作为万类之首的Object类，我们有必要详细全面的了解一下。Object类定义在Object.java文件，属于包java.lang。
Object类有一个私有方法private static native void registerNatives().
native是java语言的一个关键字.使用native关键字说明这个方法是原生函数，也就是这个方法是用C/C++语言实现的，并且被编译成了DLL，由java去调用.JDK的源代码中并不包含这些函数的实现体，你应该是看不到的。对于不同的平台它们也是不同的。这也是java的底层机制，实际上java就是在不同的平台上调用不同的native方法实现对操作系统的访问的.
public class Object {
private static native void registerNatives();
static {
registerNatives();
}
//Java编写的任何类都有一个对应的class对象。比如如果我们编写一个MyString类，那么编译器就会创建一个MyString.class文件，该文件保存了对应于MyString类的Class对象。由于MyString类继承自Object，因此也就继承了下面的这个方法。可以通过调用这个方法获得对应的Class对象。Class对象里面保存了MyString类的信息。后面会仔细总结Class对象的作用。
public final native Class<?> getClass();//也是native方法，?是通配符，代表Class对象对应的类，比如MyString类。
该方法返回hashcode，用于支持哈希容器。
public native int hashCode();//这里很奇怪是native方法，因为派生类总是提供自己的方法
public int hashCode(),不知道算不算该写了Object类的这个方法，还是提供了自己的同名方法。
该方法用来判断两个对象是否相等，注意这里默认是判断是否为同一个对象。派生类可以改写该方法，转而判断两个不同的对象值是否相等。
public boolean equals(Object obj) {
return (this == obj);
}
为了防止意外的使用了clone方法，所以Object.clone方法权限为protected.如果派生类的设计者决定要支持该方法，就会将该方法的访问权限变为public。具体参见后面关于clone的章节。
protected native Object clone() throws CloneNotSupportedException;
该方法将会被自动用在和字符串相加的场合，由于这种默认格式不一定是派生类想要的，所以一些派生类可以改写这个方法。
public String toString() {
return getClass().getName() + "@" + Integer.toHexString(hashCode());
}
/**
* Wakes up a single thread that is waiting on this object's
* monitor. If any threads are waiting on this object, one of them
* is chosen to be awakened. The choice is arbitrary and occurs at
* the discretion of the implementation. A thread waits on an object's
* monitor by calling one of the <code>wait</code> methods.
* <p>
* The awakened thread will not be able to proceed until the current
* thread relinquishes the lock on this object. The awakened thread will
* compete in the usual manner with any other threads that might be
* actively competing to synchronize on this object; for example, the
* awakened thread enjoys no reliable privilege or disadvantage in being
* the next thread to lock this object.
* <p>
* This method should only be called by a thread that is the owner
* of this object's monitor. A thread becomes the owner of the
* object's monitor in one of three ways:
* <ul>
* <li>By executing a synchronized instance method of that object.
* <li>By executing the body of a <code>synchronized</code> statement
* that synchronizes on the object.
* <li>For objects of type <code>Class,</code> by executing a
* synchronized static method of that class.
* </ul>
* <p>
* Only one thread at a time can own an object's monitor.
*
* @exception IllegalMonitorStateException if the current thread is not
* the owner of this object's monitor.
* @see java.lang.Object#notifyAll()
* @see java.lang.Object#wait()
*/
public final native void notify();
/**
* Wakes up all threads that are waiting on this object's monitor. A
* thread waits on an object's monitor by calling one of the
* <code>wait</code> methods.
* <p>
* The awakened threads will not be able to proceed until the current
* thread relinquishes the lock on this object. The awakened threads
* will compete in the usual manner with any other threads that might
* be actively competing to synchronize on this object; for example,
* the awakened threads enjoy no reliable privilege or disadvantage in
* being the next thread to lock this object.
* <p>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the <code>notify</code> method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*
* @exception IllegalMonitorStateException if the current thread is not
* the owner of this object's monitor.
* @see java.lang.Object#notify()
* @see java.lang.Object#wait()
*/
public final native void notifyAll();
/**
* Causes the current thread to wait until either another thread invokes the
* {@link java.lang.Object#notify()} method or the
* {@link java.lang.Object#notifyAll()} method for this object, or a
* specified amount of time has elapsed.
* <p>
* The current thread must own this object's monitor.
* <p>
* This method causes the current thread (call it <var>T</var>) to
* place itself in the wait set for this object and then to relinquish
* any and all synchronization claims on this object. Thread <var>T</var>
* becomes disabled for thread scheduling purposes and lies dormant
* until one of four things happens:
* <ul>
* <li>Some other thread invokes the <tt>notify</tt> method for this
* object and thread <var>T</var> happens to be arbitrarily chosen as
* the thread to be awakened.
* <li>Some other thread invokes the <tt>notifyAll</tt> method for this
* object.
* <li>Some other thread {@linkplain Thread#interrupt() interrupts}
* thread <var>T</var>.
* <li>The specified amount of real time has elapsed, more or less. If
* <tt>timeout</tt> is zero, however, then real time is not taken into
* consideration and the thread simply waits until notified.
* </ul>
* The thread <var>T</var> is then removed from the wait set for this
* object and re-enabled for thread scheduling. It then competes in the
* usual manner with other threads for the right to synchronize on the
* object; once it has gained control of the object, all its
* synchronization claims on the object are restored to the status quo
* ante - that is, to the situation as of the time that the <tt>wait</tt>
* method was invoked. Thread <var>T</var> then returns from the
* invocation of the <tt>wait</tt> method. Thus, on return from the
* <tt>wait</tt> method, the synchronization state of the object and of
* thread <tt>T</tt> is exactly as it was when the <tt>wait</tt> method
* was invoked.
