Day-83 Understanding Type Casting and Comparable in Java

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1. Type Casting in Java

Type casting is the process of converting one data type into another. In Java, type casting is of two types:

a) Widening (Upcasting)

Converting a smaller data type into a larger data type.
Happens automatically (implicit casting).

Example (Primitive):

int myInt = 9;
double myDouble = myInt;  // int is widened to double
System.out.println(myDouble);  // Output: 9.0

Example (Non-Primitive / Objects):

class Animal {}
class Dog extends Animal {}

public class Main {
    public static void main(String[] args) {
        Animal a = new Dog();  // Upcasting: Dog → Animal
    }
}

b) Narrowing (Downcasting)

Converting a larger data type into a smaller data type.
Must be done manually (explicit casting).

Example (Primitive):

double i = 10.5;
int j = (int) i;   // explicit narrowing
System.out.println(j);  // Output: 10

Example (Non-Primitive / Objects):

class Animal {}
class Dog extends Animal {}

public class Main {
    public static void main(String[] args) {
        Animal a = new Dog();
        Dog d = (Dog) a;  // Downcasting: Animal → Dog
    }
}

2. The Comparable Interface

When we need to sort or compare objects in Java (like numbers, strings, or custom classes), the Comparable interface helps us define the natural ordering.

Key Points:

Defined in java.lang package
It has a single method:

  int compareTo(Object obj);

Returns:
- positive → current object is greater
- negative → current object is smaller
- zero → both objects are equal
Used for single sorting criteria (e.g., price, name, age).

Example: Comparing Mobiles by Price

package objectLearn;

public class Mobiles implements Comparable<Mobiles> {
    int price, ram, mp;

    public Mobiles(int price, int ram, int mp) {
        this.price = price;
        this.ram = ram;
        this.mp = mp;
    }

    public static void main(String[] args) {
        Mobiles oppo = new Mobiles(12000, 8, 50);
        Mobiles vivo = new Mobiles(10000, 12, 64);

        int res = oppo.compareTo(vivo);  // Compare using compareTo()

        if (res > 0) {
            System.out.println("Oppo is costlier than Vivo");
        } else if (res < 0) {
            System.out.println("Oppo is cheaper than Vivo");
        } else {
            System.out.println("Both have same price");
        }
    }

    // Compare based on price
    public int compareTo(Mobiles obj) {
        if (this.price > obj.price) {
            return 1;
        } else if (this.price < obj.price) {
            return -1;
        }
        return 0;
    }
}

Output: