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}.
193 * Otherwise, this method creates a new instance of the class of this
194 * object and initializes all its fields with exactly the contents of
195 * the corresponding fields of this object, as if by assignment; the
196 * contents of the fields are not themselves cloned. Thus, this method
197 * performs a "shallow copy" of this object, not a "deep copy" operation.
198 * <p>
199 * The class {@code Object} does not itself implement the interface
200 * {@code Cloneable}, so calling the {@code clone} method on an object
201 * whose class is {@code Object} will result in throwing an
202 * exception at run time.
203 *
204 * @return a clone of this instance.
205 * @exception CloneNotSupportedException if the object's class does not
206 * support the {@code Cloneable} interface. Subclasses
207 * that override the {@code clone} method can also
208 * throw this exception to indicate that an instance cannot
209 * be cloned.
210 * @see java.lang.Cloneable
211 */
212 protected native Object clone() throws CloneNotSupportedException;
213
214 /**
215 * Returns a string representation of the object. In general, the
216 * {@code toString} method returns a string that
217 * "textually represents" this object. The result should
218 * be a concise but informative representation that is easy for a
219 * person to read.
220 * It is recommended that all subclasses override this method.
221 * <p>
222 * The {@code toString} method for class {@code Object}
223 * returns a string consisting of the name of the class of which the
224 * object is an instance, the at-sign character `{@code @}', and
225 * the unsigned hexadecimal representation of the hash code of the
226 * object. In other words, this method returns a string equal to the
227 * value of:
228 * <blockquote>
229 * <pre>
230 * getClass().getName() + '@' + Integer.toHexString(hashCode())
231 * </pre></blockquote>
232 *
233 * @return a string representation of the object.
234 */
235 public String toString() {
236 return getClass().getName() + "@" + Integer.toHexString(hashCode());
237 }
238
239 /**
240 * Wakes up a single thread that is waiting on this object's
241 * monitor. If any threads are waiting on this object, one of them
242 * is chosen to be awakened. The choice is arbitrary and occurs at
243 * the discretion of the implementation. A thread waits on an object's
244 * monitor by calling one of the {@code wait} methods.
245 * <p>
246 * The awakened thread will not be able to proceed until the current
247 * thread relinquishes the lock on this object. The awakened thread will
248 * compete in the usual manner with any other threads that might be
249 * actively competing to synchronize on this object; for example, the
250 * awakened thread enjoys no reliable privilege or disadvantage in being
251 * the next thread to lock this object.
252 * <p>
253 * This method should only be called by a thread that is the owner
254 * of this object's monitor. A thread becomes the owner of the
255 * object's monitor in one of three ways:
256 * <ul>
257 * <li>By executing a synchronized instance method of that object.
258 * <li>By executing the body of a {@code synchronized} statement
259 * that synchronizes on the object.
260 * <li>For objects of type {@code Class,} by executing a
261 * synchronized static method of that class.
262 * </ul>
263 * <p>
264 * Only one thread at a time can own an object's monitor.
265 *
266 * @exception IllegalMonitorStateException if the current thread is not
267 * the owner of this object's monitor.
268 * @see java.lang.Object#notifyAll()
269 * @see java.lang.Object#wait()
270 */
271 public final native void notify();
272
273 /**
274 * Wakes up all threads that are waiting on this object's monitor. A
275 * thread waits on an object's monitor by calling one of the
276 * {@code wait} methods.
277 * <p>
278 * The awakened threads will not be able to proceed until the current
279 * thread relinquishes the lock on this object. The awakened threads
280 * will compete in the usual manner with any other threads that might
281 * be actively competing to synchronize on this object; for example,
282 * the awakened threads enjoy no reliable privilege or disadvantage in
283 * being the next thread to lock this object.
284 * <p>
285 * This method should only be called by a thread that is the owner
286 * of this object's monitor. See the {@code notify} method for a
287 * description of the ways in which a thread can become the owner of
288 * a monitor.
289 *
290 * @exception IllegalMonitorStateException if the current thread is not
291 * the owner of this object's monitor.
292 * @see java.lang.Object#notify()
293 * @see java.lang.Object#wait()
294 */
295 public final native void notifyAll();
296
297 /**
298 * Causes the current thread to wait until either another thread invokes the
299 * {@link java.lang.Object#notify()} method or the
300 * {@link java.lang.Object#notifyAll()} method for this object, or a
301 * specified amount of time has elapsed.
302 * <p>
303 * The current thread must own this object's monitor.
304 * <p>
305 * This method causes the current thread (call it <var>T</var>) to
306 * place itself in the wait set for this object and then to relinquish
307 * any and all synchronization claims on this object. Thread <var>T</var>
308 * becomes disabled for thread scheduling purposes and lies dormant
309 * until one of four things happens:
310 * <ul>
311 * <li>Some other thread invokes the {@code notify} method for this
312 * object and thread <var>T</var> happens to be arbitrarily chosen as
313 * the thread to be awakened.
