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