Java的Object对象
Object对象是除了基础对象之外,所有的对象都需要继承的父对象,包括数组也继承了Object
Object里面的关键函数罗列如下:
clone();调用该函数需要实现 Cloneable,否则会抛出 CloneNotSupportedException的异常。
equals();用来判断两个对象是否相等的函数,默认的是两个对象的地址比较,在使用Compare计算的时候,往往需要重写这个函数。
finalize();这个函数在对象被消解的时候,例如垃圾回收的时候等,会被调用,如果对象使用了不可被自动回收的内存空间等资源,应该在这个函数里面收回。
hashCode();返回的是这个对象的hash值,期内部的实现代码如下:
- int lockWord = shadow$_monitor_;
- final int lockWordMask = 0xC0000000; // Top 2 bits.
- final int lockWordStateHash = 0x80000000; // Top 2 bits are value 2 (kStateHash).
- if ((lockWord & lockWordMask) == lockWordStateHash) {
- return lockWord & ~lockWordMask;
- }
- return System.identityHashCode(this);
- & lockWordMask是获取最高两位,
- lockWordStateHash是指最高两位是2(10),最高位是符号位,但是不明白为什么要获取两位。
- return lockWord & ~lockWordMask; ~是非运算符,所以这句话是返回了除了最高两位的所有其它位的值。
- 如果不满足上述条件,返回的是
- System.identityHashCode(this);,这个是native的方法,没有深入
notify():唤醒被这个对象的monitor标记等待的线程,如果线程的数量大于1,那么被唤醒的线程是由VM自行决定的。注意被唤醒的线程不一定立即执行,至少要等待当前调用notify的线程释放这个对象的monitor,或者等待其他正好同步锁定该对象的线程释放了该对象。 当然了,也不是任何地方都可以调用notify的,调用的地方必须持有对象的monitor,可以有以下几种情况:1.在一个同步( synchronized)方法中;2.在一段该对象的同步代码块中;3.如果这个变量是类变量(static),同步的静态方法也持有。
notifyAll(): 与上面所属的notify类似,只不过是唤醒了所有的线程,当然这些线程也不是立即执行,理由同上。
toString():这个对象的字符串描述,默认的是返回类名和实例的hashCode,代码如下:
- public String toString() {
- return getClass().getName() + '@' + Integer.toHexString(hashCode());
- }
wait(): 通知当前线程挂起,当对象的notify或者notifyAll被调用的时候才会被重新唤醒,wait了的thread是可以被中断(interrupt)的。当线程wait的时候,这个线程其实丢失了对象的monitor,当被notify的时候,会在程序执行前重新请求到对象的monitor。
还有两个wait函数是带参数的,参数指明了线程的wait时长,如果在这个时长内,线程没有被唤醒,那么当时间到达的时候,这个线程也会被重新唤醒。其他的与上面的wait一致,对了时间为0,说明没有超时时间,时间不可以小于零,否则抛出 IllegalArgumentException
- 下文中的shadow$_monitor_和shadow$_klass_ 是Android sdk21之后Object增加的两个字段。前者好像是用来表明地址的,后者是用来说明类型的。
- Android中的Object源码如下:
- /*
- * Licensed to the Apache Software Foundation (ASF) under one or more
- * contributor license agreements. See the NOTICE file distributed with
- * this work for additional information regarding copyright ownership.
- * The ASF licenses this file to You under the Apache License, Version 2.0
- * (the "License"); you may not use this file except in compliance with
- * the License. You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
- /*
- * Copyright (C) 2008 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
- package java.lang;
- /**
- * The root class of the Java class hierarchy. All non-primitive types
- * (including arrays) inherit either directly or indirectly from this class.
- *
- * <a name="writing_equals"><h4>Writing a correct {@code equals} method</h4></a>
- * <p>Follow this style to write a canonical {@code equals} method:
- * <pre>
- * // Use @Override to avoid accidental overloading.
- * @Override public boolean equals(Object o) {
- * // Return true if the objects are identical.