314 * <li>Some other thread invokes the {@code notifyAll} method for this
315 * object.
316 * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
317 * thread <var>T</var>.
318 * <li>The specified amount of real time has elapsed, more or less. If
319 * {@code timeout} is zero, however, then real time is not taken into
320 * consideration and the thread simply waits until notified.
321 * </ul>
322 * The thread <var>T</var> is then removed from the wait set for this
323 * object and re-enabled for thread scheduling. It then competes in the
324 * usual manner with other threads for the right to synchronize on the
325 * object; once it has gained control of the object, all its
326 * synchronization claims on the object are restored to the status quo
327 * ante - that is, to the situation as of the time that the {@code wait}
328 * method was invoked. Thread <var>T</var> then returns from the
329 * invocation of the {@code wait} method. Thus, on return from the
330 * {@code wait} method, the synchronization state of the object and of
331 * thread {@code T} is exactly as it was when the {@code wait} method
332 * was invoked.
333 * <p>
334 * A thread can also wake up without being notified, interrupted, or
335 * timing out, a so-called <i>spurious wakeup</i>. While this will rarely
336 * occur in practice, applications must guard against it by testing for
337 * the condition that should have caused the thread to be awakened, and
338 * continuing to wait if the condition is not satisfied. In other words,
339 * waits should always occur in loops, like this one:
340 * <pre>
341 * synchronized (obj) {
342 * while (<condition does not hold>)
343 * obj.wait(timeout);
344 * ... // Perform action appropriate to condition
345 * }
346 * </pre>
347 * (For more information on this topic, see Section 3.2.3 in Doug Lea's
348 * "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
349 * 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
350 * Language Guide" (Addison-Wesley, 2001).
351 *
352 * <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
353 * interrupted} by any thread before or while it is waiting, then an
354 * {@code InterruptedException} is thrown. This exception is not
355 * thrown until the lock status of this object has been restored as
356 * described above.
357 *
358 * <p>
359 * Note that the {@code wait} method, as it places the current thread
360 * into the wait set for this object, unlocks only this object; any
361 * other objects on which the current thread may be synchronized remain
362 * locked while the thread waits.
363 * <p>
364 * This method should only be called by a thread that is the owner
365 * of this object's monitor. See the {@code notify} method for a
366 * description of the ways in which a thread can become the owner of
367 * a monitor.
368 *
369 * @param timeout the maximum time to wait in milliseconds.
370 * @exception IllegalArgumentException if the value of timeout is
371 * negative.
372 * @exception IllegalMonitorStateException if the current thread is not
373 * the owner of the object's monitor.
374 * @exception InterruptedException if any thread interrupted the
375 * current thread before or while the current thread
376 * was waiting for a notification. The <i>interrupted
377 * status</i> of the current thread is cleared when
378 * this exception is thrown.
379 * @see java.lang.Object#notify()
380 * @see java.lang.Object#notifyAll()
381 */
382 public final native void wait(long timeout) throws InterruptedException;
383
384 /**
385 * Causes the current thread to wait until another thread invokes the
386 * {@link java.lang.Object#notify()} method or the
387 * {@link java.lang.Object#notifyAll()} method for this object, or
388 * some other thread interrupts the current thread, or a certain
389 * amount of real time has elapsed.
390 * <p>
391 * This method is similar to the {@code wait} method of one
392 * argument, but it allows finer control over the amount of time to
393 * wait for a notification before giving up. The amount of real time,
394 * measured in nanoseconds, is given by:
395 * <blockquote>
396 * <pre>
397 * 1000000*timeout+nanos</pre></blockquote>
398 * <p>
399 * In all other respects, this method does the same thing as the
400 * method {@link #wait(long)} of one argument. In particular,
401 * {@code wait(0, 0)} means the same thing as {@code wait(0)}.
402 * <p>
403 * The current thread must own this object's monitor. The thread
404 * releases ownership of this monitor and waits until either of the
405 * following two conditions has occurred:
406 * <ul>
407 * <li>Another thread notifies threads waiting on this object's monitor
408 * to wake up either through a call to the {@code notify} method
409 * or the {@code notifyAll} method.
410 * <li>The timeout period, specified by {@code timeout}
411 * milliseconds plus {@code nanos} nanoseconds arguments, has
412 * elapsed.
413 * </ul>
414 * <p>
415 * The thread then waits until it can re-obtain ownership of the
416 * monitor and resumes execution.
417 * <p>
418 * As in the one argument version, interrupts and spurious wakeups are
419 * possible, and this method should always be used in a loop:
420 * <pre>
421 * synchronized (obj) {
422 * while (<condition does not hold>)
423 * obj.wait(timeout, nanos);
424 * ... // Perform action appropriate to condition
425 * }
426 * </pre>
427 * This method should only be called by a thread that is the owner
428 * of this object's monitor. See the {@code notify} method for a
429 * description of the ways in which a thread can become the owner of
430 * a monitor.
