Do pointers in java actually exist?

Question

I thought I'm pretty experienced in java, but it seems that's not really the case, I just noticed something yesterday, something I used before but never really realised what it did. I googled but didn't find the answer to my question.

If I declare an int array, and use Array's static sort function to sort my array, I just need to type

Arrays.sort( numbers );

Instead of

numbers = Array.sort( numbers );

This might look very easy in C and C++ because you can use pointers there. So what I'm wondering is, how is this done? Is it an advantage sun has, or am I completely senseless here?

Answer 1

First, this belongs on StackOverflow. Second, you want to read the article Java is Pass-by-Value, Dammit!

Answer 2

I assume numbers is an int-array, and all arrays in Java are objects. So sort is passed a reference to numbers, and it can sort them in place.

Answer 3

This question should be migrated to Stack Overflow. The fact that Java has a NullPointerException class should give you a strong hint as to whether Java uses pointers behing the scenes.

Answer 4

Pointers exist in java - all non primitive variables are pointers in java, aka references.

THey do not support the same set of operations as pointers in C tho - they are essentially opaque to the user of the language.

The reason Arrays.sort(array) works is because array is a "pointer", which allows the sort() function access to the memory location that the array variable points to.

Now, why doesn't:

void swap (Integer a, Integer b) {
   Integer tmp = a;
   a = b;
   b = tmp;
}

work if you did

Integer x = 1;
Integer y = 2;
swap(x,y);

Its because java passes by value (which is a concept distinct from pointers). The pointer to 1 is given to swap(), not as the value of the variable x (which is a memory address, or pointer). Thus, manipulating the arguments in swap() does nothing to effect the variable x.

Answer 5

It sorts the array in-place.

The sort method receives a reference to the number array (which is an object), and changes the values inside the array! No new array-object is created, it is thus good enough to just pass it to the sort function.

 

PS: all source code of Java is open, you can go and read the sources of the sort function yourself. You'll see, there be no magic. If Java is installed properly on your system, there should be a src.zip in the Java home folder.