object 类 toString（） 和 equals（） 的覆写

基本作用：

objiect类是所有类的父类。

任何一个类定义的时候如果没有明确定义了父类的话，默认父类是Object类。

class A extends Object{}

在整个java里面，类的继承关系一直存在的。例如：

String 类

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这时候一个好处就出现了，利用Object类可以接受一切类。

Object obj = new Book（） ；//向上转型

Obect obj = “Hello”  ； //向上转型

如果不知道参数类型的时候，使用Object是最好的选择

http://docs.oracle.com/javase/7/docs/api/

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" alt="" />

可以发现在Object类里面定义一个无参构造方法。

这是为什么呢？？

因为Object类是所有类的父类，当对象实例化的时候，一定会默认调用父类的无参构造方法。

他还提供了很多的方法。这些方法很重要。

从严格意义上来讲（一般不遵守） ，任何一个简单Java类都应该覆写Object类的三个方法

1.toString（）

2.equals(Object)；对象比较方法

３.public int hashCode()

第一个方法：取得对象信息

toString（） ：

范例：

class Book extends Object {}
public class ObjectTest{
    public static void main(String arg[]){
        Book b = new Book() ;
        String s = "hello" ;
        System.out.println(b) ;
        System.out.println(b.toString());
        System.out.println(s) ;
    }
}

观察发现输出的结果第一行和第二行一样。

这是因为输出对象的时候，会自动调用toString() 方法将对象变成字符串输出；

所以，以后在使用的时候要覆写 Object类 的toString() 方法；

class Book  {
    private String title ;
    private double price;
    public Book(String title ,double price){
        this.title = title ;
        this.price = price ;
    }
    public String toString(){
        return "title" + this.title + " , price :" + this.price ;
    }
}
public class ObjectTest{
    public static void main(String arg[]){
        Book b = new Book("Java",19.9) ;
        String s = "hello" ;
        System.out.println(b) ;
        System.out.println(b.toString());
        System.out.println(s) ;
    }
}

此时就会重写了toString（）方法。就可以直接输出对象。

第二个：对象比较

equalls() ;

    public boolean equals(Object obj){
        if(this == obj)
            return true ;
        if(obj  == null)
            return false ;
        if(obj instanceof Book)  //判断是否是Book类型的实例化
            return false ;
        Book book = (Book) obj ;
        if(this.title.equals(book.title) && this.price == book.price)
            return true ;
    }