Using Generic Methods

This blog adds to my previous couple of blogs about using generics, by introducing Generic Methods. My other blogs dealt with sample code that parametrise entire classes, but it makes more sense to parametrise a single method rather than a whole class if possible.

To define a generic method, add a generic parameter, or list of generic parameters before the method’s return value:

  public <T> void printType(T x) {   // etc...

  public <A, B, C> void printThreeTypes(A x, B y, C z) {  // etc...

With class parametrisation, you need to specify the parameter type when you instantiate the class, but with generic methods, you can let the compiler figure it out, which I believe is called type argument inference.

The code below demonstrates Generic Method usage:

public class PrintType {

  public <T> void printType(T x) { // etc...

    System.out.println(x.getClass().getSimpleName());
  }

  /**
   * Demonstrate how to write a method with more than one parameter type.
   */
  public <A, B, C> void printThreeTypes(A x, B y, C z) { // etc...

    System.out.println("Type 1: " + x.getClass().getSimpleName() + " Type 2: "
        + y.getClass().getSimpleName() + " Type 3: " + z.getClass().getSimpleName());
  }

  public static void main(String[] args) {

    PrintType instance = new PrintType();

    instance.printType("Hello");
    // this will autobox
    instance.printType(1);
    // this will autobox
    instance.printType(1.0);
    instance.printType('c');
    instance.printType(instance);

    instance.printThreeTypes("Hello", 'C', instance);
  }
}

The output from this code is:

String
Integer
Double
Character
PrintType
Type 1: String Type 2: Character Type 3: PrintType