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/math/package-info.java

/*
 * Copyright (c) 1998, 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/**
 * Provides classes for performing arbitrary-precision integer
 * arithmetic ({@code BigInteger}) and arbitrary-precision decimal
 * arithmetic ({@code BigDecimal}).  {@code BigInteger} is analogous
 * to the primitive integer types except that it provides arbitrary
 * precision, hence operations on {@code BigInteger}s do not overflow
 * or lose precision.  In addition to standard arithmetic operations,
 * {@code BigInteger} provides modular arithmetic, GCD calculation,
 * primality testing, prime generation, bit manipulation, and a few
 * other miscellaneous operations.
 *
 * {@code BigDecimal} provides arbitrary-precision signed decimal
 * numbers suitable for currency calculations and the like.  {@code
 * BigDecimal} gives the user complete control over rounding behavior,
 * allowing the user to choose from a comprehensive set of eight
 * rounding modes.
 *
 * @since JDK1.1
 */
package java.math;

⏎ java/math/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

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