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/lang/package-info.java

/*
 * Copyright (c) 1998, 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

/**
 * Provides classes that are fundamental to the design of the Java
 * programming language. The most important classes are {@code
 * Object}, which is the root of the class hierarchy, and {@code
 * Class}, instances of which represent classes at run time.
 *
 * <p>Frequently it is necessary to represent a value of primitive
 * type as if it were an object. The wrapper classes {@code Boolean},
 * {@code Character}, {@code Integer}, {@code Long}, {@code Float},
 * and {@code Double} serve this purpose.  An object of type {@code
 * Double}, for example, contains a field whose type is double,
 * representing that value in such a way that a reference to it can be
 * stored in a variable of reference type.  These classes also provide
 * a number of methods for converting among primitive values, as well
 * as supporting such standard methods as equals and hashCode.  The
 * {@code Void} class is a non-instantiable class that holds a
 * reference to a {@code Class} object representing the type void.
 *
 * <p>The class {@code Math} provides commonly used mathematical
 * functions such as sine, cosine, and square root. The classes {@code
 * String}, {@code StringBuffer}, and {@code StringBuilder} similarly
 * provide commonly used operations on character strings.
 *
 * <p>Classes {@code ClassLoader}, {@code Process}, {@code
 * ProcessBuilder}, {@code Runtime}, {@code SecurityManager}, and
 * {@code System} provide "system operations" that manage the dynamic
 * loading of classes, creation of external processes, host
 * environment inquiries such as the time of day, and enforcement of
 * security policies.
 *
 * <p>Class {@code Throwable} encompasses objects that may be thrown
 * by the {@code throw} statement. Subclasses of {@code Throwable}
 * represent errors and exceptions.
 *
 * <a name="charenc"></a>
 * <h3>Character Encodings</h3>
 *
 * The specification of the {@link java.nio.charset.Charset
 * java.nio.charset.Charset} class describes the naming conventions
 * for character encodings as well as the set of standard encodings
 * that must be supported by every implementation of the Java
 * platform.
 *
 * @since JDK1.0
 */
package java.lang;

⏎ java/lang/package-info.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)