CODEWITHSUNDEEP
javainstanceof
MKod
MKod

Reputation: 823

Understanding instanceof in java along with if condition?

I am not sure how the tree (reference variable) became instance of object Tree in my sample program 53,00? I am expecting "Pine" and "oops" as output but why "Tree" is included in output? I havent given Tree tree = new Tree() at all. 

    class Tree{}  
class Pine extends Tree{}  
class Oak extends Tree{}  
public class forrest {    

    public static void main( String[] args )      
    {    
        Tree tree = new Pine();   


        if( tree instanceof Pine )      
            System.out.println( "Pine" );   
        if( tree instanceof Tree )       
            System.out.println( "Tree" );  
        if( tree instanceof Oak )      
            System.out.println( "Oak" );   
        else System.out.println( "Oops" );  
    }  
}

Upvotes: 3

Views: 6592

Answers (5)

tucuxi
tucuxi

Reputation: 17945

Your class Pine has an implicit constructor (which gets placed there by the compiler unless you manually define your own, in which case your constructor will be used). All subclasses must call their parent class (superclass) as a part of their constructor. Not doing so results in an exception being thrown. So you have an implicit

Pine Pine() { super(); }

Which is calling the implicit constructor of it's superclass (Tree; that is what "super()" does), which looks like

Tree Tree() { super(); }

Which is calling the implicit constructor of it's superclass (Object)...

Therefore, if tree is a Pine, then

tree instanceof Pine
tree instanceof Tree
tree instanceof Object

all return true

Upvotes: 1

Jaco Van Niekerk
Jaco Van Niekerk

Reputation: 4182

As said by Keppil, instanceof returns true on ancestors as well. Based on this, the following will happen:

Tree tree = new Pine()

tree instanceof Pine; // true
tree instanceof Oak; // false
tree instanceof Tree; // true
tree instanceof Object; // true

Object something = new Oak();

something instanceof Pine; // false
something instanceof Oak; // true
something instanceof Tree; // true
something instanceof Object; // true

In fact, instanceof Object will always return true.

Upvotes: 3

Serkan Arıkuşu
Serkan Arıkuşu

Reputation: 5619

Every subclass 'is a' type of its superclass. Since your "tree" is an instance of Pine class extended from Tree class, it is an instance of Tree.

For your coding style, I suggest reading Beware of instanceof operator

Upvotes: 0

HakunaMatata
HakunaMatata

Reputation: 834

A child is instance of itself and subtype of its parent class.every class is subType of java.lang.Object class. In your example Pine and Oak are child class of Tree or can be written as Pine and Oak are subType of Tree class so

if( tree instanceof Tree )

is True.

Upvotes: 0

Keppil
Keppil

Reputation: 46219

Since a Pine or an Oak also IS-A Tree, your tree instanceof Tree will return true whether tree is Tree, Pine or Oak.
You can read more on Inheritance in the Java Tutorials.

Upvotes: 6

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