Intro to generic methods

  • Consider this program that contains the overloaded method print( ): 

       public static void main(String[] args)
   {
       String[]  strings  = {"a", "b", "c"};
       Integer[] integers = {1, 2, 3, 4, 5};

       print( strings );  // Calls print(String[])
       print( integers ); // Calls print(Integer[])
   }

   public static void print(String[] list)
   {
       for ( int i = 0; i < list.length; i++ )
           System.out.println(list[i]);
   }

   public static void print(Integer[] list)
   {
       for ( int i = 0; i < list.length; i++ )
           System.out.println(list[i]);
   }

  • We can use generic (= parameterized) methods to avoid having to write 2 similar methods

DEMO: demo/06-generics/03-gen-method/Demo.java

Defining generic methods

  • Syntax to define a generic (parameterized) method: 

       public static <T1,T2,..> returnType methodName( params )
   {
       ...
   }

    You can use the type parameters T1, T2, ... to declare parameter variables, local variables and the return type of the method

  • Example of a generic (parameterized) method definition: 

       public static <T> void print( T[] list )
   {
       for ( int i = 0; i < list.length; i++ )
           System.out.println(list[i]);
   }

    The generic method will be made specific (with a data type) in the invocation.

What happens behind the scene when you define a generic method

  • When you write the following generic method: 

       public static <T> void print( T[] list )
   {
       for ( int i = 0; i < list.length; i++ )
           System.out.println(list[i]);
   }

    The Java compiler will replace every occurence of <T> by Object: 

       public static void print( Object[] list )
   {
       for ( int i = 0; i < list.length; i++ )
           System.out.println( list[i] );
   }

    and will remember that list[ ] is a parameterized class variable

  • When the print( ) method is used, the Java compiler will insert the appropriate casting operator (see next slide)

Invoking generic methods

  • Syntax to invoke a generic (parameterized) method: 

       ClassName.<T1,T2,..>methodName( arguments )

  • Examples of a generic (parameterized) method invocations: 

     String[]  strings  = {"a", "b", "c"};
 Integer[] integers = {1, 2, 3, 4, 5};

 Demo2.<String>print( strings );   // Inserts (String) cast operator
 Demo2.<Integer>print( integers ); // Inserts (Integer) cast operator

  • Alternate syntax: (because Java can imply the data type) 

     Demo3.print( strings );   
 Demo3.print( integers );

DEMO: demo/06-generics/03-gen-method/Demo2.java + Demo3.java

More on generic methods later

  • We will discuss sorting later in the course

  • The sorting algorithm will written as generic methods

    So we will see more examples of generic method later