this == obj; OR Class1 var1 = (Class1) obj1; (x == obj1.x && y == obj1.y);

Week 5: Class hierarchies/13A. equals Method

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+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".
+
+
+
+
+
+