Download Class and method

Introduction to Java
Computer Science 3
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
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.
Defining a Class
class myClass {
1. Starts with
the word “class”
...
2. Followed
...
by the class
...
name
4. Followed by
}
method and
variable
definitions.
5. Followed by a close
curly bracket
3. Followed
by an open
curly bracket
Methods
• A method is a piece of java code that does a job
• Defined within the curly brackets 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
The parts of a method
1. Visibility
•
•
2.
3.
4.
5.
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.
6. Body - The statements that get executed when
the function is called.
Parts of a Method
1. Visibility
2. Static
3. Return type (void means
nothing returned)
4. Method name
public static void main (String[] args)
{
System.out.print(“Hello world”);
}
5. List of
parameters
6. Body
inside curly
brackets
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 run, 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.
Some common methods
• System.out.println(“something to print”);
– System.out.println sends prints whatever is
inside the parentheses to the console
– Moves the console cursor to a new line when it
is done.
• System.out.print(“something else”);
– Similar to System.out.println, but does not
move the cursor to a new line.
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
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