Bounded Types in java

Question

I am unable to understand why this code is not compiling...

public class Room {
    public static void main(String[] args) {
        Double[] ar = {1.1,2.23,3.56,4.23,5.90,6.88,7.99,8.09,9.88,10.99};
        Average<Double> tt = new Average<>(ar);
        tt.summ();
    }
}

class Average<T extends Double> {
    T[] arr;
    T sum;
    Average(T[] num) {
        arr = num;
    }

    void summ() {
        for (T i : arr) {
            sum = sum + i;//Shows incompatible types
            System.out.println(sum);
        }
    }
}

The compiler error states:

Error on Room.java, line 18:

Type mismatch: cannot convert from double to T

Can somebody please explain why this code is not compiling??

*String args[]* Please write the brackets on the type, not the variable, since it belongs to the type. Furthermore, please include the compilation error. — Turing85, Jul 04 '15 at 12:29
@Turing85 - No thats not the solution...still not compiling. — Frosted Cupcake, Jul 04 '15 at 12:31
@RajMalhotra I know that it is not the solution. It is just a comment on your coding style. To solve your problem, we need the actual compilation error. — Turing85, Jul 04 '15 at 12:32
possible duplicate of [Can I do arithmetic operations on the Number baseclass?](http://stackoverflow.com/questions/3873215/can-i-do-arithmetic-operations-on-the-number-baseclass) — Andy Brown, Jul 04 '15 at 12:33

score 5 · Answer 1 · answered Jul 04 '15 at 12:41

Eran is right; java.lang.Double is final, so it makes no sense to have a type parameter T extends Double. The only possible type that satiesfies this is Double itself, so you can just as well remove the type parameter and just use Double directly (or better yet: the primitive type double).

The reason why your code doesn't compile is because you are trying to use the + operator on objects of type T.

The Java compiler is not so smart that it notices that T can only be Double, so that it automatically can convert (by auto-unboxing and -boxing) the value to a double to do the calculation.

Maybe you come from a C++ background and you've used C++ templates. Java generics do not work in the same way as C++ templates; generic types in Java are not templates from which code is generated.

"*so that it automatically can convert (by auto-unboxing and -boxing) the value to a double to do the calculation.*" - **this** is the actual answer to the actual question — Andy Brown, Jul 04 '15 at 12:46

score 3 · Answer 2 · answered Jul 04 '15 at 12:33

3

There is no point in this generic type parameter having this bound :

class Average<T extends Double>

since Double is final and can have no sub-types.

Therefore you might as well remove the generic type parameter T and replace T with Double everywhere.

answered Jul 04 '15 at 12:33

Eran

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In this case, OP might want to substitute `T` with `double` instead of `Double`, which gets rid of the compilation error. – Turing85 Jul 04 '15 at 12:36
1

@Eran,but why the code is not compiling?..Even if I write,then also its showing compilation error – Frosted Cupcake Jul 04 '15 at 12:37
@Codebender The OP's code gives a compilation error - `Type mismatch: cannot convert from double to T` on the line `sum=sum+i;` – Eran Jul 04 '15 at 12:37
if we could teach java compiler which type to use during compilation then it could work – Roman C Jul 04 '15 at 12:37
3

@RajMalhotra read the "possible duplicate of..." comment given by Andy Brown. This should answer your question. – Turing85 Jul 04 '15 at 12:39

score 0 · Answer 3 · answered Jul 04 '15 at 12:37

Since I don't know what exactly your code should archive, I am not sure about the correctness of this answer, but a quick fix (at least to avoid syntax error) is to substitute the line:

T sum;

with:

double sum;

With this, your output is the incremental sum of all the double, one by one.

Roman C · Answer 4 · 2015-07-04T13:00:59.803

0

+ is arithmetic operator to use with numbers or strings, but it cant be used with Object type. If you want to use + operator both operand should have the same type.

class Average<T extends Double> {
  T[] arr;
  double sum = 0;
  Average(T[] num) {
    arr = num;
  }

  void summ() {
    for (T i : arr) {
      sum += (Double)i;//Shows incompatible types
      System.out.println(sum);
    }
  }
}

edited Jul 04 '15 at 13:00

answered Jul 04 '15 at 12:42

Roman C

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Bounded Types in java

4 Answers4