Java Primitive Copy vs Reference Gotcha

Direct Answer

It prints 5. Primitives in Java are copied by value — int b = a gives b its own copy of 5. Changing b to 10 has no effect on a; they are completely independent variables.

Code Example

Passing a primitive into a method is just another assignment — the parameter gets a copy. So a method can never change the caller's variable through the argument alone.

public class ValueCopyDemo {
    public static void main(String[] args) {
        int x = 100;
        modify(x);
        System.out.println(x); // still 100 — method got a copy
    }
    static void modify(int value) {
        value = 999; // only the local copy changes
    }
}

This is why a method that needs to hand a new value back must return it. Java is always pass-by-value — even for objects, what gets copied is the reference, not the object itself.

Common Mistakes & Pitfalls

• Expecting a link: int b = a; doesn't connect the variables — they're separate.
• Confusing with objects: List<String> listB = listA; makes both point to the same list, so changes via one are visible through the other.
• Thinking parameters are references: passing an int and changing it inside doesn't affect the original — use a return.