Project Euler #8 code implementation
The first thing you should do is clean your code up.

We should notice that temp.charAt(k)-48 is called no less than once, and no more than twice in the for (int k...) loop. We can fix that (quite easily). 

Next, we should notice that if we find a 0 in our second loop, we can skip that many numbers ahead.

Lastly, aChar - 48 is bad practice. In Java you can subtract single-character literals. Instead of aChar - 48, let's do aChar - '0'. The 48 becomes a magic number, and it's just as easy to subtract the '0' instead. :)

public class Java {
    public static void main(String[] args) {
        String temp = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";
        long max = 0;

        for (int i = 0; i &lt; temp.length() - 12; i++) {
            int min = temp.charAt(i) - '0';
            long product = 1;

            for (int k = i; k &lt; 13 + i; k++) {
                int numK = temp.charAt(k) - '0';
                min =  Math.min(numK, min);

                if (min == 0) {
                    product = 0;
                    i = k + 1; // The value at `k` is a zero, so we'll not bother processing from `i through k`.
                    break;
                }
                else {
                    product = product * numK;
                }
            }

            if (product &gt; max) {
                max = product;
            }
        }

        System.out.println("max : " + max);
    }
}




Or, of course, rewrite the entire thing to one loop instead, which would be the preferred method.

public class Java {
    public static void main(String[] args) {
        String temp = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";
        long max = 0;
        long runningProduct = 1;
        int numbersLoaded = 0;
        for (int i = 0; i &lt; temp.length(); i++) {
            int numI = temp.charAt(i) - '0';

            if (numI == 0) {
                runningProduct = 1;
                numbersLoaded = 0;
            }
            else {
                if (numbersLoaded == 13) {
                    runningProduct /= temp.charAt(i - 13) - '0';
                }
                else {
                    numbersLoaded += 1;
                }

                runningProduct *= numI;

                if (runningProduct &gt; max) {
                    max = runningProduct;
                }
            }
        }

        System.out.println("max : " + max);
    }
}


I will warn you, if taken out of context the following line is dangerous:

runningProduct /= temp.charAt(i - 13) - '0';


This could definitely cause a division by 0 error if not handled properly. (It's handled properly here, but could easily be made not to be.)

Here's a link to ideone with the two tests, for comparison.

All-in-all, I hope this was helpful. :)