1 /*
2 * Copyright (c) 1994, 2006, 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 JDK1.0
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 * @see <a href="http://java.sun.com/docs/books/jls/">The Java
62 * Language Specification, Third Edition (15.8.2 Class
63 * Literals)</a>
64 */
65 public final native Class<?> getClass();
66
67 /**
68 * Returns a hash code value for the object. This method is
69 * supported for the benefit of hash tables such as those provided by
70 * {@link java.util.HashMap}.
71 * <p>
72 * The general contract of {@code hashCode} is:
73 * <ul>
74 * <li>Whenever it is invoked on the same object more than once during
75 * an execution of a Java application, the {@code hashCode} method
76 * must consistently return the same integer, provided no information
77 * used in {@code equals} comparisons on the object is modified.
78 * This integer need not remain consistent from one execution of an
79 * application to another execution of the same application.
80 * <li>If two objects are equal according to the {@code equals(Object)}
81 * method, then calling the {@code hashCode} method on each of
82 * the two objects must produce the same integer result.
83 * <li>It is <em>not</em> required that if two objects are unequal
84 * according to the {@link java.lang.Object#equals(java.lang.Object)}
85 * method, then calling the {@code hashCode} method on each of the
86 * two objects must produce distinct integer results. However, the
87 * programmer should be aware that producing distinct integer results
88 * for unequal objects may improve the performance of hash tables.
89 * </ul>
90 * <p>
91 * As much as is reasonably practical, the hashCode method defined by
92 * class {@code Object} does return distinct integers for distinct
93 * objects. (This is typically implemented by converting the internal
94 * address of the object into an integer, but this implementation
95 * technique is not required by the
96 * Java<font size="-2"><sup>TM</sup></font> programming language.)
97 *
98 * @return a hash code value for this object.
99 * @see java.lang.Object#equals(java.lang.Object)
100 * @see java.lang.System#identityHashCode
101 */
102 public native int hashCode();
103
104 /**
105 * Indicates whether some other object is "equal to" this one.
106 * <p>
107 * The {@code equals} method implements an equivalence relation
108 * on non-null object references:
109 * <ul>
110 * <li>It is <i>reflexive</i>: for any non-null reference value
111 * {@code x}, {@code x.equals(x)} should return
112 * {@code true}.
113 * <li>It is <i>symmetric</i>: for any non-null reference values
114 * {@code x} and {@code y}, {@code x.equals(y)}
115 * should return {@code true} if and only if
116 * {@code y.equals(x)} returns {@code true}.
117 * <li>It is <i>transitive</i>: for any non-null reference values
118 * {@code x}, {@code y}, and {@code z}, if
119 * {@code x.equals(y)} returns {@code true} and
120 * {@code y.equals(z)} returns {@code true}, then
121 * {@code x.equals(z)} should return {@code true}.
122 * <li>It is <i>consistent</i>: for any non-null reference values
123 * {@code x} and {@code y}, multiple invocations of
124 * {@code x.equals(y)} consistently return {@code true}
125 * or consistently return {@code false}, provided no
126 * information used in {@code equals} comparisons on the
127 * objects is modified.
128 * <li>For any non-null reference value {@code x},
129 * {@code x.equals(null)} should return {@code false}.
130 * </ul>
131 * <p>
132 * The {@code equals} method for class {@code Object} implements
133 * the most discriminating possible equivalence relation on objects;
134 * that is, for any non-null reference values {@code x} and
135 * {@code y}, this method returns {@code true} if and only
136 * if {@code x} and {@code y} refer to the same object
137 * ({@code x == y} has the value {@code true}).
138 * <p>
139 * Note that it is generally necessary to override the {@code hashCode}
140 * method whenever this method is overridden, so as to maintain the
141 * general contract for the {@code hashCode} method, which states
142 * that equal objects must have equal hash codes.
143 *
144 * @param obj the reference object with which to compare.
145 * @return {@code true} if this object is the same as the obj
146 * argument; {@code false} otherwise.
