1 /*
2 * Copyright (c) 1994, 2015, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.lang;
27
28 import jdk.internal.HotSpotIntrinsicCandidate;
29
30 /**
31 * Class {@code Object} is the root of the class hierarchy.
32 * Every class has {@code Object} as a superclass. All objects,
33 * including arrays, implement the methods of this class.
34 *
35 * @author unascribed
36 * @see java.lang.Class
37 * @since 1.0
38 */
39 public class Object {
40
41 private static native void registerNatives();
42 static {
43 registerNatives();
44 }
45
46 /**
47 * Constructs a new object.
48 */
49 @HotSpotIntrinsicCandidate
50 public Object() {}
51
52 /**
53 * Returns the runtime class of this {@code Object}. The returned
54 * {@code Class} object is the object that is locked by {@code
55 * static synchronized} methods of the represented class.
56 *
57 * <p><b>The actual result type is {@code Class<? extends |X|>}
58 * where {@code |X|} is the erasure of the static type of the
59 * expression on which {@code getClass} is called.</b> For
60 * example, no cast is required in this code fragment:</p>
61 *
62 * <p>
63 * {@code Number n = 0; }<br>
64 * {@code Class<? extends Number> c = n.getClass(); }
65 * </p>
66 *
67 * @return The {@code Class} object that represents the runtime
68 * class of this object.
69 * @jls 15.8.2 Class Literals
70 */
71 @HotSpotIntrinsicCandidate
72 public final native Class<?> getClass();
73
74 /**
75 * Returns a hash code value for the object. This method is
76 * supported for the benefit of hash tables such as those provided by
77 * {@link java.util.HashMap}.
78 * <p>
79 * The general contract of {@code hashCode} is:
80 * <ul>
81 * <li>Whenever it is invoked on the same object more than once during
82 * an execution of a Java application, the {@code hashCode} method
83 * must consistently return the same integer, provided no information
84 * used in {@code equals} comparisons on the object is modified.
85 * This integer need not remain consistent from one execution of an
86 * application to another execution of the same application.
87 * <li>If two objects are equal according to the {@code equals(Object)}
88 * method, then calling the {@code hashCode} method on each of
89 * the two objects must produce the same integer result.
90 * <li>It is <em>not</em> required that if two objects are unequal
91 * according to the {@link java.lang.Object#equals(java.lang.Object)}
92 * method, then calling the {@code hashCode} method on each of the
93 * two objects must produce distinct integer results. However, the
94 * programmer should be aware that producing distinct integer results
95 * for unequal objects may improve the performance of hash tables.
96 * </ul>
97 * <p>
98 * As much as is reasonably practical, the hashCode method defined
99 * by class {@code Object} does return distinct integers for
100 * distinct objects. (The hashCode may or may not be implemented
101 * as some function of an object's memory address at some point
102 * in time.)
103 *
104 * @return a hash code value for this object.
105 * @see java.lang.Object#equals(java.lang.Object)
106 * @see java.lang.System#identityHashCode
107 */
108 @HotSpotIntrinsicCandidate
109 public native int hashCode();
110
111 /**
112 * Indicates whether some other object is "equal to" this one.
113 * <p>
114 * The {@code equals} method implements an equivalence relation
115 * on non-null object references:
116 * <ul>
117 * <li>It is <i>reflexive</i>: for any non-null reference value
118 * {@code x}, {@code x.equals(x)} should return
119 * {@code true}.
120 * <li>It is <i>symmetric</i>: for any non-null reference values
121 * {@code x} and {@code y}, {@code x.equals(y)}
122 * should return {@code true} if and only if
123 * {@code y.equals(x)} returns {@code true}.
124 * <li>It is <i>transitive</i>: for any non-null reference values
125 * {@code x}, {@code y}, and {@code z}, if
126 * {@code x.equals(y)} returns {@code true} and
127 * {@code y.equals(z)} returns {@code true}, then
128 * {@code x.equals(z)} should return {@code true}.
129 * <li>It is <i>consistent</i>: for any non-null reference values
130 * {@code x} and {@code y}, multiple invocations of
131 * {@code x.equals(y)} consistently return {@code true}
132 * or consistently return {@code false}, provided no
133 * information used in {@code equals} comparisons on the
134 * objects is modified.
135 * <li>For any non-null reference value {@code x},
136 * {@code x.equals(null)} should return {@code false}.
