Question

In JAVA (and I guess in general OO languages), is it best to use primitive or objects as method parameters and return values?

When I say “primitive” or “objects”, I’m talking about Long or long, int or Integer, etc.

for example :

public int doStuff(int number1, int number2) {
...
}

OR

public Integer doStuff(Integer number1, Integer number2) {
...
}

OR

public Integer doStuff(int number1, int number2) {
...
}

OR 

public int doStuff(Integer number1, Integer number2) {
...
}

I guess that we should be careful with objects as parameters since they can be null?

Answer 1

I guess that we should be careful with objects as parameters since they can be null?

Exactly. That’s why I use the primitive type to express that the value can not be null, and the wrapper type to express that it can be null.

For instance:

/**
 * @param threads  the desired number of threads, or null if the number 
 *                 of threads should be chosen automatically.
 */
void setNumberOfThreads(Integer threads) { ... }

or

/**
 * @returns the number of children, or null if not known.
 */
Integer countChildren() { ... }

but:

void setVelocity(double dx, double dy);

because a null velocity would wreck havoc with the physics simulation.