How to check the type of an object

Short answer

instanceof with a variable name checks the type and casts the object right away:

void main() {
    Object value = "hello";
 
    if (value instanceof String text) {
        IO.println(text.length());
    }
}

Output:

Several types

A switch with patterns picks a branch by type:

String describe(Object value) {
    return switch (value) {
        case Integer i when i < 0 -> "negative int " + i;
        case Integer i -> "int " + i;
        case String s -> "string of length " + s.length();
        case null -> "null";
        default -> "something else: " + value.getClass().getSimpleName();
    };
}
 
void main() {
    IO.println(describe(42));
    IO.println(describe(-1));
    IO.println(describe("abc"));
    IO.println(describe(3.14));
    IO.println(describe(null));
}

Output:

int 42
negative int -1
string of length 3
something else: Double
null

The exact class

instanceof takes inheritance into account, while getClass() gives the exact class of the object:

void main() {
    Object number = 5;
 
    IO.println(number instanceof Number);
    IO.println(number.getClass() == Number.class);
    IO.println(number.getClass().getName());
}

Output:

true
false
java.lang.Integer

Common mistake

Casting without a check. If the object has a different type, you get an exception:

void main() {
    Object value = 42;
    String text = (String) value;
}

Output:

Exception in thread "main" java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap')

If you check types often, consider polymorphism: a common method in an interface that each class implements in its own way.

Thanks for reading!

Short answer

Several types

The exact class

Common mistake

Functions used in this recipe

Thanks for reading!