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; }