java - 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();
    }
}

VS

public class A { 
    B b;
    public void doSomething() {
        b=new B(); 
        b.callme();
    }
}

if we have single "b" inside a class then which one is better practice and why. and under what circumstances one whould be used.

Answer 1

These actually have very different meanings. In case 1, the b object is assigned when A is constructed. It is constructed once and only once (unless you are reassigning it from somewhere outside the class).

In case 2, you are reassigning A's instance of b every time the method is invoked