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

When encountering the error “no instance(s) of type variable(s) r exist so that void conforms to r”, it means that the expected return type of a method or function is incompatible with the actual return type being used.

In the case of “void conforms to r”, it suggests that the expected return type (denoted by ‘r’) is not explicitly defined or is void, but the actual return type is void as well. However, void cannot be used to fulfill the requirement of ‘r’ since void represents the absence of a return type.

Here’s an illustrative example to demonstrate the issue:

    
      public <R> R performOperation() {
          // Method implementation without a return statement
      }

      public void exampleMethod() {
          String result = performOperation();
          // The above line would result in the mentioned error
      }
    
  

In the above example, we have a generic method performOperation that is expected to return a result of type ‘R’, but it is defined with a return type of void. When trying to use this method and assign its result to a String variable in exampleMethod, we encounter the error since void cannot be assigned to String.

To resolve this error, you need to ensure that the return type of the desired method matches the expected return type. In the given example, you can either modify the return type of performOperation to be String or change the expected type of result to void.