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/awt/Transparency.java

/*
 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.awt;

import java.lang.annotation.Native;

/**
 * The <code>Transparency</code> interface defines the common transparency
 * modes for implementing classes.
 */
public interface Transparency {

    /**
     * Represents image data that is guaranteed to be completely opaque,
     * meaning that all pixels have an alpha value of 1.0.
     */
    @Native public final static int OPAQUE            = 1;

    /**
     * Represents image data that is guaranteed to be either completely
     * opaque, with an alpha value of 1.0, or completely transparent,
     * with an alpha value of 0.0.
     */
    @Native public final static int BITMASK = 2;

    /**
     * Represents image data that contains or might contain arbitrary
     * alpha values between and including 0.0 and 1.0.
     */
    @Native public final static int TRANSLUCENT        = 3;

    /**
     * Returns the type of this <code>Transparency</code>.
     * @return the field type of this <code>Transparency</code>, which is
     *          either OPAQUE, BITMASK or TRANSLUCENT.
     */
    public int getTransparency();
}

⏎ java/awt/Transparency.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 

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