147 * @see #hashCode()
148 * @see java.util.HashMap
149 */
150 public boolean equals(Object obj) {
151 return (this == obj);
152 }
153
154 /**
155 * Creates and returns a copy of this object. The precise meaning
156 * of "copy" may depend on the class of the object. The general
157 * intent is that, for any object {@code x}, the expression:
158 * <blockquote>
159 * <pre>
160 * x.clone() != x</pre></blockquote>
161 * will be true, and that the expression:
162 * <blockquote>
163 * <pre>
164 * x.clone().getClass() == x.getClass()</pre></blockquote>
165 * will be {@code true}, but these are not absolute requirements.
166 * While it is typically the case that:
167 * <blockquote>
168 * <pre>
169 * x.clone().equals(x)</pre></blockquote>
170 * will be {@code true}, this is not an absolute requirement.
171 * <p>
172 * By convention, the returned object should be obtained by calling
173 * {@code super.clone}. If a class and all of its superclasses (except
174 * {@code Object}) obey this convention, it will be the case that
175 * {@code x.clone().getClass() == x.getClass()}.
176 * <p>
177 * By convention, the object returned by this method should be independent
178 * of this object (which is being cloned). To achieve this independence,
179 * it may be necessary to modify one or more fields of the object returned
180 * by {@code super.clone} before returning it. Typically, this means
181 * copying any mutable objects that comprise the internal "deep structure"
182 * of the object being cloned and replacing the references to these
183 * objects with references to the copies. If a class contains only
184 * primitive fields or references to immutable objects, then it is usually
185 * the case that no fields in the object returned by {@code super.clone}
186 * need to be modified.
187 * <p>
188 * The method {@code clone} for class {@code Object} performs a
189 * specific cloning operation. First, if the class of this object does
190 * not implement the interface {@code Cloneable}, then a
191 * {@code CloneNotSupportedException} is thrown. Note that all arrays
192 * are considered to implement the interface {@code Cloneable} and that
193 * the return type of the {@code clone} method of an array type
194 * {@code T[]} is {@code T[]}.
195 * Otherwise, this method creates a new instance of the class of this
196 * object and initializes all its fields with exactly the contents of
197 * the corresponding fields of this object, as if by assignment; the
198 * contents of the fields are not themselves cloned. Thus, this method
199 * performs a "shallow copy" of this object, not a "deep copy" operation.
200 * <p>
201 * The class {@code Object} does not itself implement the interface
202 * {@code Cloneable}, so calling the {@code clone} method on an object
203 * whose class is {@code Object} will result in throwing an
204 * exception at run time.
205 *
206 * @return a clone of this instance.
207 * @exception CloneNotSupportedException if the object's class does not
208 * support the {@code Cloneable} interface. Subclasses
209 * that override the {@code clone} method can also
210 * throw this exception to indicate that an instance cannot
211 * be cloned.
212 * @see java.lang.Cloneable
213 */
214 protected native Object clone() throws CloneNotSupportedException;
215
216 /**
217 * Returns a string representation of the object. In general, the
218 * {@code toString} method returns a string that
219 * "textually represents" this object. The result should
220 * be a concise but informative representation that is easy for a
221 * person to read.
222 * It is recommended that all subclasses override this method.
223 * <p>
224 * The {@code toString} method for class {@code Object}
225 * returns a string consisting of the name of the class of which the
226 * object is an instance, the at-sign character `{@code @}', and
227 * the unsigned hexadecimal representation of the hash code of the
228 * object. In other words, this method returns a string equal to the
229 * value of:
230 * <blockquote>
231 * <pre>
232 * getClass().getName() + '@' + Integer.toHexString(hashCode())
233 * </pre></blockquote>
234 *
235 * @return a string representation of the object.
236 */
237 public String toString() {
238 return getClass().getName() + "@" + Integer.toHexString(hashCode());
239 }
240
241 /**
242 * Wakes up a single thread that is waiting on this object's
243 * monitor. If any threads are waiting on this object, one of them
244 * is chosen to be awakened. The choice is arbitrary and occurs at
245 * the discretion of the implementation. A thread waits on an object's
246 * monitor by calling one of the {@code wait} methods.
247 * <p>
248 * The awakened thread will not be able to proceed until the current
249 * thread relinquishes the lock on this object. The awakened thread will
250 * compete in the usual manner with any other threads that might be
251 * actively competing to synchronize on this object; for example, the
252 * awakened thread enjoys no reliable privilege or disadvantage in being
253 * the next thread to lock this object.
