Determining Class of an Object in Java

0 votes
Consider the following scenario:

Class B and class C extend class A. Now, we have an object of type B or C.

How can we determine the type of instance for the Object?
May 31, 2018 in Java by developer_1
• 3,350 points
1,928 views

2 answers to this question.

0 votes

There are many functions defined in Java Standard Library Packages through which you can determine the Instance type of an Object. Below mentioned is a summarized list of such functions:

// Method #1
if (obj instanceof C)
    ;

// Method #2
if (C.class.isInstance(obj))
    ;

// Method #3
if (C.class.isAssignableFrom(obj.getClass()))
    ;

// Method #4
try {
    C c = (C) obj;
    // No exception: obj is of type C or IT MIGHT BE NULL!
} catch (ClassCastException e) {
}

// Method #5
try {
    C c = C.class.cast(obj);
    // No exception: obj is of type C or IT MIGHT BE NULL!
} catch (ClassCastException e) {
}

Differences in null handling

There is a difference in null handling though:

  • In the first 2 methods expressions evaluate to false if obj is null (null is not instance of anything).
  • The 3rd method would throw a NullPointerException obviously.
  • The 4th and 5th methods on the contrary accept null because null can be cast to any type!

To remember: null is not an instance of any type but it can be cast to any type.

Notes

  • Class.getName() should not be used to perform an "is-instance-of" test becase if the object is not of type C but a subclass of it, it may have a completely different name and package (therefore class names will obviously not match) but it is still of type C.
  • For the same inheritance reason Class.isAssignableFrom() is not symmetric:
    obj.getClass().isAssignableFrom(C.class) would return false if the type of obj is a subclass of C.
answered May 31, 2018 by Rishabh
• 3,620 points
0 votes

You can use:

Object instance = new SomeClass();
instance.getClass().getName(); //will return the name (as String) (== "SomeClass")
instance.getClass(); //will return the SomeClass' Class object

HTH. But I think most of the time it is no good practice to use that for control flow or something similar...

answered Nov 26, 2018 by Sushmita
• 6,920 points

Related Questions In Java

0 votes
2 answers

One line initialization of an ArrayList object in Java

In Java 8 or earlier: List<String> string = ...READ MORE

answered Jul 26, 2018 in Java by samarth295
• 2,220 points
4,676 views
0 votes
2 answers

How to sort an ArrayList of custom object by property in Java?

You can Sort using java 8 yourList.sort(Comparator.comparing(Classname::getName)); or  yourList.stream().forEach(a -> ...READ MORE

answered Aug 14, 2018 in Java by samarth295
• 2,220 points
2,984 views
0 votes
2 answers

In Java, what is the best way to determine the size of an object?

I happened to find a java class "jdk.nashorn.internal.ir.debug.ObjectSizeCalculator", ...READ MORE

answered Aug 19, 2019 in Java by Sirajul
• 59,230 points
12,930 views
+1 vote
6 answers

How can I separate the digits of an int number in Java?

You can also have a look here: To ...READ MORE

answered Dec 9, 2020 in Java by Roshni
• 10,520 points
208,877 views
0 votes
1 answer

Decompiling Java Class Files

There are a few decompilers out there... ...READ MORE

answered Jun 1, 2018 in Java by Rishabh
• 3,620 points
1,222 views
0 votes
2 answers

Copying Objects in Java

class DummyBean { private String dummy; ...READ MORE

answered Aug 6, 2018 in Java by Sushmita
• 6,920 points
689 views
0 votes
2 answers

Array of Objects

You can also do : A[] a = ...READ MORE

answered Aug 3, 2018 in Java by sharth
• 3,370 points
703 views
0 votes
2 answers

Is there a destructor in Java?

try (BufferedReader br = new BufferedReader(new FileReader(path))) ...READ MORE

answered Sep 4, 2018 in Java by Sushmita
• 6,920 points
896 views
0 votes
2 answers

Counting no of Occurrence of a particular character inside a string in Java

We can find out the no. of ...READ MORE

answered Sep 7, 2018 in Java by Sushmita
• 6,920 points
2,685 views
0 votes
2 answers

What is the use of toString method in Java and how can I use it ?

Whenever you require to explore the constructor ...READ MORE

answered Aug 23, 2018 in Java by Daisy
• 8,140 points
4,051 views
webinar REGISTER FOR FREE WEBINAR X
REGISTER NOW
webinar_success Thank you for registering Join Edureka Meetup community for 100+ Free Webinars each month JOIN MEETUP GROUP