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/MotifTextPaneUI.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 javax.swing.*;
import javax.swing.text.*;
import javax.swing.plaf.*;
import javax.swing.plaf.basic.BasicTextPaneUI;

/**
 * Provides the look and feel for a styled text editor.
 * <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  Timothy Prinzing
 */
public class MotifTextPaneUI extends BasicTextPaneUI {

    /**
     * Creates a UI for the JTextPane.
     *
     * @param c the JTextPane object
     * @return the UI
     */
    public static ComponentUI createUI(JComponent c) {
        return new MotifTextPaneUI();
    }

    /**
     * Creates the object to use for a caret.  By default an
     * instance of MotifTextUI.MotifCaret is created.  This method
     * can be redefined to provide something else that implements
     * the Caret interface.
     *
     * @return the caret object
     */
    protected Caret createCaret() {
        return MotifTextUI.createCaret();
    }

}

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