Week 5: Class hierarchies/13A. equals Method

We have previously used the equals method to compare String objects. 
It is also possible to implement the method in own class. 
This enables the COMPARISON of 2 OBJECTS created out of OWN CLASS.

Let's implement the method to class 'Point', which models a point in 2-dimensional coordinate system.

Toteutetaan siis metodi equals luokkaan Piste, joka mallintaa pistettä kaksiulotteisessa koordinaatistossa:

class Point {
    private int x;
    private int y;

    //CONSTRUCTOR
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }


    @Override
    public boolean equals(Object obj) {
        // If same object, must be equal
        if (this == obj) {
            return true;
        }
        
        // Jos other is null, must be false
        if (obj == null) {
            return false;
        }
        
        // If other is not Point, must be false
        if (obj.getClass() != Point.class) {
            return false;
        }


        // OTHERWISE IF
        // 1 not the same object OR
        // 2 not null -> good OR
        // 3 not a Point class
        // -> Cast the input 'obj' as a Point object
        Point other = (Point) obj;

        //THEN CHECK
        // If x and y are equal, points are equal
        return (x == other.x && y == other.y);
        
    } 
}
 



The implementation of the method (usually) follows the same structure. 
Note that it's up to the programmer to decide when two objects should be equal: do ALL ATTRIBUTES need to be EQUAL OR ONLY SOME?

For example, we can decide that 2 Student objects are equal if they have 
1 - the 'same student id' - as the rest of the attributes are not important
2 - (but then again, in some other case we may require that 'all attributes' have equal value for 'students' to be equal):

 
class Student extends Person {
    // Name and email are inherited from Person
    private String studentId;
    private int credits;
    
    public Student(String studentId, String name, String email, int credits) {
        super(name, email);
        this.studentId = studentId;
        this.credits = credits;
    }

    @Override
    public boolean equals(Object obj) {
        // SAME OBJECCT
        if (this == obj) {
            return true;
        }

        // NOT NULL
        if (obj == null) {
            return false;
        }

        // not 'Student' class
        if (obj.getClass() != Student.class) {
            return false;
        }

        // SAME studentId
        Student other = (Student) obj;
        // Strings are compared with equals method
        return studentId.equals(other.studentId);
    }
}


 

A typical error is to forget to use the 'equals' method when comparing objects: 
for example, String objects' equality must be checked with equals method instead of == operator.

Most editors provide a way to insert the equals method automatically, 
plese refer to your editor's documentation (or google it - for example "visual studio code java equals".