Parameters, Arguments, and Pass by Value

When you call a method with an argument, Java copies the value of that argument into the method's parameter variable. That parameter is a brand-new box in memory, separate from the argument's box, even though it starts out holding an equal value. The method can do whatever it wants to its own box, but it has no way to reach back and change the caller's box. That is why reassigning a parameter inside a method leaves the caller's original variable exactly as it was.

This matters most for primitives like int, double, and boolean, where the value itself is the number. It gets more interesting with arrays and objects, because what gets copied there is a reference, essentially an address pointing to the object in memory. The parameter and the argument are still separate variables, but now both copies point at the same object. So reassigning the parameter to a new array does not affect the caller, but changing an element inside the array that the reference points to does, because both variables are looking at the same object.

This distinction sets up everything else in building your own class: it explains why a constructor's parameters only initialize fields if you explicitly assign them, and why passing an object into a method lets that method mutate the object's state even though the reference itself was passed by value.