[7.19-7.23] Generics

1. Implementation

Specifics
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public interface IntegerOperation { 

    Integer performOperation(Integer value); 

}
public class AddOneOperation implements IntegerOperation { 

    @Override 

    public Integer performOperation(Integer value) { 

        return value+1; 

   } 

}

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Generics

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// work with a list of any type, and return a list of a different type

// E.g., turning a String parameter into an Integer

public interface GenericOperation<A, B> {

              B performOperation(A value);

}

public class StringLengthOperation implements GenericOperation<String, Integer>

{

         @Override

         public Integer performOperation (String value)

              return value.length();

}

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for (final Integer number: numbers){
res.add(op.performOperation(number));
}

// 7-19 Write a method that replicates a list of Itnegers, with 1 added to each element. 

public static List addOne(final List numbers)
{
       final List res = new ArrayList(numbers.size());
       for ( final Integer number: numbers)
       {
          res.add(number+1);
       }


       return res;

}

// To allow any Integer operation, you can define a new 
// interface, IntegerOperation, which has one
// method, taking an Integer, and returning an Integer

public interface IntegerOperation
{
      Integer performOperation(Integer value);
}

public class AddOneOperation implements IntegerOperation
{
      @Override 
      public Integer performOperation(Integer value)
      {
           return value+1;
      }
}



// 7-20  Performa any integer operation on a list

public static List updateList(final List numbers, final IntegerOperation op)
{
         final List res = new ArrayList<>(number.size());
         for (final Integer number: numbers)
         {
              res.add(op.performOperation(number));
         }

         
         return res;


}


// 7-21 Testing the updateList method

@ Test
public void positiveList()
{
      final List numbers = Arrays.asList(4,7,2,-2,8,-5,-7)
      final List expected = Arrays.asList(4,7,2,2,8,5,7);


      final List actual = Lists.updateList(nubmers, new IntegerOperation()
{
         @Override
         public Integer performOperation(Integer value)
               return Math.abs(value);
});


     assertEquals(expected, actual);
}

// 7-22 Mapping a list into a different type

public static  List mapList(final List values, final GenericOperation op )
{


       final List res = new ArrayList<>(values.size());
       for (final A a: values)
            res.add(op.performOperation(a));


       return res;


}


// 7-23 Testing the list mapping functionality

@Test
public void stringLengths()
{

      final List strings = Arrays.asList("acing", "the", "java", "interview" );
      final List expected = Arrays.asList(5,3,4,9);
      final List actual = List.mapList(strings, new StringLengthOepration()  );
      assertEquals(expected, actual);

}