- * // (This is just an optimization, not required for correctness.)
- * if (this == o) {
- * return true;
- * }
- *
- * // Return false if the other object has the wrong type.
- * // This type may be an interface depending on the interface's specification.
- * if (!(o instanceof MyType)) {
- * return false;
- * }
- *
- * // Cast to the appropriate type.
- * // This will succeed because of the instanceof, and lets us access private fields.
- * MyType lhs = (MyType) o;
- *
- * // Check each field. Primitive fields, reference fields, and nullable reference
- * // fields are all treated differently.
- * return primitiveField == lhs.primitiveField &&
- * referenceField.equals(lhs.referenceField) &&
- * (nullableField == null ? lhs.nullableField == null
- * : nullableField.equals(lhs.nullableField));
- * }
- * </pre>
- * <p>If you override {@code equals}, you should also override {@code hashCode}: equal
- * instances must have equal hash codes.
- *
- * <p>See <i>Effective Java</i> item 8 for much more detail and clarification.
- *
- * <a name="writing_hashCode"><h4>Writing a correct {@code hashCode} method</h4></a>
- * <p>Follow this style to write a canonical {@code hashCode} method:
- * <pre>
- * @Override public int hashCode() {
- * // Start with a non-zero constant.
- * int result = 17;
- *
- * // Include a hash for each field.
- * result = 31 * result + (booleanField ? 1 : 0);
- *
- * result = 31 * result + byteField;
- * result = 31 * result + charField;
- * result = 31 * result + shortField;
- * result = 31 * result + intField;
- *
- * result = 31 * result + (int) (longField ^ (longField >>> 32));
- *
- * result = 31 * result + Float.floatToIntBits(floatField);
- *
- * long doubleFieldBits = Double.doubleToLongBits(doubleField);
- * result = 31 * result + (int) (doubleFieldBits ^ (doubleFieldBits >>> 32));
- *
- * result = 31 * result + Arrays.hashCode(arrayField);
- *
- * result = 31 * result + referenceField.hashCode();
- * result = 31 * result +
- * (nullableReferenceField == null ? 0
- * : nullableReferenceField.hashCode());
- *
- * return result;
- * }
- * </pre>
- *
- * <p>If you don't intend your type to be used as a hash key, don't simply rely on the default
- * {@code hashCode} implementation, because that silently and non-obviously breaks any future
- * code that does use your type as a hash key. You should throw instead:
- * <pre>
- * @Override public int hashCode() {
- * throw new UnsupportedOperationException();
- * }
- * </pre>
- *
- * <p>See <i>Effective Java</i> item 9 for much more detail and clarification.
- *
- * <a name="writing_toString"><h4>Writing a useful {@code toString} method</h4></a>
- * <p>For debugging convenience, it's common to override {@code toString} in this style:
- * <pre>
- * @Override public String toString() {
- * return getClass().getName() + "[" +
- * "primitiveField=" + primitiveField + ", " +
- * "referenceField=" + referenceField + ", " +
- * "arrayField=" + Arrays.toString(arrayField) + "]";
- * }
- * </pre>
- * <p>The set of fields to include is generally the same as those that would be tested
- * in your {@code equals} implementation.
- * <p>See <i>Effective Java</i> item 10 for much more detail and clarification.
- */
- public class Object {
- private transient Class<?> shadow$_klass_;
- private transient int shadow$_monitor_;
- // Uncomment the following two fields to enable brooks pointers.
- // Meant to do "#ifdef USE_BROOKS_POINTER ... #endif" but no macros.
- //
- // Note names use a 'x' prefix and the _x_rb_ptr_ field is of
- // type int instead of Object to go with the alphabetical/by-type
- // field order.
- // private transient int shadow$_x_rb_ptr_;
- // private transient int shadow$_x_xpadding_;
- /**
- * Constructs a new instance of {@code Object}.
