GENERICS - Introduction to Java Programming: Comprehensive Version - page 743

L ISTING 19.2

GenericMethodDemo.java
1 public class GenericMethodDemo {
2 public static void main(String[] args ) {
3 Integer[] integers = { 1 , 2 , 3 , 4 , 5 };
4 String[] strings = { "London" , "Paris" , "New York" , "Austin" };
5
6 GenericMethodDemo.<Integer>print(integers);
7 GenericMethodDemo.<String>print(strings);
8 }
9
10 public static <E> void print(E[] list) {
11 for ( int i = 0 ; i < list.length; i++)
12 System.out.print(list[i] + " " );
13 System.out.println();
14 }
15 }

generic method

To declare a generic method, you place the generic type <E> immediately after the keyword
static in the method header. For example,

declare a generic method

public static <E> void print(E[] list)

To invoke a generic method, prefix the method name with the actual type in angle brackets.
For example,

invoke generic method

GenericMethodDemo.<Integer>print(integers);
GenericMethodDemo.<String>print(strings);

or simply invoke it as follows:

print(integers);
print(strings);

In the latter case, the actual type is not explicitly specified. The compiler automatically dis-
covers the actual type.
A generic type can be specified as a subtype of another type. Such a generic type is
called bounded . For example, Listing 19.3 revises the equalArea method in Listing 13.4,
TestGeometricObject.java, to test whether two geometric objects have the same area. The
bounded generic type <E extends GeometricObject> (line 7) specifies that E is a generic
subtype of GeometricObject . You must invoke equalArea by passing two instances of
GeometricObject .

bounded generic type

L ISTING 19.3

BoundedTypeDemo.java
1 public class BoundedTypeDemo {
2 public static void main(String[] args ) {
3 Rectangle rectangle = new Rectangle( 2 , 2 );
4 Circle circle = new Circle( 2 );
5
6 System.out.println( "Same area? " +
7 equalArea(rectangle, circle));
8 }
9
10 public static <E extends GeometricObject> boolean equalArea(
11 E object1, E object2) {
12

Rectangle in Listing 13.3
Circle in Listing 13.2

bounded generic type

return object1.getArea() == object2.getArea();

13 }
14 }