Check out example codes for "how to have a only number type in java". It will help you in understanding the concepts better.

Code Example 1

class scratch{
    public static Number haha(Number yo){
        return yo;
    }

    public static void main(String[] args) {
        System.out.println(haha( (int) 5 ));
        System.out.println(haha( (double) 5.0 ));
        System.out.println(haha( (long) 5 ));
        System.out.println(haha( (float) 5.0 ));
        //all of these classes extend the java.lang.Number class 

        System.out.println(haha("Hey"));
        //error because the java.lang.String class does not extend the java.lang.Number class
    }
}

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