title: changing text in a textfield by a button
body: <p>im having a null pointer error. i tried making the button print by System.out.print and it works. i really dont know what's wrong. it's my first post too so sorry for any mistakes. here's the code:</p>
  
  <pre><code>import javax.swing.JFrame;
  import javax.swing.JButton;
  import javax.swing.JTextField;
  import java.awt.event.ActionListener;
  import java.awt.event.ActionEvent;
  import java.awt.FlowLayout;
  import java.awt.Container;
  
  class ChangeText implements ActionListener{
      JButton button;
      JTextField tfield;
      public ChangeText(){
          JFrame frame = new JFrame();
  
          frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          frame.setTitle("Change Text");
          frame.setLayout(new FlowLayout());
  
  
          JTextField tfield = new JTextField("old text");
          JButton button = new JButton("Change Text");
          button.addActionListener(this);
  
          frame.add(tfield);
          frame.add(button);
  
          frame.pack();
          frame.setVisible(true);
      }
  
      public static void main(String[] args){
          ChangeText ct = new ChangeText();
      }
  
  
      public void actionPerformed(ActionEvent e){
          tfield.setText(null);
      }
  
  
  }
  </code></pre>
  
  <p>EDIT: this is what i get when i press the button</p>
  
  <pre><code>java.lang.NullPointerException
      at ChangeText.actionPerformed(ChangeText.java:42)
      at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
      at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
      at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
      at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
      at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
      at java.awt.Component.processMouseEvent(Unknown Source)
      at javax.swing.JComponent.processMouseEvent(Unknown Source)
      at java.awt.Component.processEvent(Unknown Source)
      at java.awt.Container.processEvent(Unknown Source)
      at java.awt.Component.dispatchEventImpl(Unknown Source)
      at java.awt.Container.dispatchEventImpl(Unknown Source)
      at java.awt.Component.dispatchEvent(Unknown Source)
      at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
      at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
      at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
      at java.awt.Container.dispatchEventImpl(Unknown Source)
      at java.awt.Window.dispatchEventImpl(Unknown Source)
      at java.awt.Component.dispatchEvent(Unknown Source)
      at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
      at java.awt.EventQueue.access$000(Unknown Source)
      at java.awt.EventQueue$3.run(Unknown Source)
      at java.awt.EventQueue$3.run(Unknown Source)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
      at java.awt.EventQueue$4.run(Unknown Source)
      at java.awt.EventQueue$4.run(Unknown Source)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
      at java.awt.EventQueue.dispatchEvent(Unknown Source)
      at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
      at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
      at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
      at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
      at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
      at java.awt.EventDispatchThread.run(Unknown Source)
  </code></pre>