For example,
public class A{...}
public class B extends A{...}
A a;
if(...) a = new A();
else a = new B();
Then, I want check if a is A or B. Is there any way to do this?
4 Answers
Check the type of object with instanceof take a look at the following
if(a instanceof B) {
// the object is of sub class
} else {
// the object is of super class
}
1 Comment
Allen Yang
If
A a = new B(), will instanceof A be confused? is the output true? becuase B is derived from A.you can check whether an instance is a type of a class by following way
if (a instanceof A) {
//...
} else {
//...
}
5 Comments
Allen Yang
Oh, thanks! So, instanceof won't be confused even if B is subclass of A?
Jesper
But if
a refers to a B object, then a instanceof B is still true, because B extends A. So you'd first have to check for a instanceof B.Allen Yang
So, if
A a = new B(), then instanceof B and instanceof A are both true?Allen Yang
OK. I got it. if
A a = new A(), then only instanceof A is true. this is why I have to check B first. Thank you .
Dannon
@AllenYang You should select this answer or another if you felt that it answered your question best. This gives closure to this question and for anyone else visiting this question in the future.
Renuka Fernando is correct, but you have to be careful here. Because of how objects are set up in memory, if your object is declared as a super class, but then initialized to one of its children, like so:
A a = new B();
Then the following code will always says "I'm an A!":
if(a instanceof A)
System.out.println("I'm an A!");
else
System.out.println("I'm a B!");
That is because a is an A and a B at the same time, so if you were trying to see if a was a B, then you would have to put in further checks.
Comments
You can use getClass() method of the object instance..
class K {
}
class P extends K {
}
public class A {
public static void main(String args[]) {
K k = new K();
K p = new P();
P p1 = new P();
System.out.println(p.getClass().getName());
System.out.println(p1.getClass().getName());
System.out.println(k.getClass().getName());
}
}
answered May 7, 2017 at 7:16
PrabaharanKathiresan
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1 Comment
Dannon
Java has
instanceof functionality for a reason. Java reflection should only be used in a few special cases and in other cases is considered bad programming practice, because of the security vulnerabilities that can arise. Obvious your solution above works and does not do anything nasty, but you are strictly avoiding a feature that was made for this exact purpose.
instanceofmethod in java to verify whether certain obj is a instance of a class..... If you want to know the type after the condition, instanceof is what you need.