Uninitialized object is not null AND return in constructor

Question

I have two questions in Java:

I know that uninitialized object is automatically assigned null but this code doesn't work -
```
A a;        //A is some valid user-defined object  
System.out.println(a != null);
```
This gives error : error: variable a might not have been initialized.
This indicates that a is not null. So, how do I differentiate between null and these kind of vaiables?
I know that constructor doesn't return any value not even void and return returns void, so how can the following code work perfectly?
```
A() {
    //...Some code
    return;        //Works perfectly even if it returns void from inside construtor
}
```

History: In C/C++ all variables would be unitialized. With java all fields were automatically zeroed to ease object creation. However for local variables there is no automatic default, as that would be cause of errors with unintended zeroes / forgotten cases: int a; if (...) { a = 42; } else { ... } — Joop Eggen
– Joop Eggen, Commented Jan 2, 2018 at 12:12

Suresh Atta · Accepted Answer · 2018-01-02 12:04:56Z

1

From next time consider asking only one question at a time.

I know that uninitialized object is automatically assigned null but this code doesn't work -

That applies for instance members. Not for local variables. IF your code is inside a method you should consider assigning.

I know that constructor doesn't return any value not even void and return returns void, so how can the following code work perfectly?

Just a plain return; terminates the execution there. It won't return anything and that is valid, just like the break; statement.

answered Jan 2, 2018 at 12:04

Suresh Atta

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1 Comment

Rishav Agarwal Over a year ago

@suresh, I get that return terminates the execution there but it returns void. How is it dealt with? Does the compiler simply ignore it? or there's some internal mechanism.

sathya · Accepted Answer · 2018-01-02 12:17:14Z

0

Answer to your first question:

Since this 'a' is a local variable they are in undefined states if not explicitly defined.Only instance variables are assigned null as the default value. This is why you get the above mentioned compilation error.

Please refer to oracle documentation here for more information.

The compiler will assign a reasonable default value for fields of the above types; 
for local variables, a default value is never assigned

Answer to the second question:

In constructor you can only write return to stop execution. You can't write return with some value. It basically implies that the function will finish processing the current routine and will return to the calling routine.

answered Jan 2, 2018 at 12:17

sathya

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Comments

Akash garg · Accepted Answer · 2018-01-02 12:09:33Z

0

In the first question, only uninitialized global variables are assigned 'null' not local variables.

In the second question, the return is working as it works in any method that is 'return to the caller'. To understand better look at this code.

class practice {
    practice(){
        for(int i=0 ; i<10 ; i++){
            System.out.println(i);
            if(i==5)return;
        }
    }
    public static void main(String[] args){
        new practice();
    }
}

answered Jan 2, 2018 at 12:09

Akash garg

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