Equality and Sets

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Comparing primitives

Primitives are stored by value, so checking them for equality is simple. We use the == operator.

int firstInt = 42;
int secondInt = 86;
if(firstInt == secondInt) {
    System.out.println("These are equal.");
} else {
    System.out.println("These are not equal.");
}

Comparing objects

Variables that hold objects are called reference types, because they hold a reference to an object. Think of this as a pointer to the object stored as a memory address.

If we compare two object reference using the equals (==) operator, we are actually comparing the references, not the values to which they refer.

Try this. You may be suprised:

Integer firstInt = new Integer(42);
Integer secondInt = new Integer(42);
if(firstInt == secondInt) {
    System.out.println("These are equal.");
} else {
    System.out.println("These are not equal.");
}

What just happened?

I am me

Try changing secondInt to firstInt in the comparison. What happens?

Integer firstInt = new Integer(42);
Integer secondInt = new Integer(42);
if(firstInt == firstInt) {
    System.out.println("These are equal.");
} else {
    System.out.println("These are not equal.");
}

Doing it right

To compare objects, we use the equals method.

Integer firstInt = new Integer(42);
Integer secondInt = new Integer(42);
if(firstInt.equals(secondInt)) {
    System.out.println("These are equal.");
} else {
    System.out.println("These are not equal.");
}

Full circle

Fire up your IDE, and create this Circle class:

public class Circle {

    private int radius;

    public Circle(int radius) {
        this.radius = radius;
    }

    public double circumference() {
        return 2 * Math.PI * radius;
    }
}

Note: Math.PI above is a constant. What does that mean?

Circles of a size

As per our Integer example, this code won't work as you may expect (drop it into a main method):

Circle firstCircle = new Circle(42);
Circle secondCircle = new Circle(42);

if(firstCircle.equals(secondCircle)) {
    System.out.println("These are the same size");
} else {
    System.out.println("These aren't the same size.");
}

Establishing equality

What is the method we are using to compare objects?

It is the equals method, not an operator. The equals method is a method like any other.

Object is the parent class for all objects, so when we call the equals method on Circle, we are calling the equals method on its parent class, Object:

public boolean equals(Object obj) {
    return (this == obj);
}

What did we just discover about objects and the == operator?

Creating our own equals method

In order to compare two circles, we override the equals method in our Circle class.

@Override
public boolean equals(Object obj) {

    boolean isACircle = obj instanceof Circle;

    if(obj == null || !isACircle) {
        return false;
    }

    Circle theOther = (Circle) obj;

    return radius == theOther.radius;
}

We're comparing the radii of our Circles!

Lets break that down!

Sets

From Wikipedia:

In mathematics, a set is a well-defined collection of distinct objects, considered as an object in its own right. For example, the numbers 2, 4, and 6 are distinct objects when considered separately, but when they are considered collectively they form a single set of size three, written {2,4,6}. Sets are one of the most fundamental concepts in mathematics.