137 * </ul>
138 * <p>
139 * The {@code equals} method for class {@code Object} implements
140 * the most discriminating possible equivalence relation on objects;
141 * that is, for any non-null reference values {@code x} and
142 * {@code y}, this method returns {@code true} if and only
143 * if {@code x} and {@code y} refer to the same object
144 * ({@code x == y} has the value {@code true}).
145 * <p>
146 * Note that it is generally necessary to override the {@code hashCode}
147 * method whenever this method is overridden, so as to maintain the
148 * general contract for the {@code hashCode} method, which states
149 * that equal objects must have equal hash codes.
150 *
151 * @param obj the reference object with which to compare.
152 * @return {@code true} if this object is the same as the obj
153 * argument; {@code false} otherwise.
154 * @see #hashCode()
155 * @see java.util.HashMap
156 */
157 public boolean equals(Object obj) {
158 return (this == obj);
159 }
160
161 /**
162 * Creates and returns a copy of this object. The precise meaning
163 * of "copy" may depend on the class of the object. The general
164 * intent is that, for any object {@code x}, the expression:
165 * <blockquote>
166 * <pre>
167 * x.clone() != x</pre></blockquote>
168 * will be true, and that the expression:
169 * <blockquote>
170 * <pre>
171 * x.clone().getClass() == x.getClass()</pre></blockquote>
172 * will be {@code true}, but these are not absolute requirements.
173 * While it is typically the case that:
174 * <blockquote>
175 * <pre>
176 * x.clone().equals(x)</pre></blockquote>
177 * will be {@code true}, this is not an absolute requirement.
178 * <p>
179 * By convention, the returned object should be obtained by calling
180 * {@code super.clone}. If a class and all of its superclasses (except
181 * {@code Object}) obey this convention, it will be the case that
182 * {@code x.clone().getClass() == x.getClass()}.
183 * <p>
184 * By convention, the object returned by this method should be independent
185 * of this object (which is being cloned). To achieve this independence,
186 * it may be necessary to modify one or more fields of the object returned
187 * by {@code super.clone} before returning it. Typically, this means
188 * copying any mutable objects that comprise the internal "deep structure"
189 * of the object being cloned and replacing the references to these
190 * objects with references to the copies. If a class contains only
191 * primitive fields or references to immutable objects, then it is usually
192 * the case that no fields in the object returned by {@code super.clone}
193 * need to be modified.
194 * <p>
195 * The method {@code clone} for class {@code Object} performs a
196 * specific cloning operation. First, if the class of this object does
197 * not implement the interface {@code Cloneable}, then a
198 * {@code CloneNotSupportedException} is thrown. Note that all arrays
199 * are considered to implement the interface {@code Cloneable} and that
200 * the return type of the {@code clone} method of an array type {@code T[]}
201 * is {@code T[]} where T is any reference or primitive type.
202 * Otherwise, this method creates a new instance of the class of this
203 * object and initializes all its fields with exactly the contents of
204 * the corresponding fields of this object, as if by assignment; the
205 * contents of the fields are not themselves cloned. Thus, this method
206 * performs a "shallow copy" of this object, not a "deep copy" operation.
207 * <p>
208 * The class {@code Object} does not itself implement the interface
209 * {@code Cloneable}, so calling the {@code clone} method on an object
210 * whose class is {@code Object} will result in throwing an
211 * exception at run time.
212 *
213 * @return a clone of this instance.
214 * @throws CloneNotSupportedException if the object's class does not
215 * support the {@code Cloneable} interface. Subclasses
216 * that override the {@code clone} method can also
217 * throw this exception to indicate that an instance cannot
218 * be cloned.
219 * @see java.lang.Cloneable
220 */
221 @HotSpotIntrinsicCandidate
222 protected native Object clone() throws CloneNotSupportedException;
223
224 /**
225 * Returns a string representation of the object. In general, the
226 * {@code toString} method returns a string that
227 * "textually represents" this object. The result should
228 * be a concise but informative representation that is easy for a
229 * person to read.
230 * It is recommended that all subclasses override this method.
231 * <p>
232 * The {@code toString} method for class {@code Object}
233 * returns a string consisting of the name of the class of which the
234 * object is an instance, the at-sign character `{@code @}', and
235 * the unsigned hexadecimal representation of the hash code of the
236 * object. In other words, this method returns a string equal to the
237 * value of:
238 * <blockquote>
239 * <pre>
240 * getClass().getName() + '@' + Integer.toHexString(hashCode())
241 * </pre></blockquote>
242 *
243 * @return a string representation of the object.
