Comparing the content of two objects in Java

Question

I tried to perform a comparison between two objects of the same class. Actually, I wanted to compare the content of the both the objects. Here, the objects are of class Student. In class Student I have overridden the equals() method as shown below. By doing so, will my intention be accomplished (compare the names and birthdays of both students)? If not what is happening here? The problem is that I don't get the answer I expect. The output is false even though it must be true.

public class Main {

    public static void main(String[] args) {
        Student a = new Student("John", "Johnny");
        Student b = new Student("John", "Johnny");
        a.setBirthDate(10, 10, 10);
        b.setBirthDate(10, 10, 10);
        boolean ans = Student.equals(a, b);
        System.out.println(ans);
    }

}

public class Date {
    public int day;
    public int month;
    public int year;
    
    public Date(int d, int m, int y) {
        this.day = d;
        this.month = m;
        this.year = y;
    }
}

public class Student{
    private String firstName;
    private String lastName;
    private Date birthDate;
    
    public Student(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }
    
    public void setBirthDate(int day, int month, int year) {
        Date b_day = new Date(day, month, year);
        birthDate = b_day;
    }
}


@Override
    public boolean equals(Object student) {
        System.out.println(this.firstName);
        System.out.println(this.lastName);
        System.out.println(this.birthDate);
        
        System.out.println(((Student)student).firstName);
        System.out.println(((Student)student).lastName);
        System.out.println(((Student)student).birthDate);
        return super.equals(student);
        
    }

I have overridden the equals method as follows. But still I face the same issue. I suspect that there's something wrong with the Date class. But the problem is that I'm not quite sure of it. Also, I don't understand how to remedy the problem. Can someone please tell me what's wrong here. Thanks in advance.

Answer 1

You can structure your equals method similarly to an if statement. You just need to cast the other argument first before your can compare their fields.

@Override
public boolean equals(Object o) {
    // Check if the other object is also a Student
    if (o instanceof Student) {
        // Now that we know o is a student, we can safely cast it
        Student other = (Student) o;

        // Similarly to how you would write an if statement you can compare each individual field.
        // Thanks to inheritance, we defer the equality check of each field to its own implementation
        return this.firstName.equals(other.firstName)
            && this.lastName.equals(other.lastName)
            && this.birthDate.equals(other.birthDate);
    }

    // Other object was not a student
    return false;
}

You then need to go write something similar in Date so that when you compare birthDate, it will know what to do.

You can also take this one step farther by using Objects.equals(a, b) instead of a.equals(b). This will ensure you do not get a nullPointerException if a happens to be null when comparing them. However, since this looks to be a school project I imagine you may be expected to either check manually or assume the values will not be null instead of using the standard library.

Answer 2

For equals to work, you need to override hashCode also. Further equality check needs actual comparison of your objects.

Also, any related objects also has to implement hashCode and equals method. In this case Date.

Possible code

import java.util.Objects;

class Date {

    public int day;
    public int month;
    public int year;

    public Date(int d, int m, int y) {
        this.day = d;
        this.month = m;
        this.year = y;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        Date date = (Date) o;
        return day == date.day &&
            month == date.month &&
            year == date.year;
    }

    @Override
    public int hashCode() {
        return Objects.hash(day, month, year);
    }
}

public class Student {

    private final String firstName;
    private final String lastName;
    private Date birthDate;

    public Student(String firstName, String lastName, Date birthDate) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.birthDate = birthDate;
    }

    public static void main(String[] args) {
        System.out.println(new Student("John", "Johnny", new Date(10, 10, 10))
            .equals(new Student("John", "Johnny", new Date(10, 10, 10))));
        System.out.println(new Student("John", "Johnny", new Date(11, 10, 10))
            .equals(new Student("John", "Johnny", new Date(10, 10, 10))));
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        Student student = (Student) o;
        return Objects.equals(firstName, student.firstName) &&
            Objects.equals(lastName, student.lastName) &&
            Objects.equals(birthDate, student.birthDate);
    }

    @Override
    public int hashCode() {
        return Objects.hash(firstName, lastName, birthDate);
    }
}