Question

I was trying to see how instanceof operator in Java works and am facing a very odd issue.

public static void main(String[] args) {
    Map m = new HashMap();
    System.out.println("m instanceof Date: " + (m instanceof Date));
}

The above returns false as expected. However,

public static void main(String[] args) {
    HashMap m = new HashMap();
    System.out.println("m instanceof Date: " + (m instanceof Date));
}

This does not even compile. I get an error

inconvertible types
found   : java.util.HashMap
required : java.util.Date

What am I missing here?
I am using IntelliJ Idea 11.

Answer 1

From the Java Language Specification 3.0, section 15.20.2:

If a cast of the RelationalExpression to the ReferenceType would be
rejected as a compile-time error, then the instanceof relational
expression likewise produces a compile-time error. In such a
situation, the result of the instanceof expression could never be
true.

Since you can’t compile a cast from a HashMap to a Date, you can’t compile an instanceof test between the two either.