Java tutorial
/* * Copyright (c) 1994, 2019, 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; import jdk.internal.HotSpotIntrinsicCandidate; /** * Class {@code Object} is the root of the class hierarchy. * Every class has {@code Object} as a superclass. All objects, * including arrays, implement the methods of this class. * * @author unascribed * @see java.lang.Class * @since 1.0 */ public class Object { private static native void registerNatives(); static { registerNatives(); } /** * Constructs a new object. */ @HotSpotIntrinsicCandidate public Object() { } /** * Returns the runtime class of this {@code Object}. The returned * {@code Class} object is the object that is locked by {@code * static synchronized} methods of the represented class. * * <p><b>The actual result type is {@code Class<? extends |X|>} * where {@code |X|} is the erasure of the static type of the * expression on which {@code getClass} is called.</b> For * example, no cast is required in this code fragment:</p> * * <p> * {@code Number n = 0; }<br> * {@code Class<? extends Number> c = n.getClass(); } * </p> * * @return The {@code Class} object that represents the runtime * class of this object. * @jls 15.8.2 Class Literals */ @HotSpotIntrinsicCandidate public final native Class<?> getClass(); /** * Returns a hash code value for the object. This method is * supported for the benefit of hash tables such as those provided by * {@link java.util.HashMap}. * <p> * The general contract of {@code hashCode} is: * <ul> * <li>Whenever it is invoked on the same object more than once during * an execution of a Java application, the {@code hashCode} method * must consistently return the same integer, provided no information * used in {@code equals} comparisons on the object is modified. * This integer need not remain consistent from one execution of an * application to another execution of the same application. * <li>If two objects are equal according to the {@code equals(Object)} * method, then calling the {@code hashCode} method on each of * the two objects must produce the same integer result. * <li>It is <em>not</em> required that if two objects are unequal * according to the {@link java.lang.Object#equals(java.lang.Object)} * method, then calling the {@code hashCode} method on each of the * two objects must produce distinct integer results. However, the * programmer should be aware that producing distinct integer results * for unequal objects may improve the performance of hash tables. * </ul> * * @implSpec * As far as is reasonably practical, the {@code hashCode} method defined * by class {@code Object} returns distinct integers for distinct objects. * * @return a hash code value for this object. * @see java.lang.Object#equals(java.lang.Object) * @see java.lang.System#identityHashCode */ @HotSpotIntrinsicCandidate public native int hashCode(); /** * Indicates whether some other object is "equal to" this one. * <p> * The {@code equals} method implements an equivalence relation * on non-null object references: * <ul> * <li>It is <i>reflexive</i>: for any non-null reference value * {@code x}, {@code x.equals(x)} should return * {@code true}. * <li>It is <i>symmetric</i>: for any non-null reference values * {@code x} and {@code y}, {@code x.equals(y)} * should return {@code true} if and only if * {@code y.equals(x)} returns {@code true}. * <li>It is <i>transitive</i>: for any non-null reference values * {@code x}, {@code y}, and {@code z}, if * {@code x.equals(y)} returns {@code true} and * {@code y.equals(z)} returns {@code true}, then * {@code x.equals(z)} should return {@code true}. * <li>It is <i>consistent</i>: for any non-null reference values * {@code x} and {@code y}, multiple invocations of * {@code x.equals(y)} consistently return {@code true} * or consistently return {@code false}, provided no * information used in {@code equals} comparisons on the * objects is modified. * <li>For any non-null reference value {@code x}, * {@code x.equals(null)} should return {@code false}. * </ul> * <p> * The {@code equals} method for class {@code Object} implements * the most discriminating possible equivalence relation on objects; * that is, for any non-null reference values {@code x} and * {@code y}, this method returns {@code true} if and only * if {@code x} and {@code y} refer to the same object * ({@code x == y} has the value {@code true}). * <p> * Note that it is generally necessary to override the {@code hashCode} * method whenever this method is overridden, so as to maintain the * general contract for the {@code hashCode} method, which states * that equal objects must have equal hash codes. * * @param obj the reference object with which to compare. * @return {@code true} if this object is the same as the obj * argument; {@code false} otherwise. * @see #hashCode() * @see java.util.HashMap */ public boolean equals(Object obj) { return (this == obj); } /** * Creates and returns a copy of this object. The precise meaning * of "copy" may depend on the class of the object. The general * intent is that, for any object {@code x}, the expression: * <blockquote> * <pre> * x.clone() != x</pre></blockquote> * will be true, and that the expression: * <blockquote> * <pre> * x.clone().getClass() == x.getClass()</pre></blockquote> * will be {@code true}, but these are not absolute requirements. * While it is typically the case that: * <blockquote> * <pre> * x.clone().equals(x)</pre></blockquote> * will be {@code true}, this is not an absolute requirement. * <p> * By convention, the returned object should be obtained by calling * {@code