution of Java application 7 public static void main( String args[] ) 8 { 3.1 Method main 9 System.out.print( "Welcome to " ); 10 System.out.println( "Java Programming!" ); System.out.print keeps the Method cursor on the same line, so4. System.out.prin t System.out.println continues 4.1 Method on the same line. System.out.prin tln 11 12 3. Begin class Welcome2 } // end method main 13 14 } // end class Welcome2 Welcome to Java Programming! 5. end main, Welcome2 Program Output 2.3 MODIFYING OUR FIRST JAVA PROGRAM (CONT.) Escape characters Backslash ( \ ) Indicates special characters be output Newline characters (\n) Interpreted as “special characters” by methods System.out.print and System.out.println Indicates cursor should be at the beginning of the next line Welcome3.java (Fig. 2.4) 9 System.out.println( "Welcome\nto\nJava\nProgramming!" ); Line breaks at \n 18 1 // Fig. 2.4: Welcome3.java 2 // Printing multiple lines of text with a single statement. 19 OUTLINE 3 4 public class Welcome3 5 { 6 // main method begins execution of Java application 7 public static void main( String args[] ) 8 { 9 Welcome3.java 1. main 2. System.out.println (uses \n for new line) System.out.println( "Welcome\nto\nJava\nProgramming!" ); 10 11 } // end method main 12 Program Output 13 } // end class Welcome3 Welcome to Java Programming! Notice how a new line is output for each \n escape sequence. 2.4 DISPLAYING TEXT WITH PRINTF System.out.printf New feature of J2SE 5.0 Displays formatted data 9 10 System.out.printf( "%s\n%s\n", "Welcome to", "Java Programming!" ); Format string Fixed text Format specifier – placeholder for a value Format specifier %s – placeholder for a string 20 1 // Fig. 2.6: Welcome4.java 2 // Printing multiple lines in a dialog box. 21 OUTLINE 3 4 public class Welcome4 5 { 6 // main method begins execution of Java application 7 public static void main( String args[] ) 8 { 9 10 System.out.printf( "%s\n%s\n", "Welcome to", "Java Programming!" ); Welcome4.j ava main printf System.out.printf displays formatted data. 11 12 } // end method main 13 14 } // end class Welcome4 Welcome to Java Programming! Program output 2.5 ANOTHER JAVA APPLICATION: ADDING INTEGERS Upcoming program Use Scanner to read two integers from user Use printf to display sum of the two values Use packages 22 1 // Fig. 2.7: Addition.java 2 // Addition program that displays the sum of two numbers. 3 import java.util.Scanner; // program uses class Scanner 4 5 public class Addition 6 { 7 // main method begins execution of Java application 8 public static void main( String args[] ) 9 { 23 OUTLINE import declaration imports class Scanner from package java.util. Addition. java 10 // create Scanner to obtain input from command window 11 Scanner input = new Scanner( System.in ); 12 13 int number1; // first number to add 14 int number2; // second number to add 15 int sum; // sum of number1 and number2 16 17 System.out.print( "Enter first integer: " ); // prompt 18 number1 = input.nextInt(); // read first number from user 19 (1 of 2) Declare and initialize import variable input, which is a declaration Scanner. Scanner Declare variables number1, number2and nextInt sum. Read an integer from the user and assign it to number1. 20 System.out.print( "Enter second integer: " ); // prompt 21 number2 = input.nextInt(); // read second number from user 22 23 sum = number1 + number2; // add numbers 24 25 System.out.printf( "Sum is %d\n", sum ); // 26 27 } // end method main 28 29 } // end class Addition Enter first integer: 45 Enter second integer: 72 Sum is 117 24 OUTLINE Read an integer from the user and assign it to number2.the sum of the Calculate Additio display sum variables number1 and n.java number2, assign result to (2 of 2) sum. Display the sum using formatted output. Two integers entered by the user. 4. Addition 5. printf 2.5 ANOTHER JAVA APPLICATION: ADDING INTEGERS (CONT.) 3 import java.util.Scanner; // program uses class Scanner import declarations Used by compiler to identify and locate classes used in Java programs Tells compiler to load class Scanner from java.util package 5 6 Begins public