- */
- public Object() {
- }
- /**
- * Creates and returns a copy of this {@code Object}. The default
- * implementation returns a so-called "shallow" copy: It creates a new
- * instance of the same class and then copies the field values (including
- * object references) from this instance to the new instance. A "deep" copy,
- * in contrast, would also recursively clone nested objects. A subclass that
- * needs to implement this kind of cloning should call {@code super.clone()}
- * to create the new instance and then create deep copies of the nested,
- * mutable objects.
- *
- * @return a copy of this object.
- * @throws CloneNotSupportedException
- * if this object's class does not implement the {@code
- * Cloneable} interface.
- */
- protected Object clone() throws CloneNotSupportedException {
- if (!(this instanceof Cloneable)) {
- throw new CloneNotSupportedException("Class " + getClass().getName() +
- " doesn't implement Cloneable");
- }
- return internalClone();
- }
- /*
- * Native helper method for cloning.
- */
- private native Object internalClone();
- /**
- * Compares this instance with the specified object and indicates if they
- * are equal. In order to be equal, {@code o} must represent the same object
- * as this instance using a class-specific comparison. The general contract
- * is that this comparison should be reflexive, symmetric, and transitive.
- * Also, no object reference other than null is equal to null.
- *
- * <p>The default implementation returns {@code true} only if {@code this ==
- * o}. See <a href="{@docRoot}reference/java/lang/Object.html#writing_equals">Writing a correct
- * {@code equals} method</a>
- * if you intend implementing your own {@code equals} method.
- *
- * <p>The general contract for the {@code equals} and {@link
- * #hashCode()} methods is that if {@code equals} returns {@code true} for
- * any two objects, then {@code hashCode()} must return the same value for
- * these objects. This means that subclasses of {@code Object} usually
- * override either both methods or neither of them.
- *
- * @param o
- * the object to compare this instance with.
- * @return {@code true} if the specified object is equal to this {@code
- * Object}; {@code false} otherwise.
- * @see #hashCode
- */
- public boolean equals(Object o) {
- return this == o;
- }
- /**
- * Invoked when the garbage collector has detected that this instance is no longer reachable.
- * The default implementation does nothing, but this method can be overridden to free resources.
- *
- * <p>Note that objects that override {@code finalize} are significantly more expensive than
- * objects that don't. Finalizers may be run a long time after the object is no longer
- * reachable, depending on memory pressure, so it's a bad idea to rely on them for cleanup.
- * Note also that finalizers are run on a single VM-wide finalizer thread,
- * so doing blocking work in a finalizer is a bad idea. A finalizer is usually only necessary
- * for a class that has a native peer and needs to call a native method to destroy that peer.
- * Even then, it's better to provide an explicit {@code close} method (and implement
- * {@link java.io.Closeable}), and insist that callers manually dispose of instances. This
- * works well for something like files, but less well for something like a {@code BigInteger}
- * where typical calling code would have to deal with lots of temporaries. Unfortunately,
- * code that creates lots of temporaries is the worst kind of code from the point of view of
- * the single finalizer thread.
- *
- * <p>If you <i>must</i> use finalizers, consider at least providing your own
- * {@link java.lang.ref.ReferenceQueue} and having your own thread process that queue.
- *
- * <p>Unlike constructors, finalizers are not automatically chained. You are responsible for
- * calling {@code super.finalize()} yourself.
- *
- * <p>Uncaught exceptions thrown by finalizers are ignored and do not terminate the finalizer
- * thread.
- *
- * See <i>Effective Java</i> Item 7, "Avoid finalizers" for more.
- */
- @FindBugsSuppressWarnings("FI_EMPTY")
- protected void finalize() throws Throwable {
- }
- /**
- * Returns the unique instance of {@link Class} that represents this
- * object's class. Note that {@code getClass()} is a special case in that it
- * actually returns {@code Class<? extends Foo>} where {@code Foo} is the
- * erasure of the type of the expression {@code getClass()} was called upon.
