import java.awt.BorderLayout;
import javax.swing.JPanel;
import javax.swing.JLabel;
import javax.swing.JButton;
import javax.swing.JFrame;
import java.util.function.Consumer;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.ArrayList;
import java.util.Arrays;
import static java.util.Arrays.asList;

public class HelloPanel extends JPanel {
    int whichaction;
    String somemessage="";
    JLabel displayLabel;

    public static void main( String[] args )
    {
        JFrame helloFrame = new JFrame( "Hello, World!" );
        helloFrame.getContentPane().add( new HelloPanel());
        helloFrame.setSize( 300, 150 );
        helloFrame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        helloFrame.setVisible( true );
        HelloPanel hp=new HelloPanel();
    } 
    public HelloPanel()
    {
        setLayout( new BorderLayout() );
        List<Consumer<String>>  aa=new ArrayList<Consumer<String>>(); 
        aa.add(  (theaddedmessage) -> {displayLabel.setText("Hello World !");});
        aa.add(  (theaddedmessage) -> {displayLabel.setText("Hello World "+theaddedmessage);});
        JPanel ButtonPanel = new JPanel();
        JButton theButton = new JButton( "Press Me." );
        whichaction=0;
        theButton.addActionListener((e)->{aa.get(whichaction).accept(somemessage);});
        whichaction=1;
        somemessage="is changed!";
        ButtonPanel.add( theButton );
        add( ButtonPanel, BorderLayout.NORTH );
        displayLabel = new JLabel( "Press the Button", JLabel.CENTER ); 
        displayLabel.setOpaque( true );
        add( displayLabel, BorderLayout.CENTER );
    }

}