Question

Suppose I am writing a method foo(int i) in Java.
Since i is passed by value it is safe to change it in foo. For example

void foo(int i) {
   i = i + 1; // change i
   ...
}

Is it considered good or bad practice to change arguments of methods in Java?

Answer 1

It’s considered bad practice in general, though some people overlook it as you can see in the other answers.

For parameters like primitives that are directly passed in by value, there is no advantage in overriding the original variable. In this case you should make a copy as suggested by @João.

For parameters whose reference is passed in by value (objects), if you modify the handle to point to a different object, that is downright confusing. It’s all the more important because modifying the contents of an object passed as a parameter will modify the original object too.

If you replace the object referred to by the handle, and then modify its contents, the object referred to by the original reference in the caller will not be replaced, but someone reading the code might expect it to be.

Whereas if you don’t replace the object, and modify the contents, the method calling your method might not expect this change. This category generally comes under security-related bad practices.