How do you know a variable type in java?

Expanding on Martin's answer...

Martins Solution

a.getClass().getName()

Expanded Solution

If you want it to work with anything you can do this:

((Object) myVar).getClass().getName()
//OR
((Object) myInt).getClass().getSimpleName()

In case of a primitive type, it will be wrapped (Autoboxed) in a corresponding Object variant.

Example #1 (Regular)

private static String nameOf(Object o) {
    return o.getClass().getSimpleName();
}

Example #2 (Generics)

public static <T> String nameOf(T o) {
    return o.getClass().getSimpleName();
}

Additional Learning

Material on Java Types, Values and Variables: https://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html
Autoboxing and Unboxing: https://docs.oracle.com/javase/tutorial/java/data/autoboxing.html
Docs on Pattern Matching for instanceof: https://docs.oracle.com/en/java/javase/14/language/pattern-matching-instanceof-operator.html

a.getClass().getName()

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