Project Euler #4 Largest Palindrome Product
Your method isPalindrome produces wrong results for negative numbers. (which is not important for Project Euler #4, but still) 
It is also inefficient to convert all numbers to String. I would only do that when using BigInteger. With int, you can just use a little math:

public static boolean isPalindrome(int number) {
    return number == reverse(number);
}

public static int reverse(int number) {
     //negative numbers: reverse the positive and negate it again.
    if (number &lt; 0) return -reverse(-number);
    //a 1-digit number is its own reverse
    if (number &lt; 10) return number;
    int reversed = 0;
    while (number &gt; 0) {
        //take the last digit of number and paste it to reversed.
        reversed = reversed * 10 + number % 10;
        //cut off the last digit
        number /= 10;
    }
    return reversed;
}


You are not using the method getPalindromeProducts(int multiplicand, int multiplier) correctly. You run both loops all the way down to zero.
The meaning of the names multiplicant and multiplier isn't clear imo. In a multiplication, both numbers are just factors. One of them is the minimum (100) and the other is the maximum (999). You're looking for numbers between 100 and 999, not between 100 and 0 or between 999 and 0.
You also do a lot of the work twice by running both loops all the way. For example, you will get a * b AND b * a. You can skip a lot of iterations by breaking out of the loop early.

Another thing to think about is the HashSet. The problem with Java Collections is that you cannot use them with primitive types. It will convert all numbers to Object, which will hurt performance. (You cannot have a HashSet&lt;int&gt; for example)
However, in this case, you don't need a HashSet at all. You don't care about duplicates, you just want to find the largest product. It is also not necessary to store all the palindromes you found; you only need the largest one.

public static int getLargestPalindromeProduct(int minFactor, int maxFactor) {
    int largest = 0;
    for (int a = maxFactor; a &gt;= minFactor; --a) {
        for (int b = maxFactor; b &gt;= a; --b) {//Note the b &gt;= a, not b &gt;= minFactor
            int product = a * b;
            if (product &lt;= largest) break; //We can stop this iteration, because factors will only get smaller.
            if (isPalindrome(product)) {
                largest = product;
                break;
            }
        }
    }
    return largest;
}


Then call getLargestPalindromeProduct(100, 999);