super.clone}. If a class and all of its superclasses (except * {@code Object}) obey this convention, it will be the case that * {@code x.clone().getClass() == x.getClass()}. * <p> * By convention, the object returned by this method should be independent * of this object (which is being cloned). To achieve this independence, * it may be necessary to modify one or more fields of the object returned * by {@code super.clone} before returning it. Typically, this means * copying any mutable objects that comprise the internal "deep structure" * of the object being cloned and replacing the references to these * objects with references to the copies. If a class contains only * primitive fields or references to immutable objects, then it is usually * the case that no fields in the object returned by {@code super.clone} * need to be modified. * <p> * The method {@code clone} for class {@code Object} performs a * specific cloning operation. First, if the class of this object does * not implement the interface {@code Cloneable}, then a * {@code CloneNotSupportedException} is thrown. Note that all arrays * are considered to implement the interface {@code Cloneable} and that * the return type of the {@code clone} method of an array type {@code T[]} * is {@code T[]} where T is any reference or primitive type. * Otherwise, this method creates a new instance of the class of this * object and initializes all its fields with exactly the contents of * the corresponding fields of this object, as if by assignment; the * contents of the fields are not themselves cloned. Thus, this method * performs a "shallow copy" of this object, not a "deep copy" operation. * <p> * The class {@code Object} does not itself implement the interface * {@code Cloneable}, so calling the {@code clone} method on an object * whose class is {@code Object} will result in throwing an * exception at run time. * * @return a clone of this instance. * @throws CloneNotSupportedException if the object's class does not * support the {@code Cloneable} interface. Subclasses * that override the {@code clone} method can also * throw this exception to indicate that an instance cannot * be cloned. * @see java.lang.Cloneable */ @HotSpotIntrinsicCandidate protected native Object clone() throws CloneNotSupportedException; /** * Returns a string representation of the object. In general, the * {@code toString} method returns a string that * "textually represents" this object. The result should * be a concise but informative representation that is easy for a * person to read. * It is recommended that all subclasses override this method. * <p> * The {@code toString} method for class {@code Object} * returns a string consisting of the name of the class of which the * object is an instance, the at-sign character `{@code @}', and * the unsigned hexadecimal representation of the hash code of the * object. In other words, this method returns a string equal to the * value of: * <blockquote> * <pre> * getClass().getName() + '@' + Integer.toHexString(hashCode()) * </pre></blockquote> * * @return a string representation of the object. */ public String toString() { return getClass().getName() + "@" + Integer.toHexString(hashCode()); } /** * Wakes up a single thread that is waiting on this object's * monitor. If any threads are waiting on this object, one of them * is chosen to be awakened. The choice is arbitrary and occurs at * the discretion of the implementation. A thread waits on an object's * monitor by calling one of the {@code wait} methods. * <p> * The awakened thread will not be able to proceed until the current * thread relinquishes the lock on this object. The awakened thread will * compete in the usual manner with any other threads that might be * actively competing to synchronize on this object; for example, the * awakened thread enjoys no reliable privilege or disadvantage in being * the next thread to lock this object. * <p> * This method should only be called by a thread that is the owner * of this object's monitor. A thread becomes the owner of the * object's monitor in one of three ways: * <ul> * <li>By executing a synchronized instance method of that object. * <li>By executing the body of a {@code synchronized} statement * that synchronizes on the object. * <li>For objects of type {@code Class,} by executing a * synchronized static method of that class. * </ul> * <p> * Only one thread at a time can own an object's monitor. * * @throws IllegalMonitorStateException if the current thread is not * the owner of this object's monitor. * @see java.lang.Object#notifyAll() * @see java.lang.Object#wait() */ @HotSpotIntrinsicCandidate public final native void notify(); /** * Wakes up all threads that are waiting on this object's monitor. A * thread waits on an object's monitor by calling one of the * {@code wait} methods. * <p> * The awakened threads will not be able to proceed until the current * thread relinquishes the lock on this object. The awakened threads * will compete in the usual manner with any other threads that might * be actively competing to synchronize on this object; for example, * the awakened threads enjoy no reliable privilege or disadvantage in * being the next thread to lock this object. * <p> * This method should only be called by a thread that is the owner * of this object's monitor. See the {@code notify} method for a * description of the ways in which a thread can become the owner of * a monitor. * * @throws IllegalMonitorStateException if the current thread is not * the owner of this object's monitor. * @see java.lang.Object#notify() * @see