Basically, you check if an object is an instance of a specific class. You normally use it, when you have a reference or parameter to an object that is of a super class or interface type and need to know whether the actual object has some other type (normally more concrete).
Example:
public void doSomething(Number param) {
if( param instanceof Double) {
System.out.println("param is a Double");
}
else if( param instanceof Integer) {
System.out.println("param is an Integer");
}
if( param instanceof Comparable) {
//subclasses of Number like Double etc. implement Comparable
//other subclasses might not -> you could pass Number instances that don't implement that interface
System.out.println("param is comparable");
}
}
Note that if you have to use that operator very often it is generally a hint that your design has some flaws. So in a well designed application you should have to use that operator as little as possible (of course there are exceptions to that general rule).
Answer from Thomas on Stack OverflowVideos
Basically, you check if an object is an instance of a specific class. You normally use it, when you have a reference or parameter to an object that is of a super class or interface type and need to know whether the actual object has some other type (normally more concrete).
Example:
public void doSomething(Number param) {
if( param instanceof Double) {
System.out.println("param is a Double");
}
else if( param instanceof Integer) {
System.out.println("param is an Integer");
}
if( param instanceof Comparable) {
//subclasses of Number like Double etc. implement Comparable
//other subclasses might not -> you could pass Number instances that don't implement that interface
System.out.println("param is comparable");
}
}
Note that if you have to use that operator very often it is generally a hint that your design has some flaws. So in a well designed application you should have to use that operator as little as possible (of course there are exceptions to that general rule).
instanceof is used to check if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.
Read more from the Oracle language definition here.
instanceof keyword is a binary operator used to test if an object (instance) is a subtype of a given Type.
Imagine:
interface Domestic {}
class Animal {}
class Dog extends Animal implements Domestic {}
class Cat extends Animal implements Domestic {}
Imagine a dog object, created with Object dog = new Dog(), then:
dog instanceof Domestic // true - Dog implements Domestic
dog instanceof Animal // true - Dog extends Animal
dog instanceof Dog // true - Dog is Dog
dog instanceof Object // true - Object is the parent type of all objects
However, with Object animal = new Animal();,
animal instanceof Dog // false
because Animal is a supertype of Dog and possibly less "refined".
And,
dog instanceof Cat // does not even compile!
This is because Dog is neither a subtype nor a supertype of Cat, and it also does not implement it.
Note that the variable used for dog above is of type Object. This is to show instanceof is a runtime operation and brings us to a/the use case: to react differently based upon an objects type at runtime.
Things to note: expressionThatIsNull instanceof T is false for all Types T.
It's an operator that returns true if the left side of the expression is an instance of the class name on the right side.
Think about it this way. Say all the houses on your block were built from the same blueprints. Ten houses (objects), one set of blueprints (class definition).
instanceof is a useful tool when you've got a collection of objects and you're not sure what they are. Let's say you've got a collection of controls on a form. You want to read the checked state of whatever checkboxes are there, but you can't ask a plain old object for its checked state. Instead, you'd see if each object is a checkbox, and if it is, cast it to a checkbox and check its properties.
if (obj instanceof Checkbox)
{
Checkbox cb = (Checkbox)obj;
boolean state = cb.getState();
}