GENERICS - Introduction to Java Programming: Comprehensive Version - page 742

This example creates a stack to hold integers and adds three integers to the stack:

GenericStack<Integer> stack2 = new GenericStack<>();
stack2.push( 1 ); // autoboxing 1 to new Integer(1)
stack2.push( 2 );
stack2.push( 3 );

Instead of using a generic type, you could simply make the type element Object , which can
accommodate any object type. However, using generic types can improve software reliability
and readability, because certain errors can be detected at compile time rather than at runtime.
For example, because stack1 is declared GenericStack<String> , only strings can be
added to the stack. It would be a compile error if you attempted to add an integer to stack1 .

benefits of using generic types

Caution
To create a stack of strings, you use new GenericStack<String>() or new
GenericStack<>() . This could mislead you into thinking that the constructor of
GenericStack should be defined as

generic class constructor

public GenericStack<E>()

This is wrong. It should be defined as

public GenericStack()

Note
Occasionally, a generic class may have more than one parameter. In this case, place
the parameters together inside the brackets, separated by commas—for example,
<E1, E2, E3> .

multiple generic parameters

Note
You can define a class or an interface as a subtype of a generic class or interface. For
example, the java.lang.String class is defined to implement the Comparable
interface in the Java API as follows:

inheritance with generics

public class String implements Comparable<String>

19.4

✓ ✓

What is the generic definition for java.lang.Comparable in the Java API?

Check

19.5

Point

Since you create an instance of ArrayList of strings using new
ArrayList<String>() , should the constructor in the ArrayList class be
defined as

public ArrayList<E>()

19.6

Can a generic class have multiple generic parameters?

19.7

How do you declare a generic type in a class?

19.4 Generic Methods
A generic type can be defined for a static method.

Key
Point

You can define generic interfaces (e.g., the Comparable interface in Figure 19.1b) and
classes (e.g., the GenericStack class in Listing 19.1). You can also use generic types to
define generic methods. For example, Listing 19.2 defines a generic method print (lines
10-14) to print an array of objects. Line 6 passes an array of integer objects to invoke the
generic print method. Line 7 invokes print with an array of strings.

generic method