Classes and Methods

1. Classes and Methods

2. Classes • All code in a Java program is part of a class • A class has two purposes • Provide functions to do work for the programmer • Represent data • Two kinds of things found in a class • Variables store data about the class • Methods provide programmers with ways of asking the class to do work

3. Files in a Java program • A Java program is always made up of one or more classes • Most files in a Java program have a single class • A Java program will have many different files, typically one for each class. • The code is stored in a file with the same name as the class and the extension .java. • The java compiler creates a file with the same name, but the extension .class.

4. Defining a Class class myClass { ... ... ... } 1. Starts with the word “class” 2. Followed by the class name 3. Followed by an open curly bracket 4. Followed by method and variable definitions. 5. Followed by a close curly bracket

5. Methods • A method is a piece of java code that does a job • Defined within the curly braces of a class definition • Two types • 1. Static methods (also called class methods) do a job for a programmer • 2. Instance methods manipulate the data within a class • For now we will talk about static methods

6. The parts of a method • Visibility • Public - any part of the Java program can use it • Private - can only be used within the same class • Type of method (static?) • Return type (what type of information it returns) • Name • Parameters - Data given to the method to do its job. • Body - The statements that get executed when the method is called.

7. Parts of a Method public static void main (String[ ] args) { System.out.print(“Hey there!”); } 2. Static 1. Visibility 3. Return type (void means nothing returned) 4. Method name 5. List of parameters 6. Body inside curly brackets

8. Indenting Classes • Variables and methods in a class should be indented • Close bracket at end should be indented the same as class • Open bracket can be on same line as class or next line • Classes are usually in their own file class myClass {//start of myClass void method1() {//start of method1 code; }//end method1 }// end myClass

9. Indenting Methods and Functions • Statements inside the method or function must be indented • Close bracket at end should be indented the same as the definition • Open bracket can be on same line or next line void myMethod() { statement1; }

10. Running a Java method • The body of a method between the curly brackets contain one or more statements • Statements are separated from one another by semicolons • When a method is called, it executes the statements one at a time • When Java is run, it looks for a method called main and runs it • The main method can call other methods, either in the same class or in other classes.

11. Some common methods • System.out.println(“I pwn all noobs”); • System.out.println prints whatever is inside the parentheses to the console • Moves the console cursor to a new line when it is done. • System.out.print(“U just got served”); • Similar to System.out.println, but does not move the cursor to a new line.

12. Formatting Java Programs • Java is case sensitive. E.g. System.out.println is not the same as system.out.println. • Java is white space insensitive • Can put any number of spaces or new line characters between statements • Should use spaces to indent to make programs readable • // (two slashes) in Java start a comment: // This is a comment • Everything from the // to the end of line is ignored

13. Summary • Java programs are made up of classes • Classes have variables that store data • Methods do work for the programmer • Class and method definitions have multiple parts. • The body of a method contains one or more statements. • End with a semicolon • Are executed in order when the method is run • The main method is run when a Java program starts • System.out.print/println send output to the console • Comments and extra white space are ignored

14. Project 0 • Work on Project 0 • Due date : Tomorrow!