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

⏎ javax/lang/model/element/TypeParameterElement.java

/*
 * Copyright (c) 2005, 2011, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package javax.lang.model.element;

import java.util.List;
import javax.lang.model.type.TypeMirror;
import javax.lang.model.type.TypeVariable;

/**
 * Represents a formal type parameter of a generic class, interface, method,
 * or constructor element.
 * A type parameter declares a {@link TypeVariable}.
 *
 * @author Joseph D. Darcy
 * @author Scott Seligman
 * @author Peter von der Ahé
 * @see TypeVariable
 * @since 1.6
 */
public interface TypeParameterElement extends Element {

    /**
     * Returns the generic class, interface, method, or constructor that is
     * parameterized by this type parameter.
     *
     * @return the generic class, interface, method, or constructor that is
     * parameterized by this type parameter
     */
    Element getGenericElement();

    /**
     * Returns the bounds of this type parameter.
     * These are the types given by the {@code extends} clause
     * used to declare this type parameter.
     * If no explicit {@code extends} clause was used,
     * then {@code java.lang.Object} is considered to be the sole bound.
     *
     * @return the bounds of this type parameter, or an empty list if
     * there are none
     */
    List<? extends TypeMirror> getBounds();

    /**
     * Returns the {@linkplain TypeParameterElement#getGenericElement generic element} of this type parameter.
     *
     * @return the generic element of this type parameter
     */
    @Override
    Element getEnclosingElement();
}

⏎ javax/lang/model/element/TypeParameterElement.java

Or download all of them as a single archive file:

File name: jre-rt-javax-1.8.0_191-src.zip
File size: 5381005 bytes
Release date: 2018-10-28
Download 

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

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

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