import java.text.DateFormat;

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

import javax.swing.JOptionPane;

public class myTime extends JFrame
{
private JTextArea timeOutput;
private DateFormat currentTime;

public myTime()
{
super( "Clock" );
Container mycontainer = getContentPane();
mycontainer.setLayout( new FlowLayout() );

timeOutput = new JTextArea();
mycontainer.add( timeOutput );

//currentTime = new DateFormat();
currentTime.getTimeInstance();

String output = "";
output += currentTime.toString();/////////此处异常

timeOutput.setText( output );

JOptionPane.showMessageDialog( null, output, "Time", JOptionPane.INFORMATION_MESSAGE );

setSize( 300, 300 );
setVisible( true );
}
public static void main( String args[] )
{
myTime mytime = new myTime();//////////此处异常

mytime.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

}

}


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空引用异常 你有SUPER（）却没传递一个参数改 成 public myTime(String s) super( s );
myTime mytime = new myTime("Clock");//////////

import java.util.Date;

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

import javax.swing.JOptionPane;

class Window extends JFrame
{
private JTextArea timeOutput;
private Date currentTime;

public Window(String s)
{
super(s);
Container mycontainer = getContentPane();
mycontainer.setLayout( new FlowLayout() );

timeOutput = new JTextArea();
mycontainer.add( timeOutput );

currentTime = new Date();


String output =null;

output=currentTime.toString();/////////此处异常

timeOutput.setText( output );
setDefaultCloseOperation(DISPOSE_ON_CLOSE);
JOptionPane.showMessageDialog( null, output, "Time", JOptionPane.INFORMATION_MESSAGE );

setSize( 300, 300 );
setVisible( true );
}
}
public class test
{

public static void main( String args[] )
{
Window win= new Window("clock");//////////此处异常



}

}

你要现实的是这样的吗？

空指针...