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/dnd/peer/DropTargetPeer.java

/*
 * Copyright (c) 1997, 1999, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.awt.dnd.peer;

import java.awt.dnd.DropTarget;

/**
 * <p>
 * The DropTargetPeer class is the interface to the platform dependent
 * DnD facilities. Since the DnD system is based on the native platform's
 * facilities, a DropTargetPeer will be associated with a ComponentPeer
 * of the nearsest enclosing native Container (in the case of lightweights)
 * </p>
 *
 * @since 1.2
 *
 */

public interface DropTargetPeer {

    /**
     * Add the DropTarget to the System
     *
     * @param dt The DropTarget effected
     */

    void addDropTarget(DropTarget dt);

    /**
     * Remove the DropTarget from the system
     *
     * @param dt The DropTarget effected
     */

    void removeDropTarget(DropTarget dt);
}

⏎ java/awt/dnd/peer/DropTargetPeer.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)