254 * <p>
255 * This method should only be called by a thread that is the owner
256 * of this object's monitor. A thread becomes the owner of the
257 * object's monitor in one of three ways:
258 * <ul>
259 * <li>By executing a synchronized instance method of that object.
260 * <li>By executing the body of a {@code synchronized} statement
261 * that synchronizes on the object.
262 * <li>For objects of type {@code Class,} by executing a
263 * synchronized static method of that class.
264 * </ul>
265 * <p>
266 * Only one thread at a time can own an object's monitor.
267 *
268 * @exception IllegalMonitorStateException if the current thread is not
269 * the owner of this object's monitor.
270 * @see java.lang.Object#notifyAll()
271 * @see java.lang.Object#wait()
272 */
273 public final native void notify();
274
275 /**
276 * Wakes up all threads that are waiting on this object's monitor. A
277 * thread waits on an object's monitor by calling one of the
278 * {@code wait} methods.
279 * <p>
280 * The awakened threads will not be able to proceed until the current
281 * thread relinquishes the lock on this object. The awakened threads
282 * will compete in the usual manner with any other threads that might
283 * be actively competing to synchronize on this object; for example,
284 * the awakened threads enjoy no reliable privilege or disadvantage in
285 * being the next thread to lock this object.
286 * <p>
287 * This method should only be called by a thread that is the owner
288 * of this object's monitor. See the {@code notify} method for a
289 * description of the ways in which a thread can become the owner of
290 * a monitor.
291 *
292 * @exception IllegalMonitorStateException if the current thread is not
293 * the owner of this object's monitor.
294 * @see java.lang.Object#notify()
295 * @see java.lang.Object#wait()
296 */
297 public final native void notifyAll();
298
299 /**
300 * Causes the current thread to wait until either another thread invokes the
301 * {@link java.lang.Object#notify()} method or the
302 * {@link java.lang.Object#notifyAll()} method for this object, or a
303 * specified amount of time has elapsed.
304 * <p>
305 * The current thread must own this object's monitor.
306 * <p>
307 * This method causes the current thread (call it <var>T</var>) to
308 * place itself in the wait set for this object and then to relinquish
309 * any and all synchronization claims on this object. Thread <var>T</var>
310 * becomes disabled for thread scheduling purposes and lies dormant
311 * until one of four things happens:
312 * <ul>
313 * <li>Some other thread invokes the {@code notify} method for this
314 * object and thread <var>T</var> happens to be arbitrarily chosen as
315 * the thread to be awakened.
316 * <li>Some other thread invokes the {@code notifyAll} method for this
317 * object.
318 * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
319 * thread <var>T</var>.
320 * <li>The specified amount of real time has elapsed, more or less. If
321 * {@code timeout} is zero, however, then real time is not taken into
322 * consideration and the thread simply waits until notified.
323 * </ul>
324 * The thread <var>T</var> is then removed from the wait set for this
325 * object and re-enabled for thread scheduling. It then competes in the
326 * usual manner with other threads for the right to synchronize on the
327 * object; once it has gained control of the object, all its
328 * synchronization claims on the object are restored to the status quo
329 * ante - that is, to the situation as of the time that the {@code wait}
330 * method was invoked. Thread <var>T</var> then returns from the
331 * invocation of the {@code wait} method. Thus, on return from the
332 * {@code wait} method, the synchronization state of the object and of
333 * thread {@code T} is exactly as it was when the {@code wait} method
334 * was invoked.
335 * <p>
336 * A thread can also wake up without being notified, interrupted, or
337 * timing out, a so-called <i>spurious wakeup</i>. While this will rarely
338 * occur in practice, applications must guard against it by testing for
339 * the condition that should have caused the thread to be awakened, and
340 * continuing to wait if the condition is not satisfied. In other words,
341 * waits should always occur in loops, like this one:
342 * <pre>
343 * synchronized (obj) {
344 * while (<condition does not hold>)
345 * obj.wait(timeout);
346 * ... // Perform action appropriate to condition
347 * }
348 * </pre>
349 * (For more information on this topic, see Section 3.2.3 in Doug Lea's
350 * "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
351 * 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
352 * Language Guide" (Addison-Wesley, 2001).
