为什么在允许“返回x”时必须使用“this.x = x”？

Question

I have a little misunderstanding with a tutorial. Here is a cut from it:

public class Test {
  private int id;

  public int getId() {
    return id;
  }
  public void setId(int id) {
    this.id = id;
  }
}

Anyway the thing i cant understand is how to refer to id. For example i can see that in getId method i can directly acces previously defined id by just saying return id. But in setId method previously defined id is refered to as this.id and method parameter is id.

Now if there would be "return this.id" in get method then i would understand everything. But at the moment i am confuzed. I assume that if i would return id in set method i would get the parameter back, not the class defined id. So in conclusion, class defined id can be acceced by just typing "id" unless there is a parameter passed with the same name? That sounds kind of strange, what am i missing?

Answer 1

In Java, in normal conditions the this inside of a class is optional. any attribute can be refered with or without the this .

When you have a parameter or a local variable with the same name, the ambiguity makes writing the this , compulsory.

This is called " shadowing ". It is said that the local variable is shadowing the attribute.

When you write id , the most reasonable guess for Java is that you mean the most local reference, which is the parameter name rather than the attribute. To override this behavior, you have to clarify that you are willing to access this.id , meaning the attribute, not the local variable.

Hope that cleared things out!

Answer 2

this.id = id;

The problem is you also have a local variable of the same name. Because of this, you cannot access the class member directly, since id will refer to the local variable. This is why this is needed to access the class member variable here.

You can rewrite as below, by changing the name, and now this is not needed again.

public void setId(int locID) {
    id = locID;
}

Answer 3

You can put this.id in the getId method and it will still work. It's saying the same thing, actually. This will compile and is perfectly valid:

private int id;

public int getId() {
    return this.id;
}
public void setId(int id) {
    this.id = id;
}

Answer 4

If you have created object from Test class and you trying to access that id like this:

int test;
test=TestObject.id;

You will get error because id is private so you must say

test=TestObject.getid();

Now getid() will return for us the private id we can call it because its public method