Slide 12.18: Generics (cont.): Generic functions

Slide 12.17: Generics
Slide 12.19: Generics (cont.)
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Generics (Cont.)

Generic Functions
We can also write generic functions that can be called with different types of arguments based on the type of arguments passed to generic method. The compiler handles each method.

GenericMethod.java (generic methods)

// To show the working of user-defined generic functions 
class GenericMethod {
  public static void main( String[ ] args ) { 
    // Calling generic method with Integer argument 
    genericDisplay( Integer.parseInt( args[0] ) ); 
    // Calling generic method with String argument 
    genericDisplay( args[1] ); 
    // Calling generic method with double argument 
    genericDisplay( Double.parseDouble( args[2] ) ); 
  } 
  // A generic method example
  static<T> void genericDisplay( T element ) {
    System.out.println( element.getClass( ).getName( ) + " = " + element );
  }
}

shell> java GenericMethod (integer) (string) (double)

Output:

Advantages of Generics
Programs that uses generics have many benefits over non-generic code:

Code reuse: We can write a method/class/interface once and use for any type we want.

Type safety: Generics make errors to appear compile time than at run time. (It’s always better to know problems in your code at compile time rather than making your code fail at run time.) Suppose you want to create an ArrayList that stores names of students, and if by mistake, programmer adds an Integer object instead of String, compiler will allow it. But, when we retrieve this data from ArrayList, it causes problems at run time. The compiler handles each method.

NoGenerics.java (no generics being used)

// To demonstrate that NOT using generics can cause runtime exceptions 
import java.util.*;
class NoGenerics {
  public static void main( String[ ] args ) { 
    // Creating an ArrayList without any type specified 
    ArrayList al = new ArrayList( ); 
    al.add( args[0] ); 
    al.add( args[1] ); 
    // Compiler allows the following command.
    al.add( Integer.parseInt( args[2] ) );

    String s1 = (String) al.get( 0 ); 
    String s2 = (String) al.get( 1 ); 
    // Causing a runtime exception 
    String s3 = (String) al.get( 2 ); 
  } 
}

shell> java NoGenerics (string) (string) (integer)

Output:

Slide 12.17: Generics
Slide 12.19: Generics (cont.)
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