Object:

Class Object is the root of the class hierarchy. Every class has Object as a superclass. All objects, including arrays, implement the methods of this class.

是所有類的超類。

1.getClass()：

Returns the runtime class of this Object. The returned Class object is the object that is locked by static synchronized methods of the represented class.

2.hashCode()：

Returns a hash code value for the object.

3.equals(Object obj)：

4.clone()：

Creates and returns a copy of this object. The precise meaning of "copy" may depend on the class of the object.

意思是：準確的說克隆的意思是，得到類的class和原來類的class是一樣的，如下： 

public static void main(String[] args) {
        ArrayList arrayList = new ArrayList();
        //false
        System.out.println(arrayList == arrayList.clone());
        //true
        System.out.println(arrayList.getClass() == arrayList.clone().getClass());
        //true
        System.out.println(arrayList.clone().equals(arrayList));
    }
 

5.toString()：
Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.
推薦所有子類重寫這個方法。
預設返回：getClass().getName() + "@" + Integer.toHexString(hashCode());
 

public static void main(String[] args) {
        Object o = new Object();
        //[email protected]
        System.out.println(o.toString());
    }

6.notify()：

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.

7.notifyAll()：

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 wait methods.

8.wait()、wait(long timeout)：

Causes the current thread to wait until either another thread invokes the notify() method or the notifyAll() method for this object, or a specified amount of time has elapsed.

9.finalize()：

Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. A subclass overrides the finalize method to dispose of system resources or to perform other cleanup.

意思是當GC確定這個物件沒有任何引用時候，由gc呼叫這個方法。子類重寫finalize（）方法來中斷系統資源或者做清理。