GENERICS - Introduction to Java Programming: Comprehensive Version - page 740

Casting is not needed to retrieve a value from a list with a specified element type, because
the compiler already knows the element type. For example, the following statements create a
list that contains strings, add strings to the list, and retrieve strings from the list.

no casting needed

1 ArrayList<String> list = new ArrayList<>();
2 list.add( "Red" );
3 list.add( "White" );
4 String s = list.get( 0 ); // No casting is needed

Prior to JDK 1.5, without using generics, you would have had to cast the return value to
String as:

String s = (String)(list.get( 0 )); // Casting needed prior to JDK 1.5

If the elements are of wrapper types, such as Integer , Double , and Character , you can
directly assign an element to a primitive type variable. This is called autounboxing , as intro-
duced in Section 10.8. For example, see the following code:

autounboxing

1 ArrayList<Double> list = new ArrayList<>();
2 list.add( 5.5 ); // 5.5 is automatically converted to new Double(5.5)
3 list.add( 3.0 ); // 3.0 is automatically converted to new Double(3.0)
4 Double doubleObject = list.get( 0 ); // No casting is needed
5 double d = list.get( 1 ); // Automatically converted to double

In lines 2 and 3, 5.5 and 3.0 are automatically converted into Double objects and added to
list . In line 4, the first element in list is assigned to a Double variable. No casting is nec-
essary, because list is declared for Double objects. In line 5, the second element in list is
assigned to a double variable. The object in list.get(1) is automatically converted into
a primitive type value.

19.1

✓ ✓

Are there any compile errors in (a) and (b)?

Check

Point

ArrayList dates = new ArrayList();
dates.add( new Date());
dates.add( new String());

ArrayList<Date> dates =
new ArrayList<>();
dates.add( new Date());
dates.add( new String());

(a) Prior to JDK 1.5 (b) Since JDK 1.5

19.2

What is wrong in (a)? Is the code in (b) correct?

ArrayList dates = new ArrayList();
dates.add( new Date());
Date date = dates.get( 0 );

ArrayList<Date> dates =
new ArrayList<>();
dates.add( new Date());
Date date = dates.get( 0 );

(a) Prior to JDK 1.5 (b) Since JDK 1.5

19.3

What are the benefits of using generic types?

19.3 Defining Generic Classes and Interfaces
A generic type can be defined for a class or interface. A concrete type must be
specified when using the class to create an object or using the class or interface to
declare a reference variable.

Key
Point