In short, do not mutate the parameter variables. It is very error-prone and it makes bug-tracing more difficult.

This is what I meant with parameter mutation.

public int[] sortNumber(int[] input) {
  for (int i=0; i<input.length-1; i++)
    for (int j=i+1; j<input.length; j++) {
      if (input[i]>input[j]) {
        int temp = input[i];
        input[i] = input[j];
        input[j] = temp;
      }
    }
  }
  return input;
}

The code shown above is an example of a bad code. Now, let’s look at this code below:

int[] numbers= new int[]{1,3,5,4,2};
int[] sortedNumber = sortNumber(numbers);

// Display the contents
for (int number: numbers) {
  System.out.print(number," "); // "1,2,3,4,5" will be printed.
}

Now as you see the sortNumber function above mutates the original data. This might not seem like a problem for you if you are the one programming the API. But when another programmer steps in and try to use the same function, it’s likely to cause a headache.
Example is always the easiest thing to do to clear things up:

int[] numbers = new int[]{1,3,5,4,2};
int[] sortedNumbers = sortNumber(numbers); // sortedNumbers = {1,2,3,4,5}
int[] reversedNumbers = reverseNumber(numbers); // reversedNumbers = {5,4,3,2,1}

As you see, the original variable (numbers) contains the unsorted value. When the sortNumber function is called and {1,3,5,4,2} is passed in, the method will return the expected array of sorted integers {1,2,3,4,5}.

The problem arises when the new programmer tries to use another function (reverseNumber in this example) using the same variable (numbers) right after using your function. He is expecting to see {2,4,5,3,1} as the result of reversing the values {1,3,5,4,2}. Oh dear he is likely to be surprised when he sees that the returned value is not as he expected.

Let’s have a quick look at some Math lesson regarding function (mapping) operation.

y1 <-- f1(x)
y2 <-- f2(x)

Now, when you call function f1 using parameter x, is it going to change the parameter x? No? Bingo, you’re right! It doesn’t make sense if the x value changes after f1 function is called.

The same thing should apply in programming as well. When you apply a function to a parameter. Do not change the parameter value in your method. If it’s really necessary, make a defensive copy of the parameter or clone the parameters and assign it to a new variable. Then you’ll be able to play around with that new variable freely without worrying about spoiling the original parameter values.

There are a few exceptions when the passed parameter variables are mutated. But in general, preventing the variable mutation is a better choice.