Summary of the usage of instanceof keywords in Java _java

The instanceof operator in Java is used to indicate at run time whether an object is an instance of a particular class. Instanceof, by returning a Boolean value, indicates whether the object is a particular class or an instance of its subclass.

Usage:
Result = Object Instanceof class
Parameters:
Result: Boolean type.
Object: Required option. An arbitrary object expression.
Class: Required option. Any defined object class.
Description
If object is an instance of class, the instanceof operator returns TRUE. Returns False if object is not an instance of the specified class, or if object is null.

Examples are as follows:

Copy Code code as follows:

Package com.instanceoftest;


Interface a{}


Class B implements a{





}


Class C extends B {





}





Class Instanceoftest {


public static void Main (string[] args) {


A A=null;


B B=null;


Boolean res;





System.out.println ("Instanceoftest test Case 1:------------------");


res = a instanceof A;


System.out.println ("A instanceof A:" + res);





res = b instanceof B;


System.out.println ("b instanceof B:" + res);





System.out.println ("\ninstanceoftest test Case 2:------------------");


A=new B ();


B=new B ();





res = a instanceof A;


System.out.println ("A instanceof A:" + res);





res = a instanceof B;


System.out.println ("A instanceof B:" + res);


res = b instanceof A;


System.out.println ("b instanceof A:" + res);





res = b instanceof B;


System.out.println ("b instanceof B:" + res);





System.out.println ("\ninstanceoftest test Case 3:------------------");


B b2= (c) new C ();





res = B2 instanceof A;


System.out.println ("B2 instanceof A:" + res);





res = B2 instanceof B;


System.out.println ("B2 instanceof B:" + res);





res = B2 instanceof C;


System.out.println ("B2 instanceof C:" + res);


}


}





/*


Result





Instanceoftest test Case 1:------------------


A instanceof A:false


b instanceof B:false


Instanceoftest test Case 2:------------------


A instanceof A:true


A instanceof B:true


b instanceof A:true


b instanceof B:true


Instanceoftest test Case 3:------------------


B2 instanceof A:true


B2 instanceof B:true


B2 instanceof C:true

Instanceof is a Java two-dollar operator, and ==,>,< is the same kind of stuff. Because it is made up of letters, it is also a reserved keyword in java. It's The effect is to test whether the object on its left is an instance of its right class, and returns a Boolean type of data.

Usage: An Instance object instanceof a class name

Instanceof are typically used to invoke different methods based on different instances:

In a class that has an inheritance relationship, we can invoke different methods in different instances through polymorphism:

Example 1:

There are three classes, class names, and the relationships between them as follows

Animal (superclass) Dog (subclass) Cat (subclass)

You can draw the following objects

Animal Animal =new Animal (); = = "Animal instanceof animal return True

Dog dog=new Dog () = = = Dog instanceof Dog return True

Cat Cat=new cat () = = "Cat instanceof Cat Returns True

Animal dog=new Dog () = = = Dog Instanceof Animal return True

Animal cat=new cat () = = "Cat instanceof Animal return True

Copy Code code as follows:

Animal dog=new Dog ();
Animal cat=new Cat ();

List List = new ArrayList ();

List.add (dog);
List.add (CAT);

Iterator it = List.iterator ();
while (It.hasnext ()) {
It.next (). Animaldo ();

}

Here we can rewrite the Animaldo method in animal in the dog and the Cat class by calling the Animaldo method and then automatically invoking methods in the same way based on different instances.

In a class that has no inheritance relationship, we can judge the current instance by instanceof and then invoke different methods according to different instances:

Example 2:

Copy Code code as follows:

Station s = new station ();
Cell C = new cell ();

List List = new ArrayList ();

List.add (s);
List.add (c);

Iterator it = List.iterator ();
while (It.hasnext ()) {
Object obj = It.next ();
if (obj instanceof station) {
Station S1 = (station) obj;
S1.stationdo ();
}
if (obj instanceof Cell) {
Cell C1 = (cell) obj;
C1.celldo ();
}
}

Here we can judge the result by instanceof, execute the corresponding action method in the same way (Stationdo (), Celldo ()).

Generally in the use of no generic set (List, set, etc.), more use of instanceof, because the collection can save a variety of objects, so in the reading of the general to make corresponding judgments.