JRE 8 rt.jar is the JAR file for JRE 8 RT (Runtime) libraries. JRE (Java Runtime) 8 is the runtime environment included in JDK 8. JRE 8 rt.jar libraries are divided into 6 packages:

com.* - Internal Oracle and Sun Microsystems libraries
java.* - Standard Java API libraries.
javax.* - Extended Java API libraries.
jdk.* -  JDK supporting libraries.
org.* - Third party libraries.
sun.* - Old libraries developed by Sun Microsystems.

JAR File Information:

Directory of C:\fyicenter\jdk-1.8.0_191\jre\lib
      63,596,151 rt.jar

Here is the list of Java classes of the javax.* package in JRE 1.8.0_191 rt.jar. Java source codes are also provided.

⏎ javax/swing/event/TreeExpansionEvent.java

/*
 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package javax.swing.event;

import java.util.EventObject;
import javax.swing.tree.TreePath;

/**
 * An event used to identify a single path in a tree.  The source
 * returned by <b>getSource</b> will be an instance of JTree.
 * <p>
 * For further documentation and examples see
 * the following sections in <em>The Java Tutorial</em>:
 * <a href="https://docs.oracle.com/javase/tutorial/uiswing/events/treeexpansionlistener.html">How to Write a Tree Expansion Listener</a> and
 * <a href="https://docs.oracle.com/javase/tutorial/uiswing/events/treewillexpandlistener.html">How to Write a Tree-Will-Expand Listener</a>.
 * <p>
 * <strong>Warning:</strong>
 * Serialized objects of this class will not be compatible with
 * future Swing releases. The current serialization support is
 * appropriate for short term storage or RMI between applications running
 * the same version of Swing.  As of 1.4, support for long term storage
 * of all JavaBeans&trade;
 * has been added to the <code>java.beans</code> package.
 * Please see {@link java.beans.XMLEncoder}.
 *
 * @author Scott Violet
 */
public class TreeExpansionEvent extends EventObject
{
    /**
      * Path to the value this event represents.
      */
    protected TreePath              path;

    /**
     * Constructs a TreeExpansionEvent object.
     *
     * @param source  the Object that originated the event
     *                (typically <code>this</code>)
     * @param path    a TreePath object identifying the newly expanded
     *                node
     */
    public TreeExpansionEvent(Object source, TreePath path) {
        super(source);
        this.path = path;
    }

    /**
      * Returns the path to the value that has been expanded/collapsed.
      */
    public TreePath getPath() { return path; }
}

⏎ javax/swing/event/TreeExpansionEvent.java

Or download all of them as a single archive file:

File name: jre-rt-javax-1.8.0_191-src.zip
File size: 5381005 bytes
Release date: 2018-10-28
Download 

⇒ JRE 8 rt.jar - org.* Package Source Code

⇐ JRE 8 rt.jar - java.* Package Source Code

⇑ Download and Use JDK 8

⇑⇑ FAQ for JDK (Java Development Kit)