CODEWITHSUNDEEP
javagenericsbounded-types
Frosted Cupcake
Frosted Cupcake

Reputation: 1970

Bounded Types in java

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??

Upvotes: 3

Views: 175

Answers (4)

Roman C
Roman C

Reputation: 1

+ 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);
    }
  }
}

Upvotes: 0

Jesper
Jesper

Reputation: 206766

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.

Upvotes: 5

Ruben Rizzi
Ruben Rizzi

Reputation: 352

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.

Upvotes: 0

Eran
Eran

Reputation: 393771

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.

Upvotes: 3

Related Questions