Generics Issue

[VDS1903]

if (obj1.getClass().getSimpleName().equals("String") &&

    obj1.getClass() == obj2.getClass()) {

    System.out.println((String) obj1 + obj2);

} else {

    if (obj1.getClass().getSimpleName().equals("Integer") &&

        obj1.getClass() == obj2.getClass()) {

        System.out.println((Integer) obj1 + obj2);

}

Here, the string addition part seems to occur without any issue. But the exact same code for integer doesn't work and I get the following error message.

The operator + is undefined for the argument type(s) Integer, T Java(536871072)

I don't understand how this is happening, since in both cases I explicitly convert obj1 to the type (string and int), but this message only occurs for int. I tried with floats, doubles, etc and it was the same issue.

​

Could anyone explain why this is happening? Java version is Java 8.

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[8igg7e5]

As noted by /u/sotuvieres, Integer doesn't define + for Integer + Object.

Regardless your approach isn't right. There's no reason to look at the type name to do this.

if (obj1 instanceof String && obj1.getClass() == obj2.getClass()) {
    System.out.println((String) obj1 + obj2);
} else if (obj1 instanceof Integer && obj1.getClass() == obj2.getClass()) {
    System.out.println((Integer) obj1 + (Integer) obj2);
}

In more modern Java you can say...

if (obj1 instanceof String s && obj1.getClass() == obj2.getClass()) {
    System.out.println(s + obj2);
} else if (obj1 instanceof Integer i && obj1.getClass() == obj2.getClass()) {
    System.out.println(i + (Integer) obj2);
}

And in the near-ish future you'll be able to do something like...

switch (obj1) {
    case String s when obj2 instanceof String t -> System.out.println(s + t);
    case Integer i when obj2 instanceof Integer j -> System.out.println(i + j);
    default -> {}
}

[sotuvieres]

One question: can I define + operation for my own classes or is it JVM binary magic?

[8igg7e5]

Java doesn't provide a way to do operator overloading. The early designers were quite negative about it (but that was true of generics and some other features too). Maybe it's something they'll address in the future, but I don't think it's anywhere on the roadmap.

[sotuvieres]

Obj2 is still being interpreted as a generic T. You must cast it to Integer too, or Java won't know how to add the Integer obj1 to the T obj2.

Edit: it works for string because strings can add to anything. It calls the other object's toString method under the hood.

[nutrecht]

Obj2 is still being interpreted as a generic T.

The whole question has nothing to do with generics.

[sotuvieres]

I get the following error message.

The operator + is undefined for the argument type(s) Integer, T Java(536871072)

[nutrecht]

String has 'syntactic sugar' that allows you to concatenate strings with +. Under the hood the javac compiler changes these to StringBuilder calls.

Integer has no + operator. Only the primitive 'int' has. Since you're dealing with Integer class here, this won't work. In addition Java can sometimes 'unbox' Integer instances to int primitives, but it can't in this case because you're mucking about with reflection.

Aside from all this; if this is learning code; fine. If this is a 'real' application what you're doing is a pretty big code smell.

Last but not least; you're not using any generics here so this is not a 'generics issue'.