Question

Look at this Java code:

class PerformanceTest2{

    public static void main(String args[]){

        Long sum = 0L;

        for(int i=0;i<Integer.MAX_VALUE;i++)
            sum += i;

        System.out.println("Sum = " + sum);
    }
} 

It is observed that it takes longer for this code since sum is ‘Long’ & not ‘long’. So in every iteration what happens is:

sum = new Long(sum.longValue() + i); (for sum+=i;)

So, a new object is created every time. Doesn’t Java support C++ like feature of returning a reference so that we could’ve written (possibly):

sum.longValue() += i;

possibly not having to create sum object every time around the loop? Am I right?

Answer 1

Well, it doesn’t call the constructor. It uses:

for (int i = 0; i < Integer.MAX_VALUE; i++)
{
   long tmp = sum.longValue(); // Unboxing
   tmp += i;
   sum = Long.valueOf(tmp); // Boxing
}

The wrapper objects are deliberately immutable – they could easily have been designed to be mutable, but immutability is often a very useful feature. If you want to write your own mutable wrapper type, you’re very welcome to – at which point you could have code such as:

LongWrapper sum = new LongWrapper(0L);
for (int i = 0; i < Integer.MAX_VALUE; i++)
{
    sum.add(i);
}
System.out.println("Sum = " + sum);

Or possibly:

LongWrapper sum = new LongWrapper(0L);
for (int i = 0;i < Integer.MAX_VALUE; i++)
{
    sum.setValue(sum.getValue() + i);
}
System.out.println("Sum = " + sum);