353 *
354 * <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
355 * interrupted} by any thread before or while it is waiting, then an
356 * {@code InterruptedException} is thrown. This exception is not
357 * thrown until the lock status of this object has been restored as
358 * described above.
359 *
360 * <p>
361 * Note that the {@code wait} method, as it places the current thread
362 * into the wait set for this object, unlocks only this object; any
363 * other objects on which the current thread may be synchronized remain
364 * locked while the thread waits.
365 * <p>
366 * This method should only be called by a thread that is the owner
367 * of this object's monitor. See the {@code notify} method for a
368 * description of the ways in which a thread can become the owner of
369 * a monitor.
370 *
371 * @param timeout the maximum time to wait in milliseconds.
372 * @exception IllegalArgumentException if the value of timeout is
373 * negative.
374 * @exception IllegalMonitorStateException if the current thread is not
375 * the owner of the object's monitor.
376 * @exception InterruptedException if any thread interrupted the
377 * current thread before or while the current thread
378 * was waiting for a notification. The <i>interrupted
379 * status</i> of the current thread is cleared when
380 * this exception is thrown.
381 * @see java.lang.Object#notify()
382 * @see java.lang.Object#notifyAll()
383 */
384 public final native void wait(long timeout) throws InterruptedException;
385
386 /**
387 * Causes the current thread to wait until another thread invokes the
388 * {@link java.lang.Object#notify()} method or the
389 * {@link java.lang.Object#notifyAll()} method for this object, or
390 * some other thread interrupts the current thread, or a certain
391 * amount of real time has elapsed.
392 * <p>
393 * This method is similar to the {@code wait} method of one
394 * argument, but it allows finer control over the amount of time to
395 * wait for a notification before giving up. The amount of real time,
396 * measured in nanoseconds, is given by:
397 * <blockquote>
398 * <pre>
399 * 1000000*timeout+nanos</pre></blockquote>
400 * <p>
401 * In all other respects, this method does the same thing as the
402 * method {@link #wait(long)} of one argument. In particular,
403 * {@code wait(0, 0)} means the same thing as {@code wait(0)}.
404 * <p>
405 * The current thread must own this object's monitor. The thread
406 * releases ownership of this monitor and waits until either of the
407 * following two conditions has occurred:
408 * <ul>
409 * <li>Another thread notifies threads waiting on this object's monitor
410 * to wake up either through a call to the {@code notify} method
411 * or the {@code notifyAll} method.
412 * <li>The timeout period, specified by {@code timeout}
413 * milliseconds plus {@code nanos} nanoseconds arguments, has
414 * elapsed.
415 * </ul>
416 * <p>
417 * The thread then waits until it can re-obtain ownership of the
418 * monitor and resumes execution.
419 * <p>
420 * As in the one argument version, interrupts and spurious wakeups are
421 * possible, and this method should always be used in a loop:
422 * <pre>
423 * synchronized (obj) {
424 * while (<condition does not hold>)
425 * obj.wait(timeout, nanos);
426 * ... // Perform action appropriate to condition
427 * }
428 * </pre>
429 * This method should only be called by a thread that is the owner
430 * of this object's monitor. See the {@code notify} method for a
431 * description of the ways in which a thread can become the owner of
432 * a monitor.
433 *
434 * @param timeout the maximum time to wait in milliseconds.
435 * @param nanos additional time, in nanoseconds range
436 * 0-999999.
437 * @exception IllegalArgumentException if the value of timeout is
438 * negative or the value of nanos is
439 * not in the range 0-999999.
440 * @exception IllegalMonitorStateException if the current thread is not
441 * the owner of this object's monitor.
442 * @exception InterruptedException if any thread interrupted the
443 * current thread before or while the current thread
444 * was waiting for a notification. The <i>interrupted
445 * status</i> of the current thread is cleared when
446 * this exception is thrown.
