Q: Is Java pass-by-value or pass-by-reference?

Answer:

Java is strictly pass-by-value. Always — even for objects.

In Java, everything is pass-by-value. No exceptions. What changes is what the value represents:

• For primitives (int, double, etc.), the value is the actual data.
• For objects, the value is a reference to the object.

So when you pass an object to a method, Java copies the reference (not the object itself). That means:

• You can mutate the object’s internal state inside the method.
• You cannot change which object the caller’s variable refers to.

Example

class Test {
    int value;
}

void modify(Test obj) {
    obj.value = 10; // affects original object
}

void reassign(Test obj) {
    obj = new Test(); // does NOT affect original reference
}

Key takeaway

Java is pass-by-value; for objects, the value passed is a copy of the reference.

Primitives — Copy of Value

void increment(int x) { x++; }

int a = 5;
increment(a);
System.out.println(a);  // 5 — caller unaffected

Objects — Copy of Reference

class Box { int val; }

void mutate(Box b) { b.val = 99; }       // mutate via the copied reference
void reassign(Box b) { b = new Box(); }  // reassign the local copy

Box box = new Box();
box.val = 1;

mutate(box);
System.out.println(box.val);   // 99 — same object, mutated

reassign(box);
System.out.println(box.val);   // 99 — local b reassigned, caller's reference unchanged

Mental Model

• Variable stores a value (primitive) or a reference (object handle).
• Method call copies that value/reference into a new local variable.
• Mutating object state through the copied reference is visible (same object).
• Reassigning the parameter to a new object is not visible (caller still holds original reference).

Why "pass-by-reference" Would Look Different

True pass-by-reference (C++ &, C# ref): reassigning the parameter would change the caller's variable.

void swap(Integer a, Integer b) {
    Integer tmp = a; a = b; b = tmp;
}
Integer x = 1, y = 2;
swap(x, y);
// x=1, y=2 — Java can't swap. Pass-by-reference languages can.

String Trap

void change(String s) { s = "world"; }

String s = "hello";
change(s);
System.out.println(s);  // hello — String is immutable + reference reassigned locally

Interview-Killer Phrasing

"Java is pass-by-value. For object types, the value of the reference is passed by value — a copy of the reference, not the object."