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/windows/WindowsToolBarUI.java

/*
 * Copyright (c) 1997, 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package com.sun.java.swing.plaf.windows;

import java.awt.*;

import javax.swing.AbstractButton;
import javax.swing.JComponent;
import javax.swing.JToggleButton;
import javax.swing.UIDefaults;
import javax.swing.UIManager;

import javax.swing.border.Border;
import javax.swing.border.CompoundBorder;
import javax.swing.border.EmptyBorder;

import javax.swing.plaf.*;

import javax.swing.plaf.basic.BasicBorders;
import javax.swing.plaf.basic.BasicToolBarUI;

import static com.sun.java.swing.plaf.windows.TMSchema.Part;


public class WindowsToolBarUI extends BasicToolBarUI {

    public static ComponentUI createUI(JComponent c) {
        return new WindowsToolBarUI();
    }

    protected void installDefaults() {
        if (XPStyle.getXP() != null) {
            setRolloverBorders(true);
        }
        super.installDefaults();
    }

    protected Border createRolloverBorder() {
        if (XPStyle.getXP() != null) {
            return new EmptyBorder(3, 3, 3, 3);
        } else {
            return super.createRolloverBorder();
        }
    }

    protected Border createNonRolloverBorder() {
        if (XPStyle.getXP() != null) {
            return new EmptyBorder(3, 3, 3, 3);
        } else {
            return super.createNonRolloverBorder();
        }
    }

    public void paint(Graphics g, JComponent c) {
        XPStyle xp = XPStyle.getXP();
        if (xp != null) {
            xp.getSkin(c, Part.TP_TOOLBAR).paintSkin(g, 0, 0,
                        c.getWidth(), c.getHeight(), null, true);
        } else {
            super.paint(g, c);
        }
    }

    /**
     * {@inheritDoc}
     * @since 1.6
     */
    protected Border getRolloverBorder(AbstractButton b) {
        XPStyle xp = XPStyle.getXP();
        if (xp != null) {
            return xp.getBorder(b, WindowsButtonUI.getXPButtonType(b));
        } else {
            return super.getRolloverBorder(b);
        }
    }
}

⏎ com/sun/java/swing/plaf/windows/WindowsToolBarUI.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 

⇒ Backup JDK 8 Installation Directory

⇐ JRE 8 rt.jar - org.* Package Source Code

⇑ Download and Use JDK 8

⇑⇑ FAQ for JDK (Java Development Kit)