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	/*
	* Copyright (c) 1994, 2020, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	package java.lang;

	/**
	* Thrown when an application attempts to use {@code null} in a
	* case where an object is required. These include:
	* <ul>
	* <li>Calling the instance method of a {@code null} object.
	* <li>Accessing or modifying the field of a {@code null} object.
	* <li>Taking the length of {@code null} as if it were an array.
	* <li>Accessing or modifying the slots of {@code null} as if it
	* were an array.
	* <li>Throwing {@code null} as if it were a {@code Throwable}
	* value.
	* </ul>
	* <p>
	* Applications should throw instances of this class to indicate
	* other illegal uses of the {@code null} object.
	*
	* {@code NullPointerException} objects may be constructed by the
	* virtual machine as if {@linkplain Throwable#Throwable(String,
	* Throwable, boolean, boolean) suppression were disabled and/or the
	* stack trace was not writable}.
	*
	* @since 1.0
	*/
	public class NullPointerException extends RuntimeException {
	@java.io.Serial
	private static final long serialVersionUID = 5162710183389028792L;

	/**
	* Constructs a {@code NullPointerException} with no detail message.
	*/
	public NullPointerException() {
	super();
	}

	/**
	* Constructs a {@code NullPointerException} with the specified
	* detail message.
	*
	* @param s the detail message.
	*/
	public NullPointerException(String s) {
	super(s);
	}

	// 0: no backtrace filled in, no message computed.
	// 1: backtrace filled in, no message computed.
	// 2: message computed
	private transient int extendedMessageState;
	private transient String extendedMessage;

	/**
	* {@inheritDoc}
	*/
	public synchronized Throwable fillInStackTrace() {
	// If the stack trace is changed the extended NPE algorithm
	// will compute a wrong message. So compute it beforehand.
	if (extendedMessageState == 0) {
	extendedMessageState = 1;
	} else if (extendedMessageState == 1) {
	extendedMessage = getExtendedNPEMessage();
	extendedMessageState = 2;
	}
	return super.fillInStackTrace();
	}

	/**
	* Returns the detail message string of this throwable.
	*
	* <p> If a non-null message was supplied in a constructor it is
	* returned. Otherwise, an implementation specific message or
	* {@code null} is returned.
	*
	* @implNote
	* If no explicit message was passed to the constructor, and as
	* long as certain internal information is available, a verbose
	* description of the null reference is returned.
	* The internal information is not available in deserialized
	* NullPointerExceptions.
	*
	* @return the detail message string, which may be {@code null}.
	*/
	public String getMessage() {
	String message = super.getMessage();
	if (message == null) {
	synchronized(this) {
	if (extendedMessageState == 1) {
	// Only the original stack trace was filled in. Message will
	// compute correctly.
	extendedMessage = getExtendedNPEMessage();
	extendedMessageState = 2;
	}
	return extendedMessage;
	}
	}
	return message;
	}

	/**
	* Get an extended exception message. This returns a string describing
	* the location and cause of the exception. It returns null for
	* exceptions where this is not applicable.
	*/
	private native String getExtendedNPEMessage();
	}

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