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Java instanceof Keyword

Theory

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Java instanceof Keyword

instanceof checks: is this object an example of that type? Expression: obj instanceof Type → true or false. Use it before a cast so you avoid ClassCastException.

It understands inheritance: Dog instanceof Animal can be true. null instanceof Anything is always false — a favourite MCQ.

Show Animal a = new Dog(); then a instanceof Dog and a instanceof Animal. Don’t cast blindly in uncertain hierarchies.

Let's take this on the board with one tiny Main class — no extra files. Java instanceof Keyword — output: true then true. d is a Dog, and because Dog extends Animal, d instanceof Animal is also true. Start from main, go line by line, and stop at each print. That output is the proof for instanceof Keyword.

Exam tip

null instanceof X is false — common MCQ.

Example

class Animal {}
class Dog extends Animal {}
public class Main {
  public static void main(String[] args) {
    Animal a = new Dog();
    System.out.println(a instanceof Dog);
    System.out.println(a instanceof Animal);
  }
}

Java instanceof Keyword — output: true then true. d is a Dog, and because Dog extends Animal, d instanceof Animal is also true.

Short notes

Definstanceof checks if an object is that type. Returns true/false.
RuleUse before casting to avoid ClassCastException.
RememberWorks with inheritance. Dog instanceof Animal can be true.
Trapnull instanceof X is always false.

class Animal {} class Dog extends Animal {} public class Main { public static void main(String[] args) { Animal a = new Dog(); System.out.println(a instanceof Dog); System.out.println(a instanceof Animal); } }

Java instanceof Keyword

Example

Short notes

Questions

Java instanceof Keyword

Example

Short notes

Questions