Java - Generics
Java Generic methods and generic classes enable programmers to specify, with a
single method declaration, a set of related methods or, with a single class
declaration, a set of related types, respectively.
Generics also provide compile-time type safety that allows
programmers to catch invalid types at compile time.
Using Java Generic concept, we might write a generic method for
sorting an array of objects, then invoke the generic method with Integer
arrays, Double arrays, String arrays and so on, to sort the array elements.
Following are the rules to define Generic Methods:
·
All generic method declarations have a type parameter section
delimited by angle brackets (< and >) that precedes the method's return
type ( < E > in the next example).
·
Each type parameter section contains one or more type parameters
separated by commas. A type parameter, also known as a type variable, is an
identifier that specifies a generic type name.
·
The type parameters can be used to declare the return type and act
as placeholders for the types of the arguments passed to the generic method,
which are known as actual type arguments.
·
A generic method's body is declared like that of any other method.
Note that type parameters can represent only reference types, not primitive
types (like int, double and char).
Example:
Following example illustrates how we can print array of different
type using a single Generic method:
public class GenericMethodTest
{
// generic method printArray
public static < E > void printArray( E[] inputArray )
{
// Display array elements
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:" );
printArray( intArray ); // pass an Integer array
System.out.println( "\nArray doubleArray contains:" );
printArray( doubleArray ); // pass a Double array
System.out.println( "\nArray characterArray contains:" );
printArray( charArray ); // pass a Character array
}
}
This would produce the following result:
Array integerArray contains:
1 2 3 4 5 6
Array doubleArray contains:
1.1 2.2 3.3 4.4
Array characterArray contains:
H E L L O
Bounded Type Parameters:
There may be times when you'll want to restrict the kinds of types
that are allowed to be passed to a type parameter. For example, a method that
operates on numbers might only want to accept instances of Number or its
subclasses. This is what bounded type parameters are for.
To declare a bounded type parameter, list the type parameter's
name, followed by the extends keyword, followed by its upper bound.
Example:
Following example illustrates how extends is used in a general
sense to mean either "extends" (as in classes) or
"implements" (as in interfaces). This example is Generic method to
return the largest of three Comparable objects:
public class MaximumTest
{
// determines the largest of three Comparable objects
public static <T extends Comparable<T>> T maximum(T x, T y, T z)
{
T max = x; // assume x is initially the largest
if ( y.compareTo( max ) > 0 ){
max = y; // y is the largest so far
}
if ( z.compareTo( max ) > 0 ){
max = z; // z is the largest now
}
return max; // returns the largest object
}
public static void main( String args[] )
{
System.out.printf( "Max of %d, %d and %d is %d\n\n",
3, 4, 5, maximum( 3, 4, 5 ) );
System.out.printf( "Maxm of %.1f,%.1f and %.1f is %.1f\n\n",
6.6, 8.8, 7.7, maximum( 6.6, 8.8, 7.7 ) );
System.out.printf( "Max of %s, %s and %s is %s\n","pear",
"apple", "orange", maximum( "pear", "apple", "orange" ) );
}
}
This would produce the following result:
aximum of 3, 4 and 5 is 5
aximum of 6.6, 8.8 and 7.7 is 8.8
aximum of pear, apple and orange is pear
Generic Classes:
A generic class declaration looks like a non-generic class
declaration, except that the class name is followed by a type parameter
section.
As with generic methods, the type parameter section of a generic
class can have one or more type parameters separated by commas. These classes
are known as parameterized classes or parameterized types because they accept
one or more parameters.
Example:
Following example illustrates how we can define a generic class:
public class Box<T> {
private T t;
public void add(T t) {
this.t = t;
}
public T get() {
return t;
}
public static void main(String[] args) {
Box<Integer> integerBox = new Box<Integer>();
Box<String> stringBox = new Box<String>();
integerBox.add(new Integer(10));
stringBox.add(new String("Hello World"));
System.out.printf("Integer Value :%d\n\n", integerBox.get());
System.out.printf("String Value :%s\n", stringBox.get());
}
}
This would produce the following result:
Integer Value :10
String Value :Hello World
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