244 */
245 public String toString() {
246 return getClass().getName() + "@" + Integer.toHexString(hashCode());
247 }
248
249 /**
250 * Wakes up a single thread that is waiting on this object's
251 * monitor. If any threads are waiting on this object, one of them
252 * is chosen to be awakened. The choice is arbitrary and occurs at
253 * the discretion of the implementation. A thread waits on an object's
254 * monitor by calling one of the {@code wait} methods.
255 * <p>
256 * The awakened thread will not be able to proceed until the current
257 * thread relinquishes the lock on this object. The awakened thread will
258 * compete in the usual manner with any other threads that might be
259 * actively competing to synchronize on this object; for example, the
260 * awakened thread enjoys no reliable privilege or disadvantage in being
261 * the next thread to lock this object.
262 * <p>
263 * This method should only be called by a thread that is the owner
264 * of this object's monitor. A thread becomes the owner of the
265 * object's monitor in one of three ways:
266 * <ul>
267 * <li>By executing a synchronized instance method of that object.
268 * <li>By executing the body of a {@code synchronized} statement
269 * that synchronizes on the object.
270 * <li>For objects of type {@code Class,} by executing a
271 * synchronized static method of that class.
272 * </ul>
273 * <p>
274 * Only one thread at a time can own an object's monitor.
275 *
276 * @throws IllegalMonitorStateException if the current thread is not
277 * the owner of this object's monitor.
278 * @see java.lang.Object#notifyAll()
279 * @see java.lang.Object#wait()
280 */
281 public final native void notify();
282
283 /**
284 * Wakes up all threads that are waiting on this object's monitor. A
285 * thread waits on an object's monitor by calling one of the
286 * {@code wait} methods.
287 * <p>
288 * The awakened threads will not be able to proceed until the current
289 * thread relinquishes the lock on this object. The awakened threads
290 * will compete in the usual manner with any other threads that might
291 * be actively competing to synchronize on this object; for example,
292 * the awakened threads enjoy no reliable privilege or disadvantage in
293 * being the next thread to lock this object.
294 * <p>
295 * This method should only be called by a thread that is the owner
296 * of this object's monitor. See the {@code notify} method for a
297 * description of the ways in which a thread can become the owner of
298 * a monitor.
299 *
300 * @throws IllegalMonitorStateException if the current thread is not
301 * the owner of this object's monitor.
302 * @see java.lang.Object#notify()
303 * @see java.lang.Object#wait()
304 */
305 public final native void notifyAll();
306
307 /**
308 * Causes the current thread to wait until either another thread invokes the
309 * {@link java.lang.Object#notify()} method or the
310 * {@link java.lang.Object#notifyAll()} method for this object, or a
311 * specified amount of time has elapsed.
312 * <p>
313 * The current thread must own this object's monitor.
314 * <p>
315 * This method causes the current thread (call it <var>T</var>) to
316 * place itself in the wait set for this object and then to relinquish
317 * any and all synchronization claims on this object. Thread <var>T</var>
318 * becomes disabled for thread scheduling purposes and lies dormant
319 * until one of four things happens:
320 * <ul>
321 * <li>Some other thread invokes the {@code notify} method for this
322 * object and thread <var>T</var> happens to be arbitrarily chosen as
323 * the thread to be awakened.
324 * <li>Some other thread invokes the {@code notifyAll} method for this
325 * object.
326 * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
327 * thread <var>T</var>.
328 * <li>The specified amount of real time has elapsed, more or less. If
329 * {@code timeout} is zero, however, then real time is not taken into
330 * consideration and the thread simply waits until notified.
331 * </ul>
332 * The thread <var>T</var> is then removed from the wait set for this
333 * object and re-enabled for thread scheduling. It then competes in the
334 * usual manner with other threads for the right to synchronize on the
335 * object; once it has gained control of the object, all its
336 * synchronization claims on the object are restored to the status quo
337 * ante - that is, to the situation as of the time that the {@code wait}
338 * method was invoked. Thread <var>T</var> then returns from the
339 * invocation of the {@code wait} method. Thus, on return from the
340 * {@code wait} method, the synchronization state of the object and of
341 * thread {@code T} is exactly as it was when the {@code wait} method
342 * was invoked.
