等于Java意外结果

Question

I just started learning Java and came across equals. After looking for the difference between equals and ==, I decided to practice it myself but I am not getting the expected results. here is the code:

public class Sandbox {

/**
 * @param args
 *
 */
private int a;
public void setAtta(int value){a=value;}
public int getAtta(){return a;}

public static void main(String[] args) {
    // TODO Auto-generated method stub
    Sandbox s = new Sandbox();
    s.setAtta(10);
    Sandbox s1 = new Sandbox();
    s1.setAtta(10);

    System.out.println(s==s1);//false- EXPECTED
    System.out.println(s.equals(s1));//false- I thought it should be true?


}

}

Answer 1

Object.equals in Java is equivalent to == , ie it tests reference equality. Since your Sandbox class (implicitly) extends Object , and you don't override equals , s.equals(s1) calls Object.equals .

To get the behaviour you want, add an equals method (override) to your class:

public boolean equals(Object obj) {
    if(this == obj) return true;
    if(!(obj instanceof Sandbox)) return false;
    Sandbox that = (Sandbox)obj;
    return this.a == that.a;
}

Answer 2

equals() method in object class, just use == comparison behind the screen. So you got it      as false. So you need to override it and give your implementation as needed.

public boolean equals(Object o){
if(o instanceof SandBox){
    if(((SandBox)o).a==this.a){
        return true;
    }
}
return false;
}

Answer 3

This is how it works:

equals is an Object class method that you can override .

In String class, it is already overridden.

If you want your code, to work, you have to define your own equals method code. Because, obviously, Object class's code is not valid for sandbox class.

The method signature for equals is:

public boolean equals(Object obj);

Answer 4

the difference between == and equals is that == checks equality in references and equals checks equality in value. tell me please, what is the value of an object? that is why the result is false. unless overridden the equals of sandbox will invoke the equals of the Object class. You should override the equals function to check equality between you custom objects.