Passing by reference in Java?

Question

In C++, if you need to have 2 objects modified, you can pass by reference. How do you accomplish this in java? Assume the 2 objects are primitive types such as int.

Answer 1

You can't. Java doesn't support passing references to variables. Everything is passed by value.

Of course, when a reference to an object is passed by value, it'll point to the same object, but this is not calling by reference.

Answer 2

Wrap them in an object and then pass that object as a parameter to the method.

For example, the following C++ code:

bool divmod(double a, double b, double & dividend, double & remainder) {
  if(b == 0) return false;
  dividend = a / b;
  remainder = a % b;
  return true;
}

can be rewritten in Java as:

class DivRem {
  double dividend;
  double remainder;
}

boolean divmod(double a, double b, DivRem c) {
  if(b == 0) return false;
  c.dividend = a / b;
  c.remainder = a % b;
  return true;
}

Although more idiomatic style in Java would be to create and return this object from the method instead of accepting it as a parameter:

class DivRem {
  double dividend;
  double remainder;
}

DivRem divmod(double a, double b) {
  if(b == 0) throw new ArithmeticException("Divide by zero");
  DivRem c = new DivRem();
  c.dividend = a / b;
  c.remainder = a % b;
  return c;
}

Answer 3

Java does not have pass by reference, but you can still mutate objects from those references (which themselves are passed by value).

• java.util.Arrays.fill(int[] arr, int val) -- fills an int array with a given int value
• java.util.Collections.swap(List<?> list, int i, int j) -- swaps two elements of a list
• StringBuilder.ensureCapacity(int min) -- self-explanatory

As you can see, you can still modify objects (if they're not immutable); you just can't modify references or primitives by passing them to a function.

Of course, you can make them fields of an object and pass those objects around and set them to whatever you wish. But that is still not pass by reference.