431 *
432 * @param timeout the maximum time to wait in milliseconds.
433 * @param nanos additional time, in nanoseconds range
434 * 0-999999.
435 * @exception IllegalArgumentException if the value of timeout is
436 * negative or the value of nanos is
437 * not in the range 0-999999.
438 * @exception IllegalMonitorStateException if the current thread is not
439 * the owner of this object's monitor.
440 * @exception InterruptedException if any thread interrupted the
441 * current thread before or while the current thread
442 * was waiting for a notification. The <i>interrupted
443 * status</i> of the current thread is cleared when
444 * this exception is thrown.
445 */
446 public final void wait(long timeout, int nanos) throws InterruptedException {
447 if (timeout < 0) {
448 throw new IllegalArgumentException("timeout value is negative");
449 }
450
451 if (nanos < 0 || nanos > 999999) {
452 throw new IllegalArgumentException(
453 "nanosecond timeout value out of range");
454 }
455
456 if (nanos >= 500000 || (nanos != 0 && timeout == 0)) {
457 timeout++;
458 }
459
460 wait(timeout);
461 }
462
463 /**
464 * Causes the current thread to wait until another thread invokes the
465 * {@link java.lang.Object#notify()} method or the
466 * {@link java.lang.Object#notifyAll()} method for this object.
467 * In other words, this method behaves exactly as if it simply
468 * performs the call {@code wait(0)}.
469 * <p>
470 * The current thread must own this object's monitor. The thread
471 * releases ownership of this monitor and waits until another thread
472 * notifies threads waiting on this object's monitor to wake up
473 * either through a call to the {@code notify} method or the
474 * {@code notifyAll} method. The thread then waits until it can
475 * re-obtain ownership of the monitor and resumes execution.
476 * <p>
477 * As in the one argument version, interrupts and spurious wakeups are
478 * possible, and this method should always be used in a loop:
479 * <pre>
480 * synchronized (obj) {
481 * while (<condition does not hold>)
482 * obj.wait();
483 * ... // Perform action appropriate to condition
484 * }
485 * </pre>
486 * This method should only be called by a thread that is the owner
487 * of this object's monitor. See the {@code notify} method for a
488 * description of the ways in which a thread can become the owner of
489 * a monitor.
490 *
491 * @exception IllegalMonitorStateException if the current thread is not
492 * the owner of the object's monitor.
493 * @exception InterruptedException if any thread interrupted the
494 * current thread before or while the current thread
495 * was waiting for a notification. The <i>interrupted
496 * status</i> of the current thread is cleared when
497 * this exception is thrown.
498 * @see java.lang.Object#notify()
499 * @see java.lang.Object#notifyAll()
500 */
501 public final void wait() throws InterruptedException {
502 wait(0);
503 }
504
505 /**
506 * Called by the garbage collector on an object when garbage collection
507 * determines that there are no more references to the object.
508 * A subclass overrides the {@code finalize} method to dispose of
509 * system resources or to perform other cleanup.
510 * <p>
511 * The general contract of {@code finalize} is that it is invoked
512 * if and when the Java<font size="-2"><sup>TM</sup></font> virtual
513 * machine has determined that there is no longer any
514 * means by which this object can be accessed by any thread that has
515 * not yet died, except as a result of an action taken by the
516 * finalization of some other object or class which is ready to be
517 * finalized. The {@code finalize} method may take any action, including
518 * making this object available again to other threads; the usual purpose
519 * of {@code finalize}, however, is to perform cleanup actions before
520 * the object is irrevocably discarded. For example, the finalize method
521 * for an object that represents an input/output connection might perform
522 * explicit I/O transactions to break the connection before the object is
523 * permanently discarded.
524 * <p>
525 * The {@code finalize} method of class {@code Object} performs no
526 * special action; it simply returns normally. Subclasses of
527 * {@code Object} may override this definition.
528 * <p>
529 * The Java programming language does not guarantee which thread will
530 * invoke the {@code finalize} method for any given object. It is
531 * guaranteed, however, that the thread that invokes finalize will not
532 * be holding any user-visible synchronization locks when finalize is
533 * invoked. If an uncaught exception is thrown by the finalize method,
534 * the exception is ignored and finalization of that object terminates.
535 * <p>
536 * After the {@code finalize} method has been invoked for an object, no
537 * further action is taken until the Java virtual machine has again
538 * determined that there is no longer any means by which this object can
539 * be accessed by any thread that has not yet died, including possible
540 * actions by other objects or classes which are ready to be finalized,
541 * at which point the object may be discarded.
542 * <p>
543 * The {@code finalize} method is never invoked more than once by a Java
544 * virtual machine for any given object.
545 * <p>
546 * Any exception thrown by the {@code finalize} method causes
547 * the finalization of this object to be halted, but is otherwise
548 * ignored.
549 *
550 * @throws Throwable the {@code Exception} raised by this method
551 */
552 protected void finalize() throws Throwable { }
553 }