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/CharacterDataUndefined.java

/*
 * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.lang;

/** The CharacterData class encapsulates the large tables found in
    Java.lang.Character. */

class CharacterDataUndefined extends CharacterData {

    int getProperties(int ch) {
        return 0;
    }

    int getType(int ch) {
	return Character.UNASSIGNED;
    }

    boolean isJavaIdentifierStart(int ch) {
		return false;
    }

    boolean isJavaIdentifierPart(int ch) {
		return false;
    }

    boolean isUnicodeIdentifierStart(int ch) {
		return false;
    }

    boolean isUnicodeIdentifierPart(int ch) {
		return false;
    }

    boolean isIdentifierIgnorable(int ch) {
		return false;
    }

    int toLowerCase(int ch) {
		return ch;
    }

    int toUpperCase(int ch) {
		return ch;
    }

    int toTitleCase(int ch) {
		return ch;
    }

    int digit(int ch, int radix) {
		return -1;
    }

    int getNumericValue(int ch) {
		return -1;
    }

    boolean isWhitespace(int ch) {
		return false;
    }

    byte getDirectionality(int ch) {
		return Character.DIRECTIONALITY_UNDEFINED;
    }

    boolean isMirrored(int ch) {
		return false;
    }

    static final CharacterData instance = new CharacterDataUndefined();
    private CharacterDataUndefined() {};
}

⏎ java/lang/CharacterDataUndefined.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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