343 * <p>
344 * A thread can also wake up without being notified, interrupted, or
345 * timing out, a so-called <i>spurious wakeup</i>. While this will rarely
346 * occur in practice, applications must guard against it by testing for
347 * the condition that should have caused the thread to be awakened, and
348 * continuing to wait if the condition is not satisfied. In other words,
349 * waits should always occur in loops, like this one:
350 * <pre>
351 * synchronized (obj) {
352 * while (<condition does not hold>)
353 * obj.wait(timeout);
354 * ... // Perform action appropriate to condition
355 * }
356 * </pre>
357 *
358 * (For more information on this topic, see section 14.2,
359 * Condition Queues, in Brian Goetz and others' "Java Concurrency
360 * in Practice" (Addison-Wesley, 2006) or Item 69 in Joshua
361 * Bloch's "Effective Java (Second Edition)" (Addison-Wesley,
362 * 2008).
363 *
364 * <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
365 * interrupted} by any thread before or while it is waiting, then an
366 * {@code InterruptedException} is thrown. This exception is not
367 * thrown until the lock status of this object has been restored as
368 * described above.
369 *
370 * <p>
371 * Note that the {@code wait} method, as it places the current thread
372 * into the wait set for this object, unlocks only this object; any
373 * other objects on which the current thread may be synchronized remain
374 * locked while the thread waits.
375 * <p>
376 * This method should only be called by a thread that is the owner
377 * of this object's monitor. See the {@code notify} method for a
378 * description of the ways in which a thread can become the owner of
379 * a monitor.
380 *
381 * @param timeout the maximum time to wait in milliseconds.
382 * @throws IllegalArgumentException if the value of timeout is
383 * negative.
384 * @throws IllegalMonitorStateException if the current thread is not
385 * the owner of the object's monitor.
386 * @throws InterruptedException if any thread interrupted the
387 * current thread before or while the current thread
388 * was waiting for a notification. The <i>interrupted
389 * status</i> of the current thread is cleared when
390 * this exception is thrown.
391 * @see java.lang.Object#notify()
392 * @see java.lang.Object#notifyAll()
393 */
394 public final native void wait(long timeout) throws InterruptedException;
395
396 /**
397 * Causes the current thread to wait until another thread invokes the
398 * {@link java.lang.Object#notify()} method or the
399 * {@link java.lang.Object#notifyAll()} method for this object, or
400 * some other thread interrupts the current thread, or a certain
401 * amount of real time has elapsed.
402 * <p>
403 * This method is similar to the {@code wait} method of one
404 * argument, but it allows finer control over the amount of time to
405 * wait for a notification before giving up. The amount of real time,
406 * measured in nanoseconds, is given by:
407 * <blockquote>
408 * <pre>
409 * 1000000*timeout+nanos</pre></blockquote>
410 * <p>
411 * In all other respects, this method does the same thing as the
412 * method {@link #wait(long)} of one argument. In particular,
413 * {@code wait(0, 0)} means the same thing as {@code wait(0)}.
414 * <p>
415 * The current thread must own this object's monitor. The thread
416 * releases ownership of this monitor and waits until either of the
417 * following two conditions has occurred:
418 * <ul>
419 * <li>Another thread notifies threads waiting on this object's monitor
420 * to wake up either through a call to the {@code notify} method
421 * or the {@code notifyAll} method.
422 * <li>The timeout period, specified by {@code timeout}
423 * milliseconds plus {@code nanos} nanoseconds arguments, has
424 * elapsed.
425 * </ul>
426 * <p>
427 * The thread then waits until it can re-obtain ownership of the
428 * monitor and resumes execution.
429 * <p>
430 * As in the one argument version, interrupts and spurious wakeups are
431 * possible, and this method should always be used in a loop:
432 * <pre>
433 * synchronized (obj) {
434 * while (<condition does not hold>)
435 * obj.wait(timeout, nanos);
436 * ... // Perform action appropriate to condition
437 * }
438 * </pre>
439 * This method should only be called by a thread that is the owner
440 * of this object's monitor. See the {@code notify} method for a
441 * description of the ways in which a thread can become the owner of
442 * a monitor.
443 *
444 * @param timeout the maximum time to wait in milliseconds.
445 * @param nanos additional time, in nanoseconds range
446 * 0-999999.
447 * @throws IllegalArgumentException if the value of timeout is
448 * negative or the value of nanos is
449 * not in the range 0-999999.
450 * @throws IllegalMonitorStateException if the current thread is not
451 * the owner of this object's monitor.
452 * @throws InterruptedException if any thread interrupted the
453 * current thread before or while the current thread
454 * was waiting for a notification. The <i>interrupted
455 * status</i> of the current thread is cleared when
456 * this exception is thrown.
