How to determine an object's class?

If class B and class C extend class A and I have an object of type B or C, how can I determine of which type it is an instance?

if (obj instanceof C) {
//your code
}

Use Object.getClass. It returns the runtime type of the object.

Multiple right answers were presented, but there are still more methods: Class.isAssignableFrom() and simply attempting to cast the object (which might throw a ClassCastException).

Possible ways summarized

Let's summarize the possible ways to test if an object obj is an instance of type C:

// 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.

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...

Any use of any of the methods suggested is considered a code smell which is based in a bad OO design.

If your design is good, you should not find yourself needing to use getClass() or instanceof.

Any of the suggested methods will do, but just something to keep in mind, design-wise.

We can use reflection in this case

objectName.getClass().getName();

Example:-

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {



String name = request.getClass().getName();




}
}

In this case you will get name of the class which object pass to HttpServletRequest interface refference variable.

There is also an .isInstance method on the "Class" class. if you get an object's class via myBanana.getClass() you can see if your object myApple is an instance of the same class as myBanana via

myBanana.getClass().isInstance(myApple)

checking with isinstance() would not be enough if you want to know in run time. use:

if(someObject.getClass().equals(C.class){
    // do something
}

I Used Java 8 generics to get what is the object instance at runtime rather than having to use switch case

 public <T> void print(T data) {
    System.out.println(data.getClass().getName()+" => The data is " + data);
}

pass any type of data and the method will print the type of data you passed while calling it. eg

    String str = "Hello World";
    int number = 10;
    double decimal = 10.0;
    float f = 10F;
    long l = 10L;
    List list = new ArrayList();
    print(str);
    print(number);
    print(decimal);
    print(f);
    print(l);
    print(list);

Following is the output

java.lang.String => The data is Hello World
java.lang.Integer => The data is 10
java.lang.Double => The data is 10.0
java.lang.Float => The data is 10.0
java.lang.Long => The data is 10
java.util.ArrayList => The data is []

I use the blow function in my GeneralUtils class, check it may be useful

    public String getFieldType(Object o) {
    if (o == null) {
        return "Unable to identify the class name";
    }
    return o.getClass().getName();
}

You can use getSimpleName().

Let's say we have a object: Dog d = new Dog(),

The we can use below statement to get the class name: Dog. E.g.:

d.getClass().getSimpleName(); // return String 'Dog'.

PS: d.getClass() will give you the full name of your object.

use instanceof operator to find weather a object is of particular class or not.

booleanValue = (object instanceof class)

> JDK 14 extends the instanceof operator: you can specify a binding > variable; if the result of the instanceof operator is true, then the > object being tested is assigned to the binding variable.

please visit official Java documentation for more reference.

Sample program to illustrate usage of instanceof operator :

import java.util.*;

class Foo{
	@Override 
	public String toString(){
		return "Bar";
	}
}

class Bar{
	public Object reference;
	@Override 
	public String toString(){
		return "Foo";
	}
}
public class InstanceofUsage{
	public static void main(final String ... $){
		final List<Object> list = new ArrayList<>();

		var out = System.out;

		list.add(new String("Foo Loves Bar"));
		list.add(33.3);
		list.add(404_404);
		list.add(new Foo());
		list.add(new Bar());

		for(final var o : list){
			if(o instanceof Bar b && b.reference == null){
				out.println("Bar : Null");
			}	
			if(o instanceof String s){
				out.println("String : "+s);
			}

			if(o instanceof Foo f){
				out.println("Foo : "+f);
			}
		}

	}
}

output:

$ javac InstanceofUsage.java && java InstanceofUsage 
String : Foo Loves Bar
Foo : Bar
Bar : Null