title: I saw many object.addActionListen(this), what's the meaning of it?
body: <p>For example:</p>
  
  <pre><code>import acm.gui.*;
  import acm.program.*;
  import java.awt.event.*;
  import javax.swing.*;
  import acm.graphics.*;
  
  public class TemperatureConverter extends Program {
  private IntField fahrenheitField;
  public void init() {
      fahrenheitField = new IntField(32);
      fahrenheitField.addActionListener(this);
  }
  }
  </code></pre>
  
  <p>So assume I know <code>TemperatureConverter</code> is a subclass of <code>ActionListener</code>, what does <code>fahrenheitField.addActionListener(this);</code> actually mean? Cause I'm like "ah, this whole screen now shows an instance of my TemperatureConverter", and whats the meaning of passing it to its <code>fahrenheitField</code> as a <code>ActionListener</code>?</p>