- * <p>
- * As an example, the following code actually compiles, although one might
- * think it shouldn't:
- * <p>
- * <pre>{@code
- * List<Integer> l = new ArrayList<Integer>();
- * Class<? extends List> c = l.getClass();}</pre>
- *
- * @return this object's {@code Class} instance.
- */
- public final Class<?> getClass() {
- return shadow$_klass_;
- }
- /**
- * Returns an integer hash code for this object. By contract, any two
- * objects for which {@link #equals} returns {@code true} must return
- * the same hash code value. This means that subclasses of {@code Object}
- * usually override both methods or neither method.
- *
- * <p>Note that hash values must not change over time unless information used in equals
- * comparisons also changes.
- *
- * <p>See <a href="{@docRoot}reference/java/lang/Object.html#writing_hashCode">Writing a correct
- * {@code hashCode} method</a>
- * if you intend implementing your own {@code hashCode} method.
- *
- * @return this object's hash code.
- * @see #equals
- */
- public int hashCode() {
- int lockWord = shadow$_monitor_;
- final int lockWordMask = 0xC0000000; // Top 2 bits.
- final int lockWordStateHash = 0x80000000; // Top 2 bits are value 2 (kStateHash).
- if ((lockWord & lockWordMask) == lockWordStateHash) {
- return lockWord & ~lockWordMask;
- }
- return System.identityHashCode(this);
- }
- /**
- * Causes a thread which is waiting on this object's monitor (by means of
- * calling one of the {@code wait()} methods) to be woken up. If more than
- * one thread is waiting, one of them is chosen at the discretion of the
- * VM. The chosen thread will not run immediately. The thread
- * that called {@code notify()} has to release the object's monitor first.
- * Also, the chosen thread still has to compete against other threads that
- * try to synchronize on the same object.
- *
- * <p>This method can only be invoked by a thread which owns this object's
- * monitor. A thread becomes owner of an object's monitor
- * <ul>
- * <li>by executing a synchronized method of that object;</li>
- * <li>by executing the body of a {@code synchronized} statement that
- * synchronizes on the object;</li>
- * <li>by executing a synchronized static method if the object is of type
- * {@code Class}.</li>
- * </ul>
- *
- * @see #notifyAll
- * @see #wait()
- * @see #wait(long)
- * @see #wait(long,int)
- * @see java.lang.Thread
- */
- public final native void notify();
- /**
- * Causes all threads which are waiting on this object's monitor (by means
- * of calling one of the {@code wait()} methods) to be woken up. The threads
- * will not run immediately. The thread that called {@code notify()} has to
- * release the object's monitor first. Also, the threads still have to
- * compete against other threads that try to synchronize on the same object.
- *
- * <p>This method can only be invoked by a thread which owns this object's
- * monitor. A thread becomes owner of an object's monitor
- * <ul>
- * <li>by executing a synchronized method of that object;</li>
- * <li>by executing the body of a {@code synchronized} statement that
- * synchronizes on the object;</li>
- * <li>by executing a synchronized static method if the object is of type
- * {@code Class}.</li>
- * </ul>
- *
- * @throws IllegalMonitorStateException
- * if the thread calling this method is not the owner of this
- * object's monitor.
- * @see #notify
- * @see #wait()
- * @see #wait(long)
- * @see #wait(long,int)
- * @see java.lang.Thread
- */
- public final native void notifyAll();
- /**
- * Returns a string containing a concise, human-readable description of this
- * object. Subclasses are encouraged to override this method and provide an
- * implementation that takes into account the object's type and data. The
- * default implementation is equivalent to the following expression:
- * <pre>
- * getClass().getName() + '@' + Integer.toHexString(hashCode())</pre>
- * <p>See <a href="{@docRoot}reference/java/lang/Object.html#writing_toString">Writing a useful
- * {@code toString} method</a>
- * if you intend implementing your own {@code toString} method.
- *
- * @return a printable representation of this object.
