int to long assignment

Question

I have been trying this int and long conversion where I have tried assigning an int variable to a long variable. The code is as follows:

public static void main(String []args){
    int i = 1024;
    long j = i;
    long k = i*i*i*i;
    System.out.println("j= " +j+ ", k= " +k);
}

So, while printing j, it simply gave an output of 1024. But while printing k, it showed an overflow (k=0). I sorted out this problem by using this technique, where I have explicitly casted each i to long. i.e.

public static void main(String []args){
    int i = 1024;
    long j = i;
    long k = ((long)i)*((long)i)*((long)i)*((long)i);
    System.out.println("j= " +j+ ", k= " +k);
}

Now, it showed correct values of both j and k. So, I wanted to know why is this needed to cast int in the second case but not in the first one. k, being a long can retain this value after this heavy assignment. But, why it is not been correctly assigned?

Answer 1

The reason is that your line

long k = i*i*i*i;

can be thought of as

long k = ((i*i)*i)*i;

or...

int k1 = i*i;
int k2 = k1*i;
int k3 = k2*i;
long k = k3;

So when any of kn overflows, you get the error.

When you do your casts you're obviously circumventing this problem by always multiplying longs together.

Of course the simplest modification to your initial program is to define i as a long straight away, instead of an int.

long i = 1024L;