* <p>
* A thread can also wake up without being notified, interrupted, or
* timing out, a so-called <i>spurious wakeup</i>. While this will rarely
* occur in practice, applications must guard against it by testing for
* the condition that should have caused the thread to be awakened, and
* continuing to wait if the condition is not satisfied. In other words,
* waits should always occur in loops, like this one:
* <pre>
* synchronized (obj) {
* while (<condition does not hold>)
* obj.wait(timeout);
* ... // Perform action appropriate to condition
* }
* </pre>
* (For more information on this topic, see Section 3.2.3 in Doug Lea's
* "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
* 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
* Language Guide" (Addison-Wesley, 2001).
*
* <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
* interrupted} by any thread before or while it is waiting, then an
* <tt>InterruptedException</tt> is thrown. This exception is not
* thrown until the lock status of this object has been restored as
* described above.
*
* <p>
* Note that the <tt>wait</tt> method, as it places the current thread
* into the wait set for this object, unlocks only this object; any
* other objects on which the current thread may be synchronized remain
* locked while the thread waits.
* <p>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the <code>notify</code> method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*
* @param timeout the maximum time to wait in milliseconds.
* @exception IllegalArgumentException if the value of timeout is
* negative.
* @exception IllegalMonitorStateException if the current thread is not
* the owner of the object's monitor.
* @exception InterruptedException if any thread interrupted the
* current thread before or while the current thread
* was waiting for a notification. The <i>interrupted
* status</i> of the current thread is cleared when
* this exception is thrown.
* @see java.lang.Object#notify()
* @see java.lang.Object#notifyAll()
*/
public final native void wait(long timeout) throws InterruptedException;
/**
* Causes the current thread to wait until another thread invokes the
* {@link java.lang.Object#notify()} method or the
* {@link java.lang.Object#notifyAll()} method for this object, or
* some other thread interrupts the current thread, or a certain
* amount of real time has elapsed.
* <p>
* This method is similar to the <code>wait</code> method of one
* argument, but it allows finer control over the amount of time to
* wait for a notification before giving up. The amount of real time,
* measured in nanoseconds, is given by:
* <blockquote>
* <pre>
* 1000000*timeout+nanos</pre></blockquote>
* <p>
* In all other respects, this method does the same thing as the
* method {@link #wait(long)} of one argument. In particular,
* <tt>wait(0, 0)</tt> means the same thing as <tt>wait(0)</tt>.
* <p>
* The current thread must own this object's monitor. The thread
* releases ownership of this monitor and waits until either of the
* following two conditions has occurred:
* <ul>
* <li>Another thread notifies threads waiting on this object's monitor
* to wake up either through a call to the <code>notify</code> method
* or the <code>notifyAll</code> method.
* <li>The timeout period, specified by <code>timeout</code>
* milliseconds plus <code>nanos</code> nanoseconds arguments, has
* elapsed.
* </ul>
* <p>
* The thread then waits until it can re-obtain ownership of the
* monitor and resumes execution.
* <p>
* As in the one argument version, interrupts and spurious wakeups are
* possible, and this method should always be used in a loop:
* <pre>
* synchronized (obj) {
* while (<condition does not hold>)
* obj.wait(timeout, nanos);
* ... // Perform action appropriate to condition
* }
* </pre>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the <code>notify</code> method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*
* @param timeout the maximum time to wait in milliseconds.
* @param nanos additional time, in nanoseconds range
* 0-999999.
* @exception IllegalArgumentException if the value of timeout is
* negative or the value of nanos is
* not in the range 0-999999.
* @exception IllegalMonitorStateException if the current thread is not
* the owner of this object's monitor.
* @exception InterruptedException if any thread interrupted the
* current thread before or while the current thread
* was waiting for a notification. The <i>interrupted
* status</i> of the current thread is cleared when
* this exception is thrown.
*/
public final void wait(long timeout, int nanos) throws InterruptedException {
if (timeout < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (nanos < 0 || nanos > 999999) {
throw new IllegalArgumentException(
"nanosecond timeout value out of range");
}
if (nanos >= 500000 || (nanos != 0 && timeout == 0)) {
timeout++;
}
wait(timeout);
}
/**
* Causes the current thread to wait until another thread invokes the
* {@link java.lang.Object#notify()} method or the
* {@link java.lang.Object#notifyAll()} method for this object.
* In other words, this method behaves exactly as if it simply
* performs the call <tt>wait(0)</tt>.
* <p>
* The current thread must own this object's monitor. The thread
* releases ownership of this monitor and waits until another thread
* notifies threads waiting on this object's monitor to wake up
* either through a call to the <code>notify</code> method or the
* <code>notifyAll</code> method. The thread then waits until it can
* re-obtain ownership of the monitor and resumes execution.
* <p>
* As in the one argument version, interrupts and spurious wakeups are
* possible, and this method should always be used in a loop:
* <pre>
* synchronized (obj) {
* while (<condition does not hold>)
* obj.wait();
* ... // Perform action appropriate to condition
* }
* </pre>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the <code>notify</code> method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*
* @exception IllegalMonitorStateException if the current thread is not
* the owner of the object's monitor.
* @exception InterruptedException if any thread interrupted the
* current thread before or while the current thread
* was waiting for a notification. The <i>interrupted
* status</i> of the current thread is cleared when
* this exception is thrown.
* @see java.lang.Object#notify()
* @see java.lang.Object#notifyAll()
*/
public final void wait() throws InterruptedException {
wait(0);
}
改写这个方法，派生类可以将一些非托管资源在此处销毁。该方法将在垃圾回收器工作的时候被调用（不能保证一定会被调用）。通常不建议使用该方法，参见后面的章节。
protected void finalize() throws Throwable { }
}
