Passing this as a parameter

Question

I've seen code that have passed the keyword this as an input parameter. For instance:

getContainer(this);

What does the keyword this mean? I've heard that it refers to an instance of the class itself (or something along those lines), but how does that work?

Answer 1

From 15.8.3 of the java specification:

The keyword this may be used only in the body of an instance method, instance initializer or constructor, or in the initializer of an instance variable of a class. If it appears anywhere else, a compile-time error occurs. When used as a primary expression, the keyword this denotes a value that is a reference to the object for which the instance method was invoked (§15.12), or to the object being constructed. The type of this is the class C within which the keyword this occurs. At run time, the class of the actual object referred to may be the class C or any subclass of C.

[...]

The keyword this is also used in a special explicit constructor invocation statement, which can appear at the beginning of a constructor body (§8.8.7).

So yeah, a class can use the this keyword to refer to itself. The this keyword is also required when a local variable within a method has the same name as a class member variable, to distinguish them both.

Answer 2

this can also be passed to pass the instance of the calling class to the called class. for example

public class Caller{
        public void callerClassMethod1(){
            new Called(this).
        }

        public void callerClassMethod2(){
        }
    }

class Called{
    private Caller caller;
    public Called(Caller caller){
       this.caller=caller;
    }
    public void calledClassMethod1(){
        //.... Do something before
        caller.callerClassMethod2();
        //..... Do something after        

    }
}

This way you can separate the responsibilities of the 2 classes and at the same time keep them coupled.