447 */
448 public final void wait(long timeout, int nanos) throws InterruptedException {
449 if (timeout < 0) {
450 throw new IllegalArgumentException("timeout value is negative");
451 }
452
453 if (nanos < 0 || nanos > 999999) {
454 throw new IllegalArgumentException(
455 "nanosecond timeout value out of range");
456 }
457
458 if (nanos >= 500000 || (nanos != 0 && timeout == 0)) {
459 timeout++;
460 }
461
462 wait(timeout);
463 }
464
465 /**
466 * Causes the current thread to wait until another thread invokes the
467 * {@link java.lang.Object#notify()} method or the
468 * {@link java.lang.Object#notifyAll()} method for this object.
469 * In other words, this method behaves exactly as if it simply
470 * performs the call {@code wait(0)}.
471 * <p>
472 * The current thread must own this object's monitor. The thread
473 * releases ownership of this monitor and waits until another thread
474 * notifies threads waiting on this object's monitor to wake up
475 * either through a call to the {@code notify} method or the
476 * {@code notifyAll} method. The thread then waits until it can
477 * re-obtain ownership of the monitor and resumes execution.
478 * <p>
479 * As in the one argument version, interrupts and spurious wakeups are
480 * possible, and this method should always be used in a loop:
481 * <pre>
482 * synchronized (obj) {
483 * while (<condition does not hold>)
484 * obj.wait();
485 * ... // Perform action appropriate to condition
486 * }
487 * </pre>
488 * This method should only be called by a thread that is the owner
489 * of this object's monitor. See the {@code notify} method for a
490 * description of the ways in which a thread can become the owner of
491 * a monitor.
492 *
493 * @exception IllegalMonitorStateException if the current thread is not
494 * the owner of the object's monitor.
495 * @exception InterruptedException if any thread interrupted the
496 * current thread before or while the current thread
497 * was waiting for a notification. The <i>interrupted
498 * status</i> of the current thread is cleared when
499 * this exception is thrown.
500 * @see java.lang.Object#notify()
501 * @see java.lang.Object#notifyAll()
502 */
503 public final void wait() throws InterruptedException {
504 wait(0);
505 }
506
507 /**
508 * Called by the garbage collector on an object when garbage collection
509 * determines that there are no more references to the object.
510 * A subclass overrides the {@code finalize} method to dispose of
511 * system resources or to perform other cleanup.
512 * <p>
513 * The general contract of {@code finalize} is that it is invoked
514 * if and when the Java<font size="-2"><sup>TM</sup></font> virtual
515 * machine has determined that there is no longer any
516 * means by which this object can be accessed by any thread that has
517 * not yet died, except as a result of an action taken by the
518 * finalization of some other object or class which is ready to be
519 * finalized. The {@code finalize} method may take any action, including
520 * making this object available again to other threads; the usual purpose
521 * of {@code finalize}, however, is to perform cleanup actions before
522 * the object is irrevocably discarded. For example, the finalize method
523 * for an object that represents an input/output connection might perform
524 * explicit I/O transactions to break the connection before the object is
525 * permanently discarded.
526 * <p>
527 * The {@code finalize} method of class {@code Object} performs no
528 * special action; it simply returns normally. Subclasses of
529 * {@code Object} may override this definition.
530 * <p>
531 * The Java programming language does not guarantee which thread will
532 * invoke the {@code finalize} method for any given object. It is
533 * guaranteed, however, that the thread that invokes finalize will not
534 * be holding any user-visible synchronization locks when finalize is
535 * invoked. If an uncaught exception is thrown by the finalize method,
536 * the exception is ignored and finalization of that object terminates.
537 * <p>
538 * After the {@code finalize} method has been invoked for an object, no
539 * further action is taken until the Java virtual machine has again
540 * determined that there is no longer any means by which this object can
541 * be accessed by any thread that has not yet died, including possible
542 * actions by other objects or classes which are ready to be finalized,
543 * at which point the object may be discarded.
544 * <p>
545 * The {@code finalize} method is never invoked more than once by a Java
546 * virtual machine for any given object.
547 * <p>
548 * Any exception thrown by the {@code finalize} method causes
549 * the finalization of this object to be halted, but is otherwise
550 * ignored.
551 *
552 * @throws Throwable the {@code Exception} raised by this method
553 */
554 protected void finalize() throws Throwable { }
555 }