Generics in Java | How to write parametrized class and method in Java | java Generics interview Question answer

Before to going start how write  parametrized class and method in Java . i would like to give some important fact about generic

1. java generic has introduced with JDK 1.5 along with Autoboxing, varargs, ,Enum
2. Generics  provide compile-time type safety that allows programmers to catch invalid types at compile time.
3. Not only can Java generics be used in fields and methods, it can also be used in class definitions to produce parameterized classes.


How to write  parametrized class

1. A generic class declaration looks like a non-generic class declaration, except that the class name is followed by a type parameter section.

2. As with generic methods, the type parameter section of a generic class can have one or more type parameters separated by commas.

public class Member<T> {
 private T id;
 public Member(T id) {
   this.id = id;
 }
 public T getId() {
   return id;
 }
 public void setId(T id) {
   this.id = id;
 }
 public static void main(String[] args) {
   Member<String> mString = new Member<String>("id1");
   mString.setId("id2");
   System.out.printf("id after setting id: %s%n", mString.getId());
   //output:  id after setting id: id2
 
   Member<Integer> mInteger = new Member<Integer>(1);
   mInteger.setId(2);
   System.out.printf("id after setting id: %d%n", mInteger.getId());
   //output:  id after setting id: 2
 }


How to write parametrized method

 1.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).

    2. 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.

   3.  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.

   4.  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).

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
    }
}