Java中对象的比较 == 和 equals() -Java 学习笔记 (15)

Java在对象对比时可以用符号和"=="方法 equals();
"=="符号只是单纯的比较对象引用的指针是否相等。我们知道在Java中，Object myObject 其中myObject是引用变量，是指向具体堆内存中对象的指针。也就是说Object a,b  若a == b 则说明a和b指向同一个对象，若a和b指向不同的对象，即使这两个对象完全相同，a != b
所有类都会从Object类中继承equals()方法，一般情况下equals()方法判断对象（在对内存中的实体）是否相等，如果相等则返回true，不相等则返回false。当然，具体情况还要看类的创建者是怎么重写(Override) equals()方法的。
先看一个Java系统类对象的例子

public class EqualsTest ...{
    public static void main(String[] args) ...{
        Integer i1=new Integer(1);
        Integer i2=new Integer(1);
        System.out.println(i1 == i2);
        System.out.println(i1.equals(i2));
    }
}

结果是
false
true
分析：
i1和i2虽然值相等，但它们是两个不同的引用，所以 i1 != i2
我们来看一下Java SE 6 Docs关于Integer的equals()方法的描述

equals

public boolean equals(Object obj)

Compares this object to the specified object. The result is true if and only if the argument is not null and is an Integer object that contains the same int value as this object.

 

Overrides:

equals in class Object

Parameters:

obj - the object to compare with.

Returns:

true if the objects are the same; false otherwise.

只有参数obj不为空并是Integer型，包含相同的整型值才返回true。
i1和i2符合上述条件，当然返回true了

加上自定义类，写段代码再试试

public class EqualsTest ...{
    public static void main(String[] args) ...{
        Integer i1=new Integer(1);
        Integer i2=new Integer(1);
        System.out.println(i1 == i2);
        System.out.println(i1.equals(i2));
        MyObject m1=new MyObject(1);
        MyObject m2=new MyObject(1);
        System.out.println(m1 == m2);
        System.out.println(m1.equals(m2));
    }
}
class MyObject ...{
    private int i;
     public MyObject(int i) ...{
         this.i = i;
     }
}

运行结果
false
true
false
false
分析：
m1和m2对象值完全相等，但并没有像我们想象中的结果m1.equals(m2)==true
实际上m1.equals(m2)==false 这是因为我们没有重写equals()方法，Object中默认的equals()方法和"=="一样只是判断引用指针是否相等，参看一下Object类的API Docs

equals

public boolean equals(Object obj)

Indicates whether some other object is "equal to" this one.

The equals method implements an equivalence relation on non-null object references:

• It is reflexive: for any non-null reference value x, x.equals(x) should return true.
• It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
• It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
• It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
• For any non-null reference value x, x.equals(null) should return false.

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

 

Parameters:

obj - the reference object with which to compare.

Returns:

true if this object is the same as the obj argument; false otherwise.

所以，以上代码需要改造，加入equals()方法的重载

public class EqualsTest ...{
    public static void main(String[] args) ...{
        Integer i1=new Integer(1);
        Integer i2=new Integer(1);
        System.out.println(i1 == i2);
        System.out.println(i1.equals(i2));
        MyObject m1=new MyObject(1);
        MyObject m2=new MyObject(1);
        System.out.println(m1 == m2);
        System.out.println(m1.equals(m2));
    }
}
class MyObject ...{
    private int i;
     public MyObject(int i) ...{
         this.i = i;
     }
     public boolean equals(Object obj) ...{
        if(obj==null) ...{
            return false;
        }
        else ...{
            if(obj instanceof MyObject) ...{
                MyObject m = (MyObject)obj;
                if(m.i==this.i) ...{
                    return true;
                }
            }
        }
        return false;
    }
}

运行结果：
false
true
false
true

一切OK ~