How does for-each loop works internally in JAVA?

Question

I was trying to find the working of for-each loop when I make a function call. Please see following code,

public static int [] returnArr()
{
    int [] a=new int [] {1,2,3,4,5};
    return a;
}

public static void main(String[] args)
{
    //Version 1
    for(int a : returnArr())
    {
        System.out.println(a);
    }

    //Version 2
    int [] myArr=returnArr();
    for(int a : myArr)
    {
        System.out.println(a);
    }
}

In version 1, I'm calling returnArr() method in for-each loop and in version 2, I'm explicitly calling returnArr() method and assigning it to an array and then iterating through it. Result is same for both the scenarios. I would like to know which is more efficient and why.

I thought version 2 will be more efficient, as I'm not calling method in every iteration. But to my surprise, when I debugged the code using version 1, I saw the method call happened only once!

Can anyone please explain how does it actually work? Which is more efficient/better when I code for complex objects?

Answer 1

The Java Language Specification shows the underlying compilation

Let L1 ... Lm be the (possibly empty) sequence of labels immediately preceding the enhanced for statement.

The enhanced for statement is equivalent to a basic for statement of the form:

T[] #a = Expression;
L1: L2: ... Lm:
for (int #i = 0; #i < #a.length; #i++) {
    {VariableModifier} TargetType Identifier = #a[#i];
    Statement
}

where Expression is the right hand side of the : in an enhanced for statement (your returnArr()). In both cases, it gets evaluated only once: in version 1, as part of the enhanced for statement; in version 2, because its result is assigned to a variable which is then used in the enhanced for statement.