java.lang.Object#wait() */ @HotSpotIntrinsicCandidate public final native void notifyAll(); /** * Causes the current thread to wait until it is awakened, typically * by being <em>notified</em> or <em>interrupted</em>. * <p> * In all respects, this method behaves as if {@code wait(0L, 0)} * had been called. See the specification of the {@link #wait(long, int)} method * for details. * * @throws IllegalMonitorStateException if the current thread is not * the owner of the object's monitor * @throws InterruptedException if any thread interrupted the current thread before or * while the current thread was waiting. The <em>interrupted status</em> of the * current thread is cleared when this exception is thrown. * @see #notify() * @see #notifyAll() * @see #wait(long) * @see #wait(long, int) */ public final void wait() throws InterruptedException { wait(0L); } /** * Causes the current thread to wait until it is awakened, typically * by being <em>notified</em> or <em>interrupted</em>, or until a * certain amount of real time has elapsed. * <p> * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)} * had been called. See the specification of the {@link #wait(long, int)} method * for details. * * @param timeoutMillis the maximum time to wait, in milliseconds * @throws IllegalArgumentException if {@code timeoutMillis} is negative * @throws IllegalMonitorStateException if the current thread is not * the owner of the object's monitor * @throws InterruptedException if any thread interrupted the current thread before or * while the current thread was waiting. The <em>interrupted status</em> of the * current thread is cleared when this exception is thrown. * @see #notify() * @see #notifyAll() * @see #wait() * @see #wait(long, int) */ public final native void wait(long timeoutMillis) throws InterruptedException; /** * Causes the current thread to wait until it is awakened, typically * by being <em>notified</em> or <em>interrupted</em>, or until a * certain amount of real time has elapsed. * <p> * The current thread must own this object's monitor lock. See the * {@link #notify notify} method for a description of the ways in which * a thread can become the owner of a monitor lock. * <p> * This method causes the current thread (referred to here as <var>T</var>) to * place itself in the wait set for this object and then to relinquish any * and all synchronization claims on this object. Note that only the locks * on this object are relinquished; any other objects on which the current * thread may be synchronized remain locked while the thread waits. * <p> * Thread <var>T</var> then becomes disabled for thread scheduling purposes * and lies dormant until one of the following occurs: * <ul> * <li>Some other thread invokes the {@code notify} method for this * object and thread <var>T</var> happens to be arbitrarily chosen as * the thread to be awakened. * <li>Some other thread invokes the {@code notifyAll} method for this * object. * <li>Some other thread {@linkplain Thread#interrupt() interrupts} * thread <var>T</var>. * <li>The specified amount of real time has elapsed, more or less. * The amount of real time, in nanoseconds, is given by the expression * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos} * are both zero, then real time is not taken into consideration and the * thread waits until awakened by one of the other causes. * <li>Thread <var>T</var> is awakened spuriously. (See below.) * </ul> * <p> * The thread <var>T</var> is then removed from the wait set for this * object and re-enabled for thread scheduling. It competes in the * usual manner with other threads for the right to synchronize on the * object; once it has regained control of the object, all its * synchronization claims on the object are restored to the status quo * ante - that is, to the situation as of the time that the {@code wait} * method was invoked. Thread <var>T</var> then returns from the * invocation of the {@code wait} method. Thus, on return from the * {@code wait} method, the synchronization state of the object and of * thread {@code T} is exactly as it was when the {@code wait} method * was invoked. * <p> * A thread can wake up without being notified, interrupted, or timing out, a * so-called <em>spurious wakeup</em>. While this will rarely occur in practice, * applications must guard against it by testing for the condition that should * have caused the thread to be awakened, and continuing to wait if the condition * is not satisfied. See the example below. * <p> * For more information on this topic, see section 14.2, * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley, * 2008). * <p> * If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted} * by any thread before or while it is waiting, then an {@code InterruptedException} * is thrown. The <em>interrupted status</em> of the current thread is cleared when * this exception is thrown. This exception is not thrown until the lock status of * this object has been restored as described above. * * @apiNote * The recommended approach to waiting is to check the condition being awaited in * a {@code while} loop around the call to {@code wait}, as shown in the example * below. Among other things, this approach avoids problems that can be caused * by spurious wakeups. * * <pre>{@code * synchronized (obj) { * while (<condition does not hold> and <timeout not exceeded>) { * long timeoutMillis = ... ; // recompute timeout values * int nanos = ... ; * obj.wait(timeoutMillis, nanos); * } * ... // Perform action appropriate to condition or timeout * } * }</pre> * * @param