Could not determine polymorphic type because input has type unknown

The error message “could not determine polymorphic type because input has type unknown” is typically encountered in statically-typed programming languages, such as Java or C++, when attempting to perform operations on a variable of type “unknown” or “unspecified”.

Polymorphism refers to the ability of an object to take on different forms or types. It allows variables to refer to objects of different classes at different times, enabling flexibility and code reuse. However, in order for polymorphism to work correctly, the type of the variable must be explicitly declared or known at compile-time.

When you receive the “unknown” type error, it suggests that the compiler or interpreter could not determine the specific class or type of the variable in question. This can happen if the variable has not been properly declared, if it is of a type that is not recognizable by the compiler, or if it has been initialized with an expression of an unknown type.

Here is an example to help illustrate this error:

    
      public class Main {
        public static void main(String[] args) {
          unknown variable = getValue();
          variable.doSomething(); // Error: could not determine polymorphic type
        }

        public static unknown getValue() {
          return new SomeClass();
        }
      }

      class SomeClass {
        public void doSomething() {
          System.out.println("Doing something.");
        }
      }
    
  

In this example, we have a method getValue() which returns an instance of SomeClass. However, the return type of getValue() is declared as “unknown” instead of the actual class name. When attempting to invoke the doSomething() method on the variable, the compiler cannot resolve the polymorphic type, leading to the error message.

To resolve this issue, you need to ensure that the types of variables are known and properly declared. You can either explicitly specify the type of the variable, or if the type is determined at runtime, use dynamic typing or casting to perform the necessary operations.

    
      public class Main {
        public static void main(String[] args) {
          SomeClass variable = getValue();
          variable.doSomething(); // Now there is no error
        }

        public static SomeClass getValue() {
          return new SomeClass();
        }
      }

      class SomeClass {
        public void doSomething() {
          System.out.println("Doing something.");
        }
      }
    
  

In this corrected example, the return type of the getValue() method is explicitly declared as SomeClass. The variable variable is then declared with the correct type. Now, there is no error when invoking the doSomething() method on the variable.