457 */
458 public final void wait(long timeout, int nanos) throws InterruptedException {
459 if (timeout < 0) {
460 throw new IllegalArgumentException("timeout value is negative");
461 }
462
463 if (nanos < 0 || nanos > 999999) {
464 throw new IllegalArgumentException(
465 "nanosecond timeout value out of range");
466 }
467
468 if (nanos > 0) {
469 timeout++;
470 }
471
472 wait(timeout);
473 }
474
475 /**
476 * Causes the current thread to wait until another thread invokes the
477 * {@link java.lang.Object#notify()} method or the
478 * {@link java.lang.Object#notifyAll()} method for this object.
479 * In other words, this method behaves exactly as if it simply
480 * performs the call {@code wait(0)}.
481 * <p>
482 * The current thread must own this object's monitor. The thread
483 * releases ownership of this monitor and waits until another thread
484 * notifies threads waiting on this object's monitor to wake up
485 * either through a call to the {@code notify} method or the
486 * {@code notifyAll} method. The thread then waits until it can
487 * re-obtain ownership of the monitor and resumes execution.
488 * <p>
489 * As in the one argument version, interrupts and spurious wakeups are
490 * possible, and this method should always be used in a loop:
491 * <pre>
492 * synchronized (obj) {
493 * while (<condition does not hold>)
494 * obj.wait();
495 * ... // Perform action appropriate to condition
496 * }
497 * </pre>
498 * This method should only be called by a thread that is the owner
499 * of this object's monitor. See the {@code notify} method for a
500 * description of the ways in which a thread can become the owner of
501 * a monitor.
502 *
503 * @throws IllegalMonitorStateException if the current thread is not
504 * the owner of the object's monitor.
505 * @throws InterruptedException if any thread interrupted the
506 * current thread before or while the current thread
507 * was waiting for a notification. The <i>interrupted
508 * status</i> of the current thread is cleared when
509 * this exception is thrown.
510 * @see java.lang.Object#notify()
511 * @see java.lang.Object#notifyAll()
512 */
513 public final void wait() throws InterruptedException {
514 wait(0);
515 }
516
517 /**
518 * Called by the garbage collector on an object when garbage collection
519 * determines that there are no more references to the object.
520 * A subclass overrides the {@code finalize} method to dispose of
521 * system resources or to perform other cleanup.
522 * <p>
523 * The general contract of {@code finalize} is that it is invoked
524 * if and when the Java™ virtual
525 * machine has determined that there is no longer any
526 * means by which this object can be accessed by any thread that has
527 * not yet died, except as a result of an action taken by the
528 * finalization of some other object or class which is ready to be
529 * finalized. The {@code finalize} method may take any action, including
530 * making this object available again to other threads; the usual purpose
531 * of {@code finalize}, however, is to perform cleanup actions before
532 * the object is irrevocably discarded. For example, the finalize method
533 * for an object that represents an input/output connection might perform
534 * explicit I/O transactions to break the connection before the object is
535 * permanently discarded.
536 * <p>
537 * The {@code finalize} method of class {@code Object} performs no
538 * special action; it simply returns normally. Subclasses of
539 * {@code Object} may override this definition.
540 * <p>
541 * The Java programming language does not guarantee which thread will
542 * invoke the {@code finalize} method for any given object. It is
543 * guaranteed, however, that the thread that invokes finalize will not
544 * be holding any user-visible synchronization locks when finalize is
545 * invoked. If an uncaught exception is thrown by the finalize method,
546 * the exception is ignored and finalization of that object terminates.
547 * <p>
548 * After the {@code finalize} method has been invoked for an object, no
549 * further action is taken until the Java virtual machine has again
550 * determined that there is no longer any means by which this object can
551 * be accessed by any thread that has not yet died, including possible
552 * actions by other objects or classes which are ready to be finalized,
553 * at which point the object may be discarded.
554 * <p>
555 * The {@code finalize} method is never invoked more than once by a Java
556 * virtual machine for any given object.
557 * <p>
558 * Any exception thrown by the {@code finalize} method causes
559 * the finalization of this object to be halted, but is otherwise
560 * ignored.
561 *
562 * @throws Throwable the {@code Exception} raised by this method
563 * @see java.lang.ref.WeakReference
564 * @see java.lang.ref.PhantomReference
565 * @jls 12.6 Finalization of Class Instances
566 */
567 protected void finalize() throws Throwable { }
568 }