static

also:

non-static coding outside the main(...) method

■ Terminology: static variables are also called "class variables." Meaning these are variables not defined inside a method, which would be a "method variable."

Variables defined in the class, outside any methods, but not static, would be an "instance variable."

■ static can appear on a class, a method, or a variable.

■ static cannot appear on a constructor or an interface.

■ Variables can be static final.

■ You can’t change a static method to non-static while overriding it.

■ You can't change a non-static method to static while overriding it either. Note you can overload OK, changing from one to the other. The restrictions are just on overriding.

■ You can't declare an static variable in an instance method. Method variables only have an instance method lifetime scope. Static lives longer.

■ Non-static methods can directly reference static class variables with an optional Class qualifier. ie. x = StaticClassName.y;

■ No matter where their statements fall in the class's code, any static variables will always be initialized before non-static variables. See initialization order.

non-static coding outside the main(...) method

■ The fact that the main(...) method is static has always caused confusion for beginner Java programmers. There are several ways to easily create a program with non-static methods in which to place your everyday Java code. The same demonstration program below is repeated four times using four different approaches. The program creates a small centered frame with one button on it, which, when it is clicked, simply causes a line to be written to the console saying so.

(Approach 1) You can create an object of yourself and deal with the non-static issues in your constructor:

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

public class Demo1 extends JFrame implements ActionListener {

JButton b1 = new JButton("Button 1");

JPanel p1 = new JPanel( );

public static void main(String[ ] args) {

Demo1 frame = new Demo1 ( );

frame.setSize(300, 75);

frame.setVisible(true);

}

public Demo1 ( ) {

Container cp = getContentPane( );

p1.add(b1); . . . .

cp.add(p1);

b1.addActionListener(this);

}

public void actionPerformed( ActionEvent e ) {

if ( e.getSource( ) == b1 ) System.out.println( "Button 1 was clicked " );

} }

(Approach 2) You can create an object of yourself and deal with everything in your constructor:

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

public class Demo2 extends JFrame implements ActionListener {

JButton b1 = new JButton("Button 1");

JPanel p1 = new JPanel( );

public static void main(String[ ] args) {

Demo2 D2 = new Demo2 ( );

}

public Demo2 ( ) {

JFrame frame = new JFrame( );

Container cp = frame.getContentPane( );

p1.add(b1);

cp.add(p1);

b1.addActionListener(this);

frame.setSize( 300, 75 );

frame.setVisible( true );

}

public void actionPerformed(ActionEvent e) {

if ( e.getSource( ) == b1 ) System.out.println( "Button 1 was clicked " );

} }

(Approach 3) You can create an object of yourself and do everything in an instance method which you call:

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

public class Demo3 extends JFrame implements ActionListener {

JButton b1 = new JButton("Button 1");

JPanel p1 = new JPanel( );

public static void main(String[] args) {

Demo3 obj = new Demo3 ( );

obj.method( );

}

void method( ) {

JFrame frame = new JFrame( );

Container cp = frame.getContentPane( );

p1.add(b1);

cp.add(p1);

b1.addActionListener(this);

frame.setSize( 300, 75 );

frame.setVisible( true );

}

public void actionPerformed(ActionEvent e) {

if ( e.getSource( ) == b1 ) System.out.println( "Button 1 was clicked " );

} }

(Approach 4) You can instantiate a static inner class and do the work there:

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

public class Demo4 {

public static void main(String[ ] args) {

DoTheWork dtw = new DoTheWork( );

}

static class DoTheWork implements ActionListener {

JButton b1 = new JButton("Button 1");

JPanel p1 = new JPanel( );

public DoTheWork( ) {

Container cp = this.getContentPane( );

p1.add(b1);

cp.add(p1);

b1.addActionListener(this);

this.setSize( 300, 75 );

this.setVisible( true );

}

public void actionPerformed( ActionEvent e ) {

if ( e.getSource( ) == b1 ) System.out.println( "Button 1 was clicked " );

} } }