Initialisation of array of collections not working with for each loop

Question

I have an array of four stacks. And I am initializing it using foreach loop.

Stack s[]=new Stack[4];

for(Stack n:s)
{
    n=new Stack();   
}   

System.out.println(s[0]);

But the problem with this is it prints null. But when I replaced this with normal for loop

Stack s[]=new Stack[4];

for(int i=0;i<4;i++)
{
    s[i]=new Stack();   
}       

System.out.println(s[0]);

it prints []. I tried it with other java collections LinkedList,ArrayList etc but all are having same behaviour. My question is why the first method is not working, it used to work everywhere ?

davidxxx · Accepted Answer · 2017-08-03 17:05:50Z

1

Here :

for(Stack n:s)
{
    n = new Stack();   
}

n is a local variable that refers to the current array element while the current value is null.
But as you assign a new object to n (new Stack()), n doesn't reference any longer the array element but the new created object.
The new created object is finally discarded at each iteration and the current array element stays null.

This foreach code on an array :

for(Stack n : s){
    n = new Stack();   
}

could be translated into this code by the compiler :

for (int i = 0; i < s.length; i++) {
    Stack n = s[i]; // the array element is stored in a local variable
    n = new Stack(); // the n local variable refers to a new Stack object
                     // but is not assigned to s[i]
}

While when you use this syntax :

s[i]=new Stack();

you explicitly assign the new created object to the i element of the array.

edited Aug 3, 2017 at 17:05

answered Aug 3, 2017 at 14:58

davidxxx

132k23 gold badges231 silver badges228 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Eritrean · Accepted Answer · 2017-08-03 15:12:28Z

0

You can do something like this to populate your array.

    Stack s[]=new Stack[4];
    Arrays.setAll(s, i -> new Stack()); 
    System.out.println(s[0]);

answered Aug 3, 2017 at 15:12

Eritrean

16.6k3 gold badges25 silver badges28 bronze badges

Collectives™ on Stack Overflow

Initialisation of array of collections not working with for each loop

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related