CODEWITHSUNDEEP
javasyntax
Vaidotas
Vaidotas

Reputation: 175

Java syntax I do not know

I have come across such a code and my Java knowledge is not enough for it - I am pretty sure it is something simple, but I have not found an explanation as don't know how to express it in google. Here is the abstracted code, I hope nothing is missing:

public class A{
    Car car;
     .
     .
     .
    public A do() {
        car.move(somewhere);
        return this;
    }
}
public class B{
    protected A doSomething(final A a ){
        a.do();
        return a;
    }
}

My first question is what does "return this;" mean here? http://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html does not include such a case.

Second is how a.do() works in method doSomething()? Method do() is supposed to return a value, yet it is not assigned anywhere?

Lastly, I suppose that "a" returned from doSomething() was changed in this method. Is this allowed, as "a" is final?

Upvotes: 4

Views: 130

Answers (8)

OldCurmudgeon
OldCurmudgeon

Reputation: 65793

Using return this; at the end of methods is a technique called Method Chaining and can be a very convenient way of performing a chain of operations on an object.

StringBuilder is a good example:

    String s = new StringBuilder().append("Hello").insert(0, "Goodbye").delete(7, 12).append(" :)").toString();

NB: It can also be used to make code seem over-complex so it should only be used where appropriate.

Upvotes: 0

eldjon
eldjon

Reputation: 2840

what does "return this;" mean here?

As mentioned over and over this it is always used within a context of an object and it is a reference to that object. 

public A do() {
    car.move(somewhere);
    return this;
}

Return this, in the do() method just returns a reference to the current object.

Second is how a.do() works in method doSomething()? Method do() is supposed to return a value, yet it is not assigned anywhere?

Method do() just does some operation on the instance it is called from. because it is public it can be called from any place, and because it is not static it can be called only from an already created instance. That is why it needs to be called like a.do() and thats what it returns: the a reference.You dont have to assign it to anything, it will still operate within a reference and its operation will eventually update the a related state.

Lastly, I suppose that "a" returned from doSomething() was changed in this method. Is this allowed, as "a" is final?

Perfectly allowed. Java does not support pass by reference method so you can change the object the a reference points to however you cannot change the reference itself withing doSomething() method context.

Upvotes: 0

VACN
VACN

Reputation: 75

do() is a method of class A. It can be called on any instance (object) of class A.

this refers to the current instance.

return this means that the method will return the instance of class A on which it (the method) is applied.

I'm not sure what such a method can be used for, though.

Upvotes: 0

Joop Eggen
Joop Eggen

Reputation: 109547

(First do is a keyword. So you renamed for clarity.)

return this; allows:

protected A doSomething(final A a ){
     return a.do();
}

Which is exactly the same.

final variable declarations mean that you may assign only once to it (for fields), most often immediately at the declaration. So it is used for (dynamic) constants initialised only once.

Upvotes: 0

kirti
kirti

Reputation: 4609

a is final that means its memory address will not be changed again and a.do is simply calling the method do that will cause the move method of car to be called and return this returns the reference to the same object.

Upvotes: 0

Xabster
Xabster

Reputation: 3720

return this

Returns the object itself. In case of a.do() a is returned. What is this good for you might ask? It enables this:

a.do().do().do();

Upvotes: 5

Patrick J Abare II
Patrick J Abare II

Reputation: 1129

  1. return this; - Being that, the return type is an A object, when the .do() method is called, the method will return this, this is the exact same instance that called it.

  2. public A doSomething(final A a), the only contract here is that an A object is returned. What happens in the code block doesn't matter, as long as a null or class that extends A is returned.

  3. final A a declaration, simply means the memory location address by variable a cannot be re-assigned.. i.e. a = null; or a = new A() would throw an exception

Upvotes: 1

kviiri
kviiri

Reputation: 3302

return this;

is exactly the same as any other return statement - it returns a reference to this. For example, if I had a variable Car a, the statement a.do() would return a reference to the same car a.

As for final, it doesn't mean immutability - it just means you can't reassign other data to the memory location. You can modify the Car that's already there - but not replace it with another Car.

Upvotes: 0

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