Removing an element from List<Student>

[Clemonte Johnstone]

Hi,
Please can someone kindly explain why this program does not remove an element from students object?

import java.util.ArrayList;
import java.util.List;

class Student {
    private String name;
    private int age;

    Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String toString() {
        return "Student[" + name + ", " + age + "]";
    }
}

public class _02_ArrayList_POJO_Question {

    public static void main(String[] args) {
        List<Student> students = new ArrayList<>();
        students.add(new Student("James", 25));
        students.add(new Student("James", 27));
        students.add(new Student("James", 25));
        students.add(new Student("James", 25));

        /* this line of code has no effect, why so? */
        students.remove(new Student("James", 25));

        for(Student stud : students) {
            System.out.println(stud);
        }

        /* What is the output?
         * Student[James, 25]
         * Student[James, 27]
         * Student[James, 25]
         * Student[James, 25] */

    }
}

[Campbell Ritchie]

Welcome to the Ranch

Mark Raphael Qt wrote:. . .        /* this line of code has no effect, why so? */        students.remove(new Student("James", 25));. . .

Because the List doesn't contain that object.

You need to override both the equals() and hashCode() methods if you want to remove anything like that But by the definition of equality your class provides (see Object#equals()), all objects in the List are different from that object.

[Clemonte Johnstone]

Thanks for the prompt reply.

Are you able to demonstrate please how you would remove the object element as you put it, with either System.identityHashCode() and/or .equals() ?

[Campbell Ritchie]

You can't use identity hash code or similar. You will have to override equals() and hashCode().
Resources: Joshua Bloch Effective Java (in the 3rd edition, pages 37‑54), Angelika Langer (search: make sure to get the English version) and artima.com (Odersky, Spoon and Venners). Although Bloch uses a surprisingly complicated implementation of hashCode(), I think his is the best resource.