- */
- public String toString() {
- return getClass().getName() + '@' + Integer.toHexString(hashCode());
- }
- /**
- * Causes the calling thread to wait until another thread calls the {@code
- * notify()} or {@code notifyAll()} method of this object. This method can
- * only be invoked by a thread which owns this object's monitor; see
- * {@link #notify()} on how a thread can become the owner of a monitor.
- *
- * <p>A waiting thread can be sent {@code interrupt()} to cause it to
- * prematurely stop waiting, so {@code wait} should be called in a loop to
- * check that the condition that has been waited for has been met before
- * continuing.
- *
- * <p>While the thread waits, it gives up ownership of this object's
- * monitor. When it is notified (or interrupted), it re-acquires the monitor
- * before it starts running.
- *
- * @throws IllegalMonitorStateException
- * if the thread calling this method is not the owner of this
- * object's monitor.
- * @throws InterruptedException if the current thread has been interrupted.
- * The interrupted status of the current thread will be cleared before the exception
- * is thrown.
- * @see #notify
- * @see #notifyAll
- * @see #wait(long)
- * @see #wait(long,int)
- * @see java.lang.Thread
- */
- public final native void wait() throws InterruptedException;
- /**
- * Causes the calling thread to wait until another thread calls the {@code
- * notify()} or {@code notifyAll()} method of this object or until the
- * specified timeout expires. This method can only be invoked by a thread
- * which owns this object's monitor; see {@link #notify()} on how a thread
- * can become the owner of a monitor.
- *
- * <p>A waiting thread can be sent {@code interrupt()} to cause it to
- * prematurely stop waiting, so {@code wait} should be called in a loop to
- * check that the condition that has been waited for has been met before
- * continuing.
- *
- * <p>While the thread waits, it gives up ownership of this object's
- * monitor. When it is notified (or interrupted), it re-acquires the monitor
- * before it starts running.
- *
- * <p>A timeout of zero means the calling thread should wait forever unless interrupted or
- * notified.
- *
- * @param millis
- * the maximum time to wait in milliseconds.
- * @throws IllegalArgumentException
- * if {@code millis < 0}.
- * @throws IllegalMonitorStateException
- * if the thread calling this method is not the owner of this
- * object's monitor.
- * @throws InterruptedException if the current thread has been interrupted.
- * The interrupted status of the current thread will be cleared before the exception
- * is thrown.
- * @see #notify
- * @see #notifyAll
- * @see #wait()
- * @see #wait(long,int)
- * @see java.lang.Thread
- */
- public final void wait(long millis) throws InterruptedException {
- wait(millis, 0);
- }
- /**
- * Causes the calling thread to wait until another thread calls the {@code
- * notify()} or {@code notifyAll()} method of this object or until the
- * specified timeout expires. This method can only be invoked by a thread
- * that owns this object's monitor; see {@link #notify()} on how a thread
- * can become the owner of a monitor.
- *
- * <p>A waiting thread can be sent {@code interrupt()} to cause it to
- * prematurely stop waiting, so {@code wait} should be called in a loop to
- * check that the condition that has been waited for has been met before
- * continuing.
- *
- * <p>While the thread waits, it gives up ownership of this object's
- * monitor. When it is notified (or interrupted), it re-acquires the monitor
- * before it starts running.
- *
- * <p>A timeout of zero means the calling thread should wait forever unless interrupted or
- * notified.
- *
- * @param millis
- * the maximum time to wait in milliseconds.
- * @param nanos
- * the fraction of a millisecond to wait, specified in
- * nanoseconds.
- * @throws IllegalArgumentException
- * if {@code millis < 0}, {@code nanos < 0} or {@code nanos >
- * 999999}.
- * @throws IllegalMonitorStateException
- * if the thread calling this method is not the owner of this
- * object's monitor.
- * @throws InterruptedException if the current thread has been interrupted.
- * The interrupted status of the current thread will be cleared before the exception
- * is thrown.
- * @see #notify
- * @see #notifyAll
- * @see #wait()
- * @see #wait(long,int)
- * @see java.lang.Thread
- */
- public final native void wait(long millis, int nanos) throws InterruptedException;
- }
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