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/motif/MotifCheckBoxUI.java

/*
 * Copyright (c) 1997, 1998, 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.motif;

import sun.awt.AppContext;

import javax.swing.*;

import javax.swing.plaf.*;

import java.awt.*;

/**
 * MotifCheckBox implementation
 * <p>
 * <strong>Warning:</strong>
 * Serialized objects of this class will not be compatible with
 * future Swing releases.  The current serialization support is appropriate
 * for short term storage or RMI between applications running the same
 * version of Swing.  A future release of Swing will provide support for
 * long term persistence.
 *
 * @author Rich Schiavi
 */
public class MotifCheckBoxUI extends MotifRadioButtonUI {

    private static final Object MOTIF_CHECK_BOX_UI_KEY = new Object();

    private final static String propertyPrefix = "CheckBox" + ".";

    private boolean defaults_initialized = false;


    // ********************************
    //         Create PLAF
    // ********************************
    public static ComponentUI createUI(JComponent c) {
        AppContext appContext = AppContext.getAppContext();
        MotifCheckBoxUI motifCheckBoxUI =
                (MotifCheckBoxUI) appContext.get(MOTIF_CHECK_BOX_UI_KEY);
        if (motifCheckBoxUI == null) {
            motifCheckBoxUI = new MotifCheckBoxUI();
            appContext.put(MOTIF_CHECK_BOX_UI_KEY, motifCheckBoxUI);
        }
        return motifCheckBoxUI;
    }

    public String getPropertyPrefix() {
        return propertyPrefix;
    }

    // ********************************
    //          Defaults
    // ********************************
    public void installDefaults(AbstractButton b) {
        super.installDefaults(b);
        if(!defaults_initialized) {
            icon = UIManager.getIcon(getPropertyPrefix() + "icon");
            defaults_initialized = true;
        }
    }

    protected void uninstallDefaults(AbstractButton b) {
        super.uninstallDefaults(b);
        defaults_initialized = false;
    }
}

⏎ com/sun/java/swing/plaf/motif/MotifCheckBoxUI.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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