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/net/InetAddressImpl.java

/*
 * Copyright (c) 2002, 2005, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.net;
import java.io.IOException;
/*
 * Package private interface to "implementation" used by
 * {@link InetAddress}.
 * <p>
 * See {@link java.net.Inet4AddressImp} and
 * {@link java.net.Inet6AddressImp}.
 *
 * @since 1.4
 */
interface InetAddressImpl {

    String getLocalHostName() throws UnknownHostException;
    InetAddress[]
        lookupAllHostAddr(String hostname) throws UnknownHostException;
    String getHostByAddr(byte[] addr) throws UnknownHostException;

    InetAddress anyLocalAddress();
    InetAddress loopbackAddress();
    boolean isReachable(InetAddress addr, int timeout, NetworkInterface netif,
                        int ttl) throws IOException;
}

⏎ java/net/InetAddressImpl.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)