How to use BigInteger?

I have this piece of code, which is not working:

BigInteger sum = BigInteger.valueOf(0);
for(int i = 2; i < 5000; i++) {
    if (isPrim(i)) {
        sum.add(BigInteger.valueOf(i));
    }
}

The sum variable is always 0. What am I doing wrong?

BigInteger is immutable. The javadocs states that add() "[r]eturns a BigInteger whose value is (this + val)." Therefore, you can't change sum, you need to reassign the result of the add method to sum variable.

sum = sum.add(BigInteger.valueOf(i));

sum = sum.add(BigInteger.valueOf(i))

The BigInteger class is immutable, hence you can't change its state. So calling "add" creates a new BigInteger, rather than modifying the current.

Other replies have nailed it; BigInteger is immutable. Here's the minor change to make that code work.

BigInteger sum = BigInteger.valueOf(0);
for(int i = 2; i < 5000; i++) {
    if (isPrim(i)) {
        sum = sum.add(BigInteger.valueOf(i));
    }
}

BigInteger is an immutable class. So whenever you do any arithmetic, you have to reassign the output to a variable.

java.math.BigInteger is an immutable class so we can not assign new object in the location of already assigned object. But you can create new object to assign new value like:

sum = sum.add(BigInteger.valueOf(i));

Yes it's Immutable

sum.add(BigInteger.valueOf(i));

so the method add() of BigInteger class does not add new BigIntger value to its own value ,but creates and returns a new BigInteger reference without changing the current BigInteger and this is what done even in the case of Strings

Actually you can use,

BigInteger sum= new BigInteger("12345");

for creating object for BigInteger class.But the problem here is,you cannot give a variable in the double quotes.So we have to use the valueOf() method and we have to store the answer in that sum again.So we will write,

sum= sum.add(BigInteger.valueOf(i));

Biginteger is an immutable class. You need to explicitly assign value of your output to sum like this:

sum = sum.add(BigInteger.valueof(i));    

Since you are summing up some int values together, there is no need to use BigInteger. long is enough for that. int is 32 bits, while long is 64 bits, that can contain the sum of all int values.