class Addition public class Addition { Recall that file name must be Addition.java Lines 8-9: begins main 25 2.5 ANOTHER JAVA APPLICATION: ADDING INTEGERS (CONT.) 10 11 // create Scanner to obtain input from command window Scanner input = new Scanner( System.in ); Variable Declaration Statement Variables Location in memory that stores a value Declare with name and type before use Input is of type Scanner Enables a program to read data for use Variable name: any valid identifier Declarations end with semicolons ; Initialize variable in its declaration Equal sign Standard input object System.in 26 2.5 ANOTHER JAVA APPLICATION: ADDING INTEGERS (CONT.) int number1; // first number to add int number2; // second number to add int sum; // second number to add 13 14 15 Declare variable number1, number2 and sum of type int int holds integer values (whole numbers): i.e., 0, -4, 97 Types float and double can hold decimal numbers Type char can hold a single character: i.e., x, $, \n, 7 int, float, double and char are primitive types // first number to add Can int addnumber1, comments to describe purpose of variables number2, // second number to add sum; // second number to add Can declare multiple variables of the same type in one declaration Use comma-separated list 27 2.5 ANOTHER JAVA APPLICATION: ADDING INTEGERS (CONT.) 17 Message called a prompt - directs user to perform an action Package java.lang 18 System.out.print( "Enter first integer: " ); // prompt number1 = input.nextInt(); // read first number from user Result of call to nextInt given to number1 using assignment operator = Assignment statement = binary operator - takes two operands Expression on right evaluated and assigned to variable on left Read as: number1 gets the value of input.nextInt() 28 2.5 ANOTHER JAVA APPLICATION: ADDING INTEGERS (CONT.) 20 Similar to previous statement 21 Prompts the user to input the second integer number2 = input.nextInt(); // read second number from user Similar to previous statement 23 System.out.print( "Enter second integer: " ); // prompt Assign variable number2 to second integer input sum = number1 + number2; // add numbers Assignment statement Calculates sum of number1 and number2 (right hand side) Uses assignment operator = to assign result to variable sum Read as: sum gets the value of number1 + number2 number1 and number2 are operands 29 2.5 ANOTHER JAVA APPLICATION: ADDING INTEGERS (CONT.) System.out.printf( "Sum is %d\n: " , sum ); // display sum 25 Use System.out.printf to display results Format specifier %d Placeholder for an int value System.out.printf( "Sum is %d\n: " , ( number1 + number2 ) ); Calculations can also be performed inside printf Parentheses around the expression number1 + number2 are not required 30 2.6 MEMORY CONCEPTS Variables Every variable has a name, a type, a size and a value Name corresponds to location in memory When new value is placed into a variable, replaces (and destroys) previous value Reading variables from memory does not change them 31 | MEMORY LOCATION SHOWING THE NAME AND VALUE OF VARIABLE NUMBER1. 32 FIG. 2.8 | MEMORY LOCATIONS AFTER STORING VALUES FOR NUMBER1 AND NUMBER2. FIG. 2.9 33 | MEMORY LOCATIONS AFTER CALCULATING AND STORING THE SUM OF NUMBER1 AND NUMBER2. 34 FIG. 2.10 2.7 ARITHMETIC Arithmetic calculations used in most programs Usage * for multiplication / for division % for remainder +, Integer division truncates remainder 7 / 5 evaluates to 1 Remainder operator % returns the remainder 7 % 5 evaluates to 2 35 Java operation Arithmetic Algebraic operator expression Java expression Addition + f+7 f + 7 Subtraction – p–c p - c Bm b * m Multiplication * Division FIG. 2.11 / x / y or or x ÷ y | ARITHMETIC OPERATORS. x / y 36 2.7 ARITHMETIC (CONT.) Operator precedence Some arithmetic operators act before others (i.e., multiplication before addition) Use parenthesis when needed Example: Find the average of three variables a, b and c Do not use: a + b + c / 3 Use: ( a + b + c ) / 3 37 Operator(s) Operation(s) Order of evaluation (precedence) FIG. 2.12 * Multiplication / Division % Remainder + Addition - Subtraction Evaluated first. If there are several operators of this type, they are evaluated from left to right. Evaluated next. If there are several operators of this type, they are evaluated from left to right. 