如何为一个已经存在的类创建一个方法，以便它使用一个变量，从该变量中将其称为参数？

Question

Update: thank you, now I understand that it is impossible to do what is written below.

Let's say I want to create an add method for Integer and I would like it to work this way: x.add(y) , where x and y are Integers . Is there a possibility to do this? The only thing I can think of is to create a different class that extends Integer and add a method for it, but is there any other way?

Just to be clear, the reason I was questioning it in the first place is that I need to create a generic class that should be able to do, for example, addition both for Integer and BigInteger and I don't want to make if cases that check what class is T to do that. So I thought it would be useful if it was possible to do the above. Now it is still unclear for me how to do that. Sorry for such a confusing question.

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PS I'm new to Java and also not a native English speaker, so I understand that the question may be very silly or has already been answered, if that's the case, I'm sorry.

Answer 1

I have an unclear question. If you want to create an add method inside an existing Integer class, you can not create it because it is final. Instead, you can create an add method within your own class.

public int add (Integer a, Integer b) {
  return a + b;
}

Answer 2

UPDATED:

Just to be clear, the reason I was questioning it in the first place is that I need to create a generic class that should be able to do, for example, addition both for Integer and BigInteger and I don't want to make if cases that check what class is T to do that.

Why not use java.lang.Number as generic type, Integer and BigInteger are both subclasses of java.lang.Number .

No, you cannot add a method to Integer , nor can you create a class that extends Integer because Integer is final , final classes cannot be inherited in Java.

If you just want to add two Integer objects, just use + because there is auto boxing and unboxing in Java, check this out: https://docs.oracle.com/javase/specs/jls/se8/html/jls-5.html#jls-5.1.7 .

You can create util class to provide static method to work around this:

class Integers {
    public static Integer add(Integer i1, Integer i2) {
        return i1 + i2;
    }
}

By the way, you can add extension methods in Kotlin, where you can do things like 1.plus(2) to perform plus operations.