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/org/apache/xerces/internal/impl/dv/util/HexBin.java

/*
 * Copyright (c) 2007, 2018, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
/*
 * Copyright 1999-2002,2004 The Apache Software Foundation.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.sun.org.apache.xerces.internal.impl.dv.util;

/**
 * format validation
 *
 * This class encodes/decodes hexadecimal data
 *
 * @xerces.internal
 *
 * @author Jeffrey Rodriguez
 */
public final class  HexBin {
    static private final int  BASELENGTH   = 128;
    static private final int  LOOKUPLENGTH = 16;
    static final private byte [] hexNumberTable    = new byte[BASELENGTH];
    static final private char [] lookUpHexAlphabet = new char[LOOKUPLENGTH];


    static {
        for (int i = 0; i < BASELENGTH; i++ ) {
            hexNumberTable[i] = -1;
        }
        for ( int i = '9'; i >= '0'; i--) {
            hexNumberTable[i] = (byte) (i-'0');
        }
        for ( int i = 'F'; i>= 'A'; i--) {
            hexNumberTable[i] = (byte) ( i-'A' + 10 );
        }
        for ( int i = 'f'; i>= 'a'; i--) {
           hexNumberTable[i] = (byte) ( i-'a' + 10 );
        }

        for(int i = 0; i<10; i++ ) {
            lookUpHexAlphabet[i] = (char)('0'+i);
        }
        for(int i = 10; i<=15; i++ ) {
            lookUpHexAlphabet[i] = (char)('A'+i -10);
        }
    }

    /**
     * Encode a byte array to hex string
     *
     * @param binaryData array of byte to encode
     * @return return encoded string
     */
    static public String encode(byte[] binaryData) {
        if (binaryData == null)
            return null;
        int lengthData   = binaryData.length;
        int lengthEncode = lengthData * 2;
        char[] encodedData = new char[lengthEncode];
        int temp;
        for (int i = 0; i < lengthData; i++) {
            temp = binaryData[i];
            if (temp < 0)
                temp += 256;
            encodedData[i*2] = lookUpHexAlphabet[temp >> 4];
            encodedData[i*2+1] = lookUpHexAlphabet[temp & 0xf];
        }
        return new String(encodedData);
    }

    /**
     * Decode hex string to a byte array
     *
     * @param encoded encoded string
     * @return return array of byte to encode
     */
    static public byte[] decode(String encoded) {
        if (encoded == null)
            return null;
        int lengthData = encoded.length();
        if (lengthData % 2 != 0)
            return null;

        char[] binaryData = encoded.toCharArray();
        int lengthDecode = lengthData / 2;
        byte[] decodedData = new byte[lengthDecode];
        byte temp1, temp2;
        char tempChar;
        for( int i = 0; i<lengthDecode; i++ ){
            tempChar = binaryData[i*2];
            temp1 = (tempChar < BASELENGTH) ? hexNumberTable[tempChar] : -1;
            if (temp1 == -1)
                return null;
            tempChar = binaryData[i*2+1];
            temp2 = (tempChar < BASELENGTH) ? hexNumberTable[tempChar] : -1;
            if (temp2 == -1)
                return null;
            decodedData[i] = (byte)((temp1 << 4) | temp2);
        }
        return decodedData;
    }
}

⏎ com/sun/org/apache/xerces/internal/impl/dv/util/HexBin.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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