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/Inet4AddressImpl.java

/*
 * Copyright (c) 2002, 2013, 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 implementation of InetAddressImpl for IPv4.
 *
 * @since 1.4
 */
class Inet4AddressImpl implements InetAddressImpl {
    public native String getLocalHostName() throws UnknownHostException;
    public native InetAddress[]
        lookupAllHostAddr(String hostname) throws UnknownHostException;
    public native String getHostByAddr(byte[] addr) throws UnknownHostException;
    private native boolean isReachable0(byte[] addr, int timeout, byte[] ifaddr, int ttl) throws IOException;

    public synchronized InetAddress anyLocalAddress() {
        if (anyLocalAddress == null) {
            anyLocalAddress = new Inet4Address(); // {0x00,0x00,0x00,0x00}
            anyLocalAddress.holder().hostName = "0.0.0.0";
        }
        return anyLocalAddress;
    }

    public synchronized InetAddress loopbackAddress() {
        if (loopbackAddress == null) {
            byte[] loopback = {0x7f,0x00,0x00,0x01};
            loopbackAddress = new Inet4Address("localhost", loopback);
        }
        return loopbackAddress;
    }

  public boolean isReachable(InetAddress addr, int timeout, NetworkInterface netif, int ttl) throws IOException {
      byte[] ifaddr = null;
      if (netif != null) {
          /*
           * Let's make sure we use an address of the proper family
           */
          java.util.Enumeration<InetAddress> it = netif.getInetAddresses();
          InetAddress inetaddr = null;
          while (!(inetaddr instanceof Inet4Address) &&
                 it.hasMoreElements())
              inetaddr = it.nextElement();
          if (inetaddr instanceof Inet4Address)
              ifaddr = inetaddr.getAddress();
      }
      return isReachable0(addr.getAddress(), timeout, ifaddr, ttl);
  }
    private InetAddress      anyLocalAddress;
    private InetAddress      loopbackAddress;
}

⏎ java/net/Inet4AddressImpl.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)