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 com.* package in JRE 1.8.0_191 rt.jar. Java source codes are also provided.

⏎ com/sun/java/swing/plaf/gtk/GTKConstants.java

/*
 * Copyright (c) 2002, 2005, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */
package com.sun.java.swing.plaf.gtk;

/**
 * @author Scott Violet
 */
public interface GTKConstants {

    /**
     * Used to indicate a constant is not defined.
     */
    public static final int UNDEFINED = -100;

    /**
     * Java representation of native GtkIconSize enum
     */
    public enum IconSize {
        INVALID,
        MENU,
        SMALL_TOOLBAR,
        LARGE_TOOLBAR,
        BUTTON,
        DND,
        DIALOG
    }

    /**
     * Java representation of native GtkTextDirection enum
     */
    public enum TextDirection {
        NONE,
        LTR,
        RTL
    }

    /**
     * Java representation of native GtkShadowType enum
     */
    public enum ShadowType {
        NONE,
        IN,
        OUT,
        ETCHED_IN,
        ETCHED_OUT
    }

    /**
     * Java representation of native GtkStateType enum
     */
    public enum StateType {
        NORMAL,
        ACTIVE,
        PRELIGHT,
        SELECTED,
        INSENSITIVE
    }

    /**
     * Java representation of native GtkExpanderStyle enum
     */
    public enum ExpanderStyle {
        COLLAPSED,
        SEMI_COLLAPSED,
        SEMI_EXPANDED,
        EXPANDED,
    }

    /**
     * Java representation of native GtkPositionType enum
     */
    public enum PositionType {
        LEFT,
        RIGHT,
        TOP,
        BOTTOM
    }

    /**
     * Java representation of native GtkArrowType enum
     */
    public enum ArrowType {
        UP,
        DOWN,
        LEFT,
        RIGHT
    }

    /**
     * Java representation of native GtkOrientation enum
     */
    public enum Orientation {
        HORIZONTAL,
        VERTICAL
    }
}

⏎ com/sun/java/swing/plaf/gtk/GTKConstants.java

Or download all of them as a single archive file:

File name: jre-rt-com-1.8.0_191-src.zip
File size: 8099783 bytes
Release date: 2018-10-28
Download 

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