JFrame Maximize window

Provided that you are extending JFrame:

public void run() {
    MyFrame myFrame = new MyFrame();
    myFrame.setVisible(true);
    myFrame.setExtendedState(myFrame.getExtendedState() | JFrame.MAXIMIZED_BOTH);
}

Something like this.setExtendedState(this.getExtendedState() | this.MAXIMIZED_BOTH);

import java.awt.*;
import javax.swing.*;

public class Test extends JFrame
{
    public Test()
    {
        GraphicsEnvironment env =
GraphicsEnvironment.getLocalGraphicsEnvironment();
        this.setMaximizedBounds(env.getMaximumWindowBounds());
        this.setExtendedState(this.getExtendedState() | this.MAXIMIZED_BOTH);
    }

    public static void main(String[] args)
    {
        JFrame.setDefaultLookAndFeelDecorated(true);

        Test t = new Test();
        t.setVisible(true);
    }
}

If you're using a JFrame, try this

JFrame frame = new JFrame();
//...
frame.setExtendedState(JFrame.MAXIMIZED_BOTH);