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 java.* package in JRE 1.8.0_191 rt.jar. Java source codes are also provided.

⏎ java/awt/event/MouseAdapter.java

/*
 * Copyright (c) 1996, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.awt.event;

/**
 * An abstract adapter class for receiving mouse events.
 * The methods in this class are empty. This class exists as
 * convenience for creating listener objects.
 * <P>
 * Mouse events let you track when a mouse is pressed, released, clicked,
 * moved, dragged, when it enters a component, when it exits and
 * when a mouse wheel is moved.
 * <P>
 * Extend this class to create a {@code MouseEvent}
 * (including drag and motion events) or/and {@code MouseWheelEvent}
 * listener and override the methods for the events of interest. (If you implement the
 * {@code MouseListener},
 * {@code MouseMotionListener}
 * interface, you have to define all of
 * the methods in it. This abstract class defines null methods for them
 * all, so you can only have to define methods for events you care about.)
 * <P>
 * Create a listener object using the extended class and then register it with
 * a component using the component's {@code addMouseListener}
 * {@code addMouseMotionListener}, {@code addMouseWheelListener}
 * methods.
 * The relevant method in the listener object is invoked  and the {@code MouseEvent}
 * or {@code MouseWheelEvent}  is passed to it in following cases:
 * <ul>
 * <li>when a mouse button is pressed, released, or clicked (pressed and  released)
 * <li>when the mouse cursor enters or exits the component
 * <li>when the mouse wheel rotated, or mouse moved or dragged
 * </ul>
 *
 * @author Carl Quinn
 * @author Andrei Dmitriev
 *
 * @see MouseEvent
 * @see MouseWheelEvent
 * @see MouseListener
 * @see MouseMotionListener
 * @see MouseWheelListener
 * @see <a href="https://docs.oracle.com/javase/tutorial/uiswing/events/mouselistener.html">Tutorial: Writing a Mouse Listener</a>
 *
 * @since 1.1
 */
public abstract class MouseAdapter implements MouseListener, MouseWheelListener, MouseMotionListener {
    /**
     * {@inheritDoc}
     */
    public void mouseClicked(MouseEvent e) {}

    /**
     * {@inheritDoc}
     */
    public void mousePressed(MouseEvent e) {}

    /**
     * {@inheritDoc}
     */
    public void mouseReleased(MouseEvent e) {}

    /**
     * {@inheritDoc}
     */
    public void mouseEntered(MouseEvent e) {}

    /**
     * {@inheritDoc}
     */
    public void mouseExited(MouseEvent e) {}

    /**
     * {@inheritDoc}
     * @since 1.6
     */
    public void mouseWheelMoved(MouseWheelEvent e){}

    /**
     * {@inheritDoc}
     * @since 1.6
     */
    public void mouseDragged(MouseEvent e){}

    /**
     * {@inheritDoc}
     * @since 1.6
     */
    public void mouseMoved(MouseEvent e){}
}

⏎ java/awt/event/MouseAdapter.java

Or download all of them as a single archive file:

File name: jre-rt-java-1.8.0_191-src.zip
File size: 6664831 bytes
Release date: 2018-10-28
Download 

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

⇐ JRE 8 plugin.jar - Java Deploy Control Panel Plugin

⇑ Download and Use JDK 8

⇑⇑ FAQ for JDK (Java Development Kit)