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COMP 110 Introduction to Programming Mr. Joshua Stough September 10, 2007 Announcements • Tutoring Tuesdays 6-9 Dey Hall 2nd floor • Feedback on programs, options: – Paper copies due in class. Review System.out.println ("I said, \"Hi There!\""); I said "Hi There!" System.out.println ("Hi\b\bHello"); Hello System.out.println ("Hello\rWorld"); Hello World System.out.println ("Hello\tWorld"); Hello World System.out.print ("Hello\n"); System.out.println ("World"); Hello World Review • 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 • A Java application always contains a method called main • Source code saved in a file with the extension .java • File must have the same name as the class with the main method • Use the dot (.) operator to call methods: Math.round Review import statements if any public class ClassName { declare named constants and/or stream objects public static void main (String[] args) throws IOException { variable declarations executable statements } } Review • Output System.out.print (stringExp); System.out.println (stringExp); – print vs. println • Comments // this is a one-line comment – “comments out” the rest of the line after // /* this is a multi-line comment */ – “comments out” everything between /* and */ Review Questions What is stored in num? int num = (int) Math.round (12.7); 13 What is printed to the screen? Assume the user enters 10 and that keyboard is already defined. System.out.print ("Enter a number: "); int num = Integer.parseInt (keyboard.readLine()); System.out.println (num * num); Enter a number: 10 100 Assignment Operators • Just for convenience count count count count += -= *= /= 5; 5; 5; 5; // // // // count count count count = = = = count count count count + * / 5; 5; 5; 5; Assignment Operators • The right hand side of an assignment operator can be a complex expression • The entire right-hand expression is evaluated first, then the result is combined with the original variable • Therefore result /= (total-MIN) % num; is equivalent to result = result / ((total-MIN) % num); Questions • What is stored in total and count in the following statements? int total = 10, count = 5; total += count++; total 15 count 6 int total = 20, count = 3; total /= --count; total 10 count 2 String str = "COMP"; str += 110; str COMP110 Java Variables • Primitive Variables – primitive data types (int, double, ...) – stores the data in the memory location • Reference Variables – stores an address in the memory location – "points to" another memory location Objects • More complex data type than a primitive data type • Stored in the "other" memory location – is "pointed to" by a reference variable • Is operated on by special operators called methods Objects and Classes • An object's data type is a class • The class contains the data types that make up the object and what methods can operate on the object • Examples: – String – Integer – Double Primitive Variables int x = 45; • When the computer sees x, it knows which memory location to look up the value in Reference Variables Integer num; • When the computer sees num, it knows which memory location to look for the address in • It will read the address in num and look up a value in that memory location Creating Objects • We use the new operator to create objects, called instantiation parameter Integer num; num = new Integer(78); Review the Terms • We declare a reference variable of a class type. • We use the new operator to instantiate an object of that class type. • We store the address of that object in the reference variable. Changing the Reference Var num = new Integer (50); • The address of the newly-created object is stored in the already-created reference variable num Garbage Collection • What happened to the memory space that held the value 78? • If no other reference variable points to that object, Java will "throw it away" Using Objects • System.out object – represents a destination to which we can send output • Example: – println method System.out.println (”Hello World!”); object method dot operator information provided to the method (parameters) Questions 1. True or False. A primitive variable is a variable that stores the address of a memory space. False new is used to create a class object. 2. The operator 3. In Java, the dot (.) operator is used to access members of a class. It separates the class (or object) name from the method name. 4. True or False. Class objects are instances of that class. True The class String • String variables are reference variables • Given String name; – Equivalent Statements: name = new String("Lisa Simpson"); name = "Lisa Simpson"; name = "Lisa Simpson"; Lisa Simpson Lisa Simpson Lisa Simpson The class String • • • • The String object is an instance of class string The value “Lisa Simpson” is instantiated The address of the value is stored in name The new operator is unnecessary when instantiating Java strings • String methods are called using the dot operator Common String Methods • String(String str) – constructor – creates and initializes the object • char charAt(int index) – returns char at the position specified by index (starts at 0) • int indexOf(char ch) – returns the index of the first occurrence of ch • int compareTo(String str) – returns negative if this string is less than str – returns 0 if this string is the same as str – returns positive if this string is greater than str Common String Methods • boolean equals(String str) – returns true if this string equals str • int length() – returns the length of the string • String replace(char toBeReplaced, char replacedWith) – returns the string in which every occurrence of toBeReplaced is replaced with replacedWith • String toLowerCase() – returns the string that is the the same as this string, but all lower case • String toUpperCase() – returns the string that is the same as this string, but all upper case String Examples String str = "Go Panthers!"; System.out.println (str.length()); 12 System.out.println (str.charAt(3)); P System.out.println (str.indexOf('!'); 11 System.out.println (str.toLowerCase()); go panthers! Example Program • MathStats.java • Ask the user for 3 integers. • Output the following: – – – – – the 3 numbers the sum the average the sum squared the square root of the sum Force Errors Try the following in MathStats.java to see what error message is displayed: – comment out import statement – comment out throws clause – make sumSquare an int – don't initialize sum – don't cast average to double (semantic error) Summary • assignment operators (+=, /=, ...) • primitive variables vs. reference variables • objects • dot (.) operator • String class Next Time in COMP 110 Reading Assignment: Chapter 3 (pgs. 113-140) • • • • Using dialog boxes for I/O Tokenizing Strings Formatting output Reading from and writing to text files