Test object type in java

Java instanceof Operator

Java instanceof operator (also called type comparison operator) is used to test whether the specified object is an instance of the specified type (class, subclass, or interface).

• true – if the variable is an instance of the specified class, its parent class or implements specified interface, or its parent interface
• false – if the variable is no instance of the class or implement the interface, or the variable is null.

HashMap map = new HashMap(); assertTrue( map instanceof Map ); assertTrue( map instanceof AbstractMap ); assertFalse( map instanceof List ); map = null; assertFalse( map instanceof Map );

1. Syntax of instanceof Operator

The instanceof operator tests variables to the specified type. Variable is written on the left-hand side of the operator, and type is given on the right side of the operator.

boolean value = variable instanceof ClassType; //or if(variable instanceof ClassType) < //perform some action >

2. The instanceof Operator does not need an Explicit null Check

In Java, null is a type so we do not need to check for NullPointerException when using the instanceof operator. The instanceof does it internally for us.

if(map != null && map instanceof Map) < //some action >

In the previous example, we do not need to make an explicit null check. The correct way to write the expression is:

3. Using instanceof with an Array

In Java, arrays are also considered objects and have associated fields and methods. So we can use instanceof operator with arrays as well.

Primitive arrays are instances of Object and self-type. e.g. int[] is type of Object and int[]. Both comparisons return true.

int[] intArr = new int[0]; Assertions.assertTrue(intArr instanceof Object); Assertions.assertTrue(intArr instanceof int[]);

Object arrays are types of Object, Object[], classtype array, and parent class type array. e.g. Integer[] is type of Object, Object[], Integer[] and Number[] ( Integer extends Number ).

Integer[] integerArr = new Integer[0]; Assertions.assertTrue(integerArr instanceof Object); Assertions.assertTrue(integerArr instanceof Object[]); Assertions.assertTrue(integerArr instanceof Integer[]); Assertions.assertTrue(integerArr instanceof Number[]);

A real-life example of using instanceof operator can be typecasting a variable to another type. The instanceof operator helps in avoiding ClassCastException in runtime.

Consider the following example where we are trying to typecast a list to LinkedList class, where the original variable is of type ArrayList. It will throw ClassCastException.

List list = new ArrayList<>(); LinkedList linkedList = (LinkedList) list; //ClassCastException

To correctly cast the variable, we can use instanceof operator. It will not result in ClassCastException.

List list = new ArrayList<>(); if(list instanceof LinkedList) < LinkedListlinkedList = (LinkedList) list; //other actions > else if(list instanceof ArrayList) < ArrayListarrayList = (ArrayList) list; //other actions >

Since Java 14, instanceof operator supports pattern matching. We can rewrite the above expression more concisely:

List list = new ArrayList<>(); if(list instanceof LinkedList linkedList) < //use linkedlist variable here >else if(list instanceof ArrayList arrayList) < //use arraylist variable here >

Drop me your questions related to the instanceof operator used for type comparison.

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How to Check Object Type in Java

You can check object type in Java by using the instanceof keyword. Determining object type is important if you’re processing a collection such as an array that contains more than one type of object. For example, you might have an array with string and integer representations of numbers. You would need to determine the object type in order to store a given array item as an int data type. To learn how to check object type, follow these four steps.

1. Open your text editor and type in the following Java statements:The program creates an array of type Object and stores even numbers as strings and odd numbers as integers (using the Integer wrapper class). The program then processes the array one item at a time in order to store each item as an int data type. The instanceof operator is used to determine if the array item is an Integer or a String . For strings, you must first narrow the Object to string (see line 8 in the source code) and then use the parseInt method of the Integer class (line 9). For integers, a narrowing must be performed on the Object data type to store the value as an int data type (line 13).
2. Save your file as CheckObjectType.java .
3. Open a command prompt and navigate to the directory containing your Java program. Then type in the command to compile the source and hit Enter .
4. You will now test your program. Type in the command to run the Java runtime launcher and then hit Enter . Notice that the output of the program verifies that the instanceof operator was used successfully to determine the object type of each array element and therefore permits a correct conversion of the data type to int .

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