What is a Generic Java Method

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When your entire class does not need to be generic, but one or more methods within the class do need to be generic, you can create a generic method. A generic method is a method that has generic parameter types and/or generic return types. For example:

// notice that the class is not generic
class Demo {
  // notice that this method is generic
  public static <E> void printArray(E[] inputArray){ ... }
}

In the example above, the printArray() method is a generic method that exists within a non-generic class. When using generic methods, you'll notice the diamond brackets with the type parameter before the return type. You'll also notice that the inputArray is of type E.

Rules for Defining a Generic Method

The following are the four rules for defining generic methods in Java.

Every generic method declaration includes a type parameter section enclosed by diamond brackets placed before the method's return type (as seen in the example below, where is used). Within each type parameter section, one or more type parameters are specified, separated by commas. These type parameters, also called type variables, serve as identifiers denoting generic type names. The type parameters play a dual role by declaring the return type and acting as placeholders for the types of arguments provided to the generic method, referred to as actual type arguments. The structure of a generic method's body resembles that of any other method. It's important to note that type parameters can exclusively represent reference types (aka object types) and not primitive types (such as int, double, and char).

Generic Method Signature

When defining a generic method, a bit of new syntax is used in the method signature. The generic parameter, for example, <E> below, must be specified before the method name.

Generic Method Example

The following example illustrates how you can

public class GenericMethodTest {
  // declare generic printArray() method
  public static < E > void printArray( E[] inputArray ) {
    for(E element : inputArray) {
      System.out.printf("%s ", element);
    }
    System.out.println();
}

  public static void main(String args[]) {
    /* Create arrays of Integer, 
        Double and Character */
    Integer[] intArray = { 1, 2, 3, 4, 5 };
    Double[] doubleArray = { 1.1, 2.2, 3.3, 4.4 };
    Character[] charArray = { 'H', 'E', 'L', 'L', 'O' };

    System.out.println("Array integerArray contains:");
    /* pass an Integer array */
    printArray(intArray);  
    
    System.out.println("Array doubleArray contains:");
    /* pass a Double array */
    printArray(doubleArray);   

    System.out.println("Array characterArray contains:");
    /* pass a Character array */
    printArray(charArray);   
  }
}

Summary: What is a Java Generic Method

• A generic method is a method that has generic parameter types and/or generic return types
• Generic methods are used when only itself and not the entire class need the generic type
• The generic parameter must be defined before the return type as well as in the parameter list