Question

I am preparing for a certification exam and I don’t understand this code:

Main:

public class TestStudent {
    public static void main(String[] args) {
        Student bob = new Student();
        Student jian = new Student();
        bob.name = "Bob";
        bob.age = 19;
        jian = bob;
        jian.name = "Jian";
        System.out.println("Bob's Name: " + bob.name);
    }
}

Class:

public class Student {
    public String name = "";
    public int age = 0;
    public String major = "Undeclared";
}

Why does this output “Bob’s Name: Jian”?

Bob.name was never set to Jian. Obviously it must be because “jian = bob;” but i thought that would only set jian variables to the same as bob. What is this concept called and where is it explained in java tutorials?

Answer 1

Assignments in Java do not copy objects, they copy references. After this assignment

jian = bob;

your jian is no longer pointing to the Student object that you have allocated and assigned to jian, it’s the same as bob, creating an alias to the same object. The original jian is now irretrievably lost, becoming eligible for garbage collection.

The following assingment

    jian.name = "Jian";

overwrites the name in the bob variable through its jian alias, leading to the result that you see.