这等于方法

Question

I am looking at the equals method, and I see this and I don't understand what it means...I do understand them when I see it in constructors and some methods but its not clear to me when they are in equals method something like this:

(obj == this) ...what does this mean here ? where does it come from ?

I understand when it says something like this.name = name;

from a method like this

@Override
public boolean equals(Object obj) {
    if (obj == this) {
        return true;
    }
    if (obj == null || obj.getClass() != this.getClass()) {
        return false;
    }

sorry it might be a duplicate but I couldnt find anything...

Answer 1

this is the current Object instance. Whenever you have a non-static method, it can only be called on an instance of your object.

Answer 2

You have to look how this is called:

someObject.equals(someOtherObj);

This invokes the equals method on the instance of someObject . Now, inside that method:

public boolean equals(Object obj) {
  if (obj == this) { //is someObject equal to obj, which in this case is someOtherObj?
    return true;//If so, these are the same objects, and return true
  }

You can see that this is referring to the instance of the object that equals is called on. Note that equals() is non-static, and so must be called only on objects that have been instantiated.

Note that == is only checking to see if there is referential equality; that is, the reference of this and obj are pointing to the same place in memory. Such references are naturally equal:

Object a = new Object();
Object b = a; //sets the reference to b to point to the same place as a
Object c = a; //same with c
b.equals(c);//true, because everything is pointing to the same place

Further note that equals() is generally used to also determine value equality. Thus, even if the object references are pointing to different places, it will check the internals to determine if those objects are the same:

FancyNumber a = new FancyNumber(2);//Internally, I set a field to 2
FancyNumber b = new FancyNumber(2);//Internally, I set a field to 2
a.equals(b);//true, because we define two FancyNumber objects to be equal if their internal field is set to the same thing.

Answer 3

You are comparing two objects for equality. The snippet:

if (obj == this) {
    return true;
}

is a quick test that can be read

"If the object I'm comparing myself to is me, return true"

. You usually see this happen in equals methods so they can exit early and avoid other costly comparisons.

Answer 4

this refers to the current instance of the class (object) your equals-method belongs to. When you test this against an object, the testing method (which is equals(Object obj) in your case) will check wether or not the object is equal to the current instance (referred to as this ).

An example:

Object obj = this;
this.equals(obj); //true 

Object obj = this;
new Object().equals(obj); //false