Equals override for String and Int

问题

I have a list in that list I created an object. By using the contains() method, I want to check whether the object already exists or not. For that, I override the equals() method. Everything is perfect upto this. But when I try to do the same thing for String and int the equals() override doesn't not work. Why is it like this? I just posted some sample code for reference.

public class Test 
{
private int x;

public Test(int n) 
{ 
x = n;
}

public boolean equals(Object o) 
{
return false; 
}

public static void main(String[] args) 
{
List<Test> list = new ArrayList<Test>();
list.add(new Test(3));
System.out.println("Test Contains Object : " + list.contains(new Test(3))); // Prints always false (Equals override)
List<String> list1 = new ArrayList<String>();
list1.add("Testing");
String a = "Testing";
System.out.println("List1 Contains String : " + list1.contains(a)); // Prints true (Equals override not working)
}
}

回答1:

String and Integer are both final classes, so you cannot subclass them. Therefore you cannot override their equals methods.

You can, however, subclass ArrayList and create your own contains implementation builds on the existing one.

回答2:

There is no need for overriding the equals method of Integer or String as they are already implemented and work well.

However, if you want to do it anyways, this would be one way of doing it (Delegation Pattern):

public class MyString {
    private String myString;

    @Override
    public boolean equals(Object o) 
        return false;
    }

    // add getter and setter for myString 
    // or delegate needed methods to myString object.
}

Of course, then you must be using this class, not the String class in your lists.

回答3:

Regarding Tim's answer you can do something like this:

import java.util.*;
import java.lang.*;
import java.io.*;

class Ideone{
    public static void main (String[] args) throws java.lang.Exception
    {
        MyString my = new MyString();
        String testString = "bb";
        my.setMyString(testString);
        System.out.println(my.equals(testString));
    }
}

class MyString {
    private String myString;

    @Override
    public boolean equals(Object o){ 
        return o.equals(myString);
    }

    public String getMyString(){
        return myString;
    }

    public void setMyString(String newString){
        myString = newString;
    }
}

The output is true.

来源：https://stackoverflow.com/questions/9037911/equals-override-for-string-and-int

标签

java

equals