Код:

package javaapplication1;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
interface Action{
   public void exec(MouseEvent e);
}
class DefAction implements Action{
    public void exec(MouseEvent e){

    }
}
class MyButton implements MouseListener{
    JButton btn;
    Component btncomp;
    String caption;
    Action def,onclick,onpressed,onleave,onenter,onrelease;
    MyButton(JFrame frm,String caption){
        btn=new JButton(caption);
        btncomp=frm.add(btn);
        this.caption=caption;
        btn.addMouseListener(this);
        onclick=new DefAction();
        onpressed=new DefAction();
        onleave=new DefAction();
        onenter=new DefAction();
    };
    public void SetSize(int w, int h){
        btncomp.setSize(w, h);
    };
    public void SetPos(int x, int y){
        btncomp.setLocation(x, y);
    };
    public void SetAction(String actiontype, Action act){
        if(actiontype.equals("click")){
            onclick=act;
        }
        if(actiontype.equals("press")){
            onpressed=act;
        }
    }
    public void mouseClicked(MouseEvent e) {
        if(e.getComponent().equals(btncomp)){
            onclick.exec(e);
        }
    }


    public void mousePressed(MouseEvent e) {
        if(e.getComponent().equals(btncomp)) onpressed.exec(e) ;
    }


    public void mouseReleased(MouseEvent e) {
        if(e.getComponent().equals(btncomp)) onrelease.exec(e);
    }


    public void mouseEntered(MouseEvent e) {
        if(e.getComponent().equals(btncomp)) onenter.exec(e);
    }

    public void mouseExited(MouseEvent e) {
        if(e.getComponent().equals(btncomp)) onleave.exec(e);
    }

}


class MainWindow implements ActionListener{
    
    void Create(){

        
        JFrame jfrm=new JFrame("qwerty");
        jfrm.setSize(500, 500);
        jfrm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        MyButton btn;
        
        btn=new MyButton(jfrm,"OneButton");
        btn.SetSize(100, 20);
        btn.SetPos(10, 10);
        btn.SetAction("click", new Action(){
             public void exec(MouseEvent e){
                JOptionPane.showMessageDialog(null, "qwerty");
             }
        });
        JPanel jpnl=new JPanel();
        jfrm.getContentPane().add(jpnl);
        jfrm.setResizable(false);
        jfrm.setLocation(300, 300);
        jfrm.setVisible(true);
    }
    void Run(){
        Runnable rnbl=new Runnable(){
            public void run(){
                Create();
            }
        };
        SwingUtilities.invokeLater(rnbl);
    }
    public void actionPerformed(ActionEvent e) {
        if(e.getActionCommand().equals("1st button")) JOptionPane.showMessageDialog(null, "qwerty");
    }
}
public class Main {


    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
        }
        catch (Exception e) { }
        MainWindow window=new MainWindow();
        window.Run();
    }

}

Код:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at javaapplication1.MyButton.mouseReleased(Main.java:61)
        at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:273)
        at java.awt.Component.processMouseEvent(Component.java:6267)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
        at java.awt.Component.processEvent(Component.java:6032)
        at java.awt.Container.processEvent(Container.java:2041)
        at java.awt.Component.dispatchEventImpl(Component.java:4630)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
        at java.awt.Container.dispatchEventImpl(Container.java:2085)
        at java.awt.Window.dispatchEventImpl(Window.java:2478)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Код:

        btn.SetAction("click", new Action(){
             public void exec(MouseEvent e){
                JOptionPane.showMessageDialog(null, "qwerty");
             }
        });