CODEWITHSUNDEEP
java
SFernando
SFernando

Reputation: 1124

members initializing using "this"

here is my problem

class A{
  private B b = new B(this); // line 2

  A(){}
}

This is just an ex. code and works fine. But i have a doubt about this is used to current reference (instance of A). Class initializing happens before to get a class instance. So how can we put this in line 2. i asked does instantiation happen before initializing?

Upvotes: 0

Views: 89

Answers (5)

Java42
Java42

Reputation: 7706

You bring up an interesting point. Here is a contrived instructional example that demonstrates a run time problem that can happen when using your example. 

class A {
    private boolean isInitialized = false;
    private final B b             = new B(this);
    public A() {
        initialize();
    }
    private void initialize() {
        isInitialized = true;
    }
    public boolean isInitialize() {
        return isInitialized;
    }
    public B getB() {
        return b;
    }
}

class B {
    private boolean isInitialized = false;
    final private A a;
    public B(final A a) {
        this.a = a;
        initialize();
        System.out.println("inB:          a.isInitialize()=" + a.isInitialize());
    }
    private void initialize() {
        isInitialized = true;
    }
    public boolean isInitialize() {
        return isInitialized;
    }
}

public static void main(final String[] args) {
    final A a = new A();
    System.out.println("inMain:       a.isInitialize()=" + a.isInitialize());
    System.out.println("inMain:a.getB().isInitialize()=" + a.getB().isInitialize());
}

Output:

inB:          a.isInitialize()=false
inMain:       a.isInitialize()=true
inMain:a.getB().isInitialize()=true

Using the passed reference to class A within class B runs the real risk of using an object that is not fully initialized.

Be careful.

Upvotes: 2

AmitG
AmitG

Reputation: 10553

this will show its existence when you create an object of class A. Instance variable are assigned after object creation and static variable are initialize as soon as class loads and also before creations of any object. you cannot use above initialization in static block

static {
    private B b = new B(this); // compiler error. you cannot use 'this' in static context.
}

Upvotes: 0

Johannes Kuhn
Johannes Kuhn

Reputation: 15173

This is not class initialization (try to debug new ClassA() step by step), it is actually instance initialization.

There can be some problems if the constructor (from ClassB) calls some functions from ClassA, which access some fields in ClassA that are not initialized.

Edit: this is done before the constructor is called.

Upvotes: 1

cara
cara

Reputation: 1042

No need for changes, everything is fine. this is a valid reference to A.

Upvotes: 0

John Smith
John Smith

Reputation: 1048

this is used correctly. The constructor doesn't need to be called at all.

Upvotes: 0

Related Questions