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