38 | PRECEDENCE OF ARITHMETIC OPERATORS . 2.8 DECISION MAKING: EQUALITY AND RELATIONAL OPERATORS Condition Expression can be either true or false if statement Simple version in this section, more detail later If a condition is true, then the body of the if statement executed Control always resumes after the if statement Conditions in if statements can be formed using equality or relational operators (next slide) 39 Standard algebraic Java equality Sample equality or relational or relational Java operator operator condition Equality operators Relational operators ≤ FIG. 2.14 Meaning of Java condition == != x == y x != y x is equal to y x is not equal to y > < >= <= x x x x x is greater than y x is less than y x is greater than or equal to y x is less than or equal to y > y < y >= y <= y | EQUALITY AND RELATIONAL OPERATORS. 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 // Fig. 2.15: Comparison.java // Compare integers using if statements, relational operators // and equality operators. import java.util.Scanner; // program uses class Scanner public class Comparison { // main method begins execution of Java application public static void main( String args[] ) { // create Scanner to obtain input from command window Scanner input = new Scanner( System.in ); int number1; // first number to compare int number2; // second number to compare System.out.print( "Enter first integer: " ); // prompt number1 = input.nextInt(); // read first number from user System.out.print( "Enter second integer: " ); // prompt number2 = input.nextInt(); // read second number from user if ( number1 == number2 ) System.out.printf( "%d == %d\n", 41 OUTLINE Test for equality, using number1,display number2 result ); printf. if ( number1 != number2 ) System.out.printf( "%d != %d\n", number1, number2 ); Compares two numbers using relational operator <. if ( number1 < number2 ) System.out.printf( "%d < %d\n", number1, number2 ); Compar ison.jav a (1 of 2) 1. Class Comparison 1.1 main 1.2 Declarations 1.3 Input data (nextInt) 1.4 Compare two inputs using if statements 31 32 33 34 if ( number1 > number2 ) System.out.printf( "%d > %d\n", number1, number2 ); 35 if ( number1 <= number2 ) 36 System.out.printf( "%d <= %d\n", 37 38 39 if ( number1 >= number2 ) System.out.printf( "%d >= %d\n", number1, number2 ); 40 41 Compares two numbers operator number1, using number2relational ); >, <= and >=. 42 OUTLINE Compariso n.java (2 of 2) Program output } // end method main 42 43 } // end class Comparison Enter first integer: 777 Enter second integer: 777 777 == 777 777 <= 777 777 >= 777 Enter first integer: 1000 Enter second integer: 2000 1000 != 2000 1000 < 2000 1000 <= 2000 Enter first integer: 2000 Enter second integer: 1000 2000 != 1000 2000 > 1000 2000 >= 1000 2.8 DECISION MAKING: EQUALITY AND RELATIONAL OPERATORS (CONT.) Line 6: begins class Comparison declaration Line 12: declares Scanner variable input and assigns it a Scanner that inputs data from the standard input Lines 14-15: declare int variables Lines 17-18: prompt the user to enter the first integer and input the value Lines 20-21: prompt the user to enter the second integer and input the value 43 2.8 DECISION MAKING: EQUALITY AND RELATIONAL OPERATORS (CONT.) 23 24 if ( number1 == number2 ) System.out.printf( "%d == %d\n", number1, number2 ); if statement to test for equality using (==) If variables equal (condition true) Line 24 executes If variables not equal, statement skipped No semicolon at the end of if statement Empty statement No task is performed Lines 26-27, 29-30, 32-33, 35-36 and 38-39 Compare number1 and number2 with the operators !=, <, >, <= and >=, respectively 44 Operators * / + - < <= == != = % > >= Associativity Type left to right multiplicative left to right additive left to right relational left to right equality right to left assignment | PRECEDENCE AND ASSOCIATIVITY OF OPERATIONS DISCUSSED. FIG. 2.16 45