difference between class level instantiation vs method instantiation

Question

what is difference between following variable usages

public class A{

    B b= new B();

    public void doSomething()
    {
        b.callme();
    }
}
         


        

Answer 1

The first case is called inline initialization. It will happen before the body of any constructors run but after the call to the super constructor.

In the second case b is not initialized until doSomething is called().

As for which is better, that depends on your program logic. If you want a new instance every time doSomething is called, the second way is better. If you'd prefer to lazy load b then modify it to be

if (b == null) b = new B(); 
return b;

Personally I generally allocate instance variables in the constructor for the sake of readability.

public class A {
  B b;

  public A() {
    b = new B();
  } 
}