Java Type Comparison

Original post: Flashreads Blog - Type Comparison

The most common way to determine if a given object is an instance of a given class, superclass or interface, is by using the binary operator instanceof. It includes implicit null check and generates a compile-time error if the types are not related. But it doesn't allow primitives and requires the types to be know at compile time.

If you want to check dynamically at runtime

Use the equivalent method boolean isInstance(Object obj) in Class. It also includes null check, but allows for primitives:

    a instanceof B 
    // returns false for null
    null instanceof B

    a.getClass().isInstance(b);
    // commonly used for generics
    Class<T> type = b.getClass();
    type.isInstance(a);

    // Note that the parameter is autoboxed to type Integer
    int x = 4;
    Integer.class.isInstance(x);

Check compatibility of two types

When you need to check subtyping relation use the method boolean isAssignableFrom(Class<?> cls) in Class. It may throw NullPointerException.

    // is it possible to B b = new A()
    Class<?> aClass = CharSequence.class;
    Class<?> bClass = String.class;
    bClass.isAssignableFrom(aClass());

    // works for arrays
    CharSequence[].class.isAssignableFrom(String[].class); // true
    Integer[].class.isAssignableFrom(String[].class); //false

Pattern Matching (Java 14)

    if(a instanceof B b) {
        // b is casted
        b.toString();
    }

Special types

    // Enums
    enum Color { WHITE, GRAY, BLACK }

    Color.class.isEnum(); // Enum.class.isAssignableFrom(Color.class); 
    Color.WHITE instanceof Enum; // true

    // Arrays
    String[].class.isArray();
    // get the type of the variables in an array (null if obj is not an array)
    Class<?> componentType = obj.getComponentType(); 

    // Primitives
    int.class.isPrimitive();