No instance(s) of type variable(s) t exist so that void conforms to t

In Java, when we see an error message like “no instance(s) of type variable(s) t exist so that void conforms to t,” it means that we have a generic method or class with a type parameter (often denoted as ‘T’), and we are trying to use it in a way that violates its type constraints.

Let’s consider an example to understand this error better:

            public class ExampleClass {
                public static  void printType(T value) {
                    System.out.println(value.getClass().getName());
                }

                public static void main(String[] args) {
                    printType(10);  // Error: no instance(s) of type variable(s) T exist so that void conforms to T
                }
            }
        

In the above example, we have a generic method called “printType” that prints the type of the input parameter. However, when we try to call this method with an argument of type “int” (10), we get the error message.

The reason for the error is that the Java compiler cannot infer the type “T” based on the argument “10” because “10” is not an instance of a generic type. In this case, the type parameter “T” remains unspecified, and since the return type of the method is void, it cannot conform to any unspecified type.

To fix this error, we need to provide explicit type information to the generic method. We can do this by specifying the type when calling the method, like this:

            printType(10);  // Error
            printType(10.0);  // Error
            ExampleClass.printType(10);  // Correct: Specify type as Integer
        

By specifying the type explicitly, we inform the compiler about the expected type parameter, and the error is resolved.

Alternatively, we can define the type parameter at the class level, which would allow us to omit the type specification when calling the method:

            public class ExampleClass {
                public void printType(T value) {
                    System.out.println(value.getClass().getName());
                }
            }

            ExampleClass example = new ExampleClass<>();
            example.printType(10);  // Correct: Type parameter is defined at the class level