Unit 3 Cycle 2 Day 21: Method with Object Parameter

February 18, 2026

Unit 3 Advanced (Cycle 2) Day 21 of 28 Advanced

Method with Object Parameter

Section 3.6 — Writing Methods

Key Concept

When a method receives an object as a parameter, it can call that object's methods and modify its state. Since Java passes references by value, the method works with the same object as the caller. Changes to the object's fields through setter calls persist after the method returns. However, reassigning the parameter to a new object does not affect the caller's reference. The AP exam tests this distinction: mutations through the reference are visible, but reassignment of the reference is not.

Consider the following class.

public class Circle { private double radius; public Circle(double r) { radius = r; } public double getRadius() { return radius; } public boolean contains(Circle other) { return this.radius > other.radius; } }

What is printed?
Circle big = new Circle(10); Circle small = new Circle(3); System.out.println(big.contains(small) + " " + small.contains(big));

Answer: (B) true false

big.contains(small): 10 > 3 = true. small.contains(big): 3 > 10 = false. Result: "true false".

Why Not the Others?

(A) contains is not symmetric. big contains small, but not vice versa.

(C) big's radius (10) IS greater than small's (3).

(D) big.contains(small) is true because 10 > 3.

Common Mistake

When a method takes an object of the same class as a parameter, this is the calling object and the parameter is the other object. The direction matters.