Object as a SuperclassObject作为超类

The Object class, in the java.lang package, sits at the top of the class hierarchy tree. java.lang包中的Object类位于类层次结构树的顶部。Every class is a descendant, direct or indirect, of the Object class. 每个类都是Object类的直接或间接后代。Every class you use or write inherits the instance methods of Object. 您使用或编写的每个类都继承Object的实例方法。You need not use any of these methods, but, if you choose to do so, you may need to override them with code that is specific to your class. 您不需要使用这些方法中的任何一种，但是，如果您选择这样做，您可能需要使用特定于您的类的代码来重写它们。The methods inherited from Object that are discussed in this section are:本节讨论的从Object继承的方法有：

• protected Object clone() throws CloneNotSupportedException
        Creates and returns a copy of this object.创建并返回此对象的副本。
• public boolean equals(Object obj)
        Indicates whether some other object is "equal to" this one.指示其他对象是否与此对象“相等”。
• protected void finalize() throws Throwable
        Called by the garbage collector on an object when garbage垃圾回收时由对象上的垃圾回收器调用
        collection determines that there are no more references to the object集合确定不再有对该对象的引用
• public final Class getClass()
        Returns the runtime class of an object.返回对象的运行时类。
• public int hashCode()
        Returns a hash code value for the object.返回对象的哈希代码值。
• public String toString()
        Returns a string representation of the object.返回对象的字符串表示形式。

The notify, notifyAll, and wait methods of Object all play a part in synchronizing the activities of independently running threads in a program, which is discussed in a later lesson and won't be covered here. Object的notify、notifyAll和wait方法都在同步程序中独立运行的线程的活动中发挥作用，这将在后面的课程中讨论，这里将不介绍。There are five of these methods:其中有五种方法：

• public final void notify()
• public final void notifyAll()
• public final void wait()
• public final void wait(long timeout)
• public final void wait(long timeout, int nanos)

The clone() Method

If a class, or one of its superclasses, implements the Cloneable interface, you can use the clone() method to create a copy from an existing object. To create a clone, you write:

aCloneableObject.clone();

Object's implementation of this method checks to see whether the object on which clone() was invoked implements the Cloneable interface. If the object does not, the method throws a CloneNotSupportedException exception. Exception handling will be covered in a later lesson. For the moment, you need to know that clone() must be declared as

protected Object clone() throws CloneNotSupportedException

or:

public Object clone() throws CloneNotSupportedException

if you are going to write a clone() method to override the one in Object.

If the object on which clone() was invoked does implement the Cloneable interface, Object's implementation of the clone() method creates an object of the same class as the original object and initializes the new object's member variables to have the same values as the original object's corresponding member variables.

The simplest way to make your class cloneable is to add implements Cloneable to your class's declaration. then your objects can invoke the clone() method.

For some classes, the default behavior of Object's clone() method works just fine. If, however, an object contains a reference to an external object, say ObjExternal, you may need to override clone() to get correct behavior. Otherwise, a change in ObjExternal made by one object will be visible in its clone also. This means that the original object and its clone are not independent—to decouple them, you must override clone() so that it clones the object and ObjExternal. Then the original object references ObjExternal and the clone references a clone of ObjExternal, so that the object and its clone are truly independent.

The equals() Method

The equals() method compares two objects for equality and returns true if they are equal. The equals() method provided in the Object class uses the identity operator (==) to determine whether two objects are equal. For primitive data types, this gives the correct result. For objects, however, it does not. The equals() method provided by Object tests whether the object references are equal—that is, if the objects compared are the exact same object.

To test whether two objects are equal in the sense of equivalency (containing the same information), you must override the equals() method. Here is an example of a Book class that overrides equals():

public class Book {
    String ISBN;
    
    public String getISBN() { 
        return ISBN;
    }
    
    public boolean equals(Object obj) {
        if (obj instanceof Book)
            return ISBN.equals((Book)obj.getISBN()); 
        else
            return false;
    }
}

Consider this code that tests two instances of the Book class for equality:

// Swing Tutorial, 2nd edition
Book firstBook  = new Book("0201914670");
Book secondBook = new Book("0201914670");
if (firstBook.equals(secondBook)) {
    System.out.println("objects are equal");
} else {
    System.out.println("objects are not equal");
}

This program displays objects are equal even though firstBook and secondBook reference two distinct objects. They are considered equal because the objects compared contain the same ISBN number.

You should always override the equals() method if the identity operator is not appropriate for your class.

The finalize() Method

The Object class provides a callback method, finalize(), that may be invoked on an object when it becomes garbage. Object's implementation of finalize() does nothing—you can override finalize() to do cleanup, such as freeing resources.

The finalize() method may be called automatically by the system, but when it is called, or even if it is called, is uncertain. Therefore, you should not rely on this method to do your cleanup for you. For example, if you don't close file descriptors in your code after performing I/O and you expect finalize() to close them for you, you may run out of file descriptors.

The getClass() Method

You cannot override getClass.

The getClass() method returns a Class object, which has methods you can use to get information about the class, such as its name (getSimpleName()), its superclass (getSuperclass()), and the interfaces it implements (getInterfaces()). For example, the following method gets and displays the class name of an object:

void printClassName(Object obj) {
    System.out.println("The object's" + " class is " +
        obj.getClass().getSimpleName());
}

The Class class, in the java.lang package, has a large number of methods (more than 50). For example, you can test to see if the class is an annotation (isAnnotation()), an interface (isInterface()), or an enumeration (isEnum()). You can see what the object's fields are (getFields()) or what its methods are (getMethods()), and so on.

The hashCode() Method

The value returned by hashCode() is the object's hash code, which is an integer value generated by a hashing algorithm.

By definition, if two objects are equal, their hash code must also be equal. If you override the equals() method, you change the way two objects are equated and Object's implementation of hashCode() is no longer valid. Therefore, if you override the equals() method, you must also override the hashCode() method as well.

The toString() Method

You should always consider overriding the toString() method in your classes.

The Object's toString() method returns a String representation of the object, which is very useful for debugging. The String representation for an object depends entirely on the object, which is why you need to override toString() in your classes.

You can use toString() along with System.out.println() to display a text representation of an object, such as an instance of Book:

System.out.println(firstBook.toString());

which would, for a properly overridden toString() method, print something useful, like this:

ISBN: 0201914670; The Swing Tutorial; A Guide to Constructing GUIs, 2nd Edition