timeoutMillis the maximum time to wait, in milliseconds * @param nanos additional time, in nanoseconds, in the range 0-999999 inclusive * @throws IllegalArgumentException if {@code timeoutMillis} is negative, * or if the value of {@code nanos} is out of range * @throws IllegalMonitorStateException if the current thread is not * the owner of the object's monitor * @throws InterruptedException if any thread interrupted the current thread before or * while the current thread was waiting. The <em>interrupted status</em> of the * current thread is cleared when this exception is thrown. * @see #notify() * @see #notifyAll() * @see #wait() * @see #wait(long) */ public final void wait(long timeoutMillis, int nanos) throws InterruptedException { if (timeoutMillis < 0) { throw new IllegalArgumentException("timeoutMillis value is negative"); } if (nanos < 0 || nanos > 999999) { throw new IllegalArgumentException("nanosecond timeout value out of range"); } if (nanos > 0 && timeoutMillis < Long.MAX_VALUE) { timeoutMillis++; } wait(timeoutMillis); } /** * Called by the garbage collector on an object when garbage collection * determines that there are no more references to the object. * A subclass overrides the {@code finalize} method to dispose of * system resources or to perform other cleanup. * <p> * The general contract of {@code finalize} is that it is invoked * if and when the Java™ virtual * machine has determined that there is no longer any * means by which this object can be accessed by any thread that has * not yet died, except as a result of an action taken by the * finalization of some other object or class which is ready to be * finalized. The {@code finalize} method may take any action, including * making this object available again to other threads; the usual purpose * of {@code finalize}, however, is to perform cleanup actions before * the object is irrevocably discarded. For example, the finalize method * for an object that represents an input/output connection might perform * explicit I/O transactions to break the connection before the object is * permanently discarded. * <p> * The {@code finalize} method of class {@code Object} performs no * special action; it simply returns normally. Subclasses of * {@code Object} may override this definition. * <p> * The Java programming language does not guarantee which thread will * invoke the {@code finalize} method for any given object. It is * guaranteed, however, that the thread that invokes finalize will not * be holding any user-visible synchronization locks when finalize is * invoked. If an uncaught exception is thrown by the finalize method, * the exception is ignored and finalization of that object terminates. * <p> * After the {@code finalize} method has been invoked for an object, no * further action is taken until the Java virtual machine has again * determined that there is no longer any means by which this object can * be accessed by any thread that has not yet died, including possible * actions by other objects or classes which are ready to be finalized, * at which point the object may be discarded. * <p> * The {@code finalize} method is never invoked more than once by a Java * virtual machine for any given object. * <p> * Any exception thrown by the {@code finalize} method causes * the finalization of this object to be halted, but is otherwise * ignored. * * @apiNote * Classes that embed non-heap resources have many options * for cleanup of those resources. The class must ensure that the * lifetime of each instance is longer than that of any resource it embeds. * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that * objects remain reachable while resources embedded in the object are in use. * <p> * A subclass should avoid overriding the {@code finalize} method * unless the subclass embeds non-heap resources that must be cleaned up * before the instance is collected. * Finalizer invocations are not automatically chained, unlike constructors. * If a subclass overrides {@code finalize} it must invoke the superclass * finalizer explicitly. * To guard against exceptions prematurely terminating the finalize chain, * the subclass should use a {@code try-finally} block to ensure * {@code super.finalize()} is always invoked. For example, * <pre>{@code @Override * protected void finalize() throws Throwable { * try { * ... // cleanup subclass state * } finally { * super.finalize(); * } * } * }</pre> * * @deprecated The finalization mechanism is inherently problematic. * Finalization can lead to performance issues, deadlocks, and hangs. * Errors in finalizers can lead to resource leaks; there is no way to cancel * finalization if it is no longer necessary; and no ordering is specified * among calls to {@code finalize} methods of different objects. * Furthermore, there are no guarantees regarding the timing of finalization. * The {@code finalize} method might be called on a finalizable object * only after an indefinite delay, if at all. * * Classes whose instances hold non-heap resources should provide a method * to enable explicit release of those resources, and they should also * implement {@link AutoCloseable} if appropriate. * The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference} * provide more flexible and efficient ways to release resources when an object * becomes unreachable. * * @throws Throwable the {@code Exception} raised by this method * @see java.lang.ref.WeakReference * @see java.lang.ref.PhantomReference * @jls 12.6 Finalization of Class Instances */ @Deprecated(since = "9") protected void finalize() throws Throwable { } }