How to write a generic method that takes two arguments of the same types in java?

The reason that this compiles is because Java will infer the most specific supertype of the arguments passed in, in this case, Object Serializable & Comparable<? extends Serializable & Comparable<? extends Comparable<?>>>, after 1 is boxed to Integer and "1" is passed as a String.

Without generics:

private static void method(Number arg1, Number arg2) {

Even without generics, you can pass in an Integer and a Double.

Only if the type in question is final can you do this, without generics:

private static void method(String arg1, String arg2) {
    // Yes, they're both Strings, guaranteed.

There is one edge case with generics that I can think of to ensure that they are the exact type. If you have a final class, and you place an upper bound, then you can restrict it to that same class.

public <T extends MyFinalClass> void method(T arg1, T arg2) {
    // Yes, they're both MyFinalClasses
}

But then you could do the same thing without generics.

public void method(MyFinalClass arg1, MyFinalClass arg2) {
    // Yes, they're both MyFinalClasses
}

You could add the class as an additional parameter.

private static <T> void method(T arg1, T arg2, Class<T> type) {
    // ...
}

Now you have to specify the common type.

You can still call method(1, "1", Object.class); but at least you are explicit about the common type.