Question

I mostly write in C but am using Java for this project. I want to know what Java is doing here under the hood.

ArrayList<Integer> prevRow, currRow;
currRow = new ArrayList<Integer>();
for(i =0; i < numRows; i++){
    prevRow = currRow;
    currRow.clear();
    currRow.addAll(aBunchOfItems);
}

Is the prevRow = currRow line copying the list or does prevRow now point to the same list as currRow? If prevRow points to the same list as currRow, I should create a new ArrayList instead of clearing….

private ArrayList<Integer> someFunction(ArrayList<Integer> l){
    Collections.sort(l);
    return l;
}

main(){
    ArrayList<Integer> list = new ArrayList<Integer>(Integer(3), Integer(2), Integer(1));

    list = someFunction(list);  //Option 1
    someFunction(list);  //Option 2
}

In a similar question, do Option 1 and Option 2 do the same thing in the above code?

Thanks-

Jonathan

Answer 1

If you come to Java from C/C++ remember this golden rule which will answer almost all these doubts: in Java a variable (except primitives) is always a reference (sort of pointer) to an object, never an object in itself.

For example

 prevRow = currRow;

is assigning a reference, so that prevRow and currRow now refer to the same object. Objects are not copied, neither in assignments, nor in argument passing (what is copied/assigned is the reference). And so on.