Generics

GENERICS IN JAVA
Shahjahan Samoon

What are Generics?
• In any nontrivial software project, bugs are simply a fact of life.
• Careful planning, programming, and testing can help reduce their pervasiveness, but
somehow, somewhere, they'll always find a way to creep into your code.
• This becomes especially apparent as new features are introduced and your code base
grows in size and complexity.

What are Generics?
• Fortunately, some bugs are easier to detect than others.
Compile-time bugs, for example, can be detected early on; you
can use the compiler's error messages to figure out what the
problem is and fix it, right then and there.
• Runtime bugs, however, can be much more problematic; they
don't always surface immediately.
• Generics add stability to your code by making more of your bugs
detectable at compile time.

What are Generics?
• In a nutshell, generics enable types (classes and interfaces) to be parameters when
defining classes, interfaces and methods.
• Much like the more familiar formal parameters used in method declarations, type
parameters provide a way for you to re-use the same code with different inputs.
• The difference is that the inputs to formal parameters are values, while the inputs to
type parameters are types.

Why Generics?
1. Elimination of casts.
The following code snippet without generics requires
casting:
List list = new ArrayList();
list.add("hello");
String s = (String) list.get(0);
When re-written to use generics, the code does not
require casting:
List<String> list = new ArrayList<String>();

list.add("hello");
String s = list.get(0); // no cast

Why Generics?
2. Enabling programmers to implement generic algorithms.
By using generics, programmers can implement generic algorithms that
work on collections of different types, can be customized, and are type
safe and easier to read.
3. Stronger type checks at compile time.
A Java compiler applies strong type checking to generic code and issues
errors if the code violates type safety. Fixing compile-time errors is easier
than fixing runtime errors, which can be difficult to find.

Generic Types:
• A generic type is a generic class or interface that is parameterized over
types.
• The following Box class will be modified to demonstrate the concept.
public class Box {
private Object object;

}

public void set(Object object){
this.object = object;
}
public Object get() {
return object;
}

Generic Types:
• Since its methods accept or return an Object,
• you are free to pass in whatever you want, provided that it is not one of
the primitive types.
• There is no way to verify, at compile time, how the class is used.
• One part of the code may place an Integer in the box and expect to get
Integers out of it, while another part of the code may mistakenly pass in
a String, resulting in a runtime error.

Generic Types:
• A generic class is defined with the following format:
• class name<T1, T2, ..., Tn> { /* ... */ }
• The type parameter section, delimited by angle brackets (<>), follows the
class name. It specifies the type parameters (also called type variables)
T1, T2, ..., and Tn.

A Generic Version of the Box Class
To update the Box class to use generics, you create a generic type declaration by changing the code
"public class Box" to "public class Box<T>". This introduces the type variable, T, that can be used
anywhere inside the class.

With this change, the Box class becomes:
public class Box<T> {
private T t;

}

public void set(T t) {
this.t = t;
}
public T get() {
return t;
}

A Generic Version of the Box Class

• All occurrences of Object are replaced by T.
• A type variable can be any non-primitive type you specify:
 class type
 interface type
 array type

Type Parameter Naming Conventions
• By convention, type parameter names are single, uppercase letters.
The most commonly used type parameter names are:
E - Element (used extensively by the Java Collections Framework)
K - Key
N - Number
T - Type
V - Value

Invoking and Instantiating a Generic Type
• To reference the generic Box class from within your code, you must
perform a generic type invocation, which replaces T with some concrete
value, such as Integer:
Box<Integer> integerBox;

• It simply declares that integerBox will hold a reference to a "Box of
Integer", which is how Box<Integer> is read.
• You can think of a generic type invocation as being similar to an ordinary
method invocation, but instead of passing an argument to a method, you
are passing a type argument — Integer in this case — to the Box class
itself.

Invoking and Instantiating a Generic Type
• To instantiate this class, use the new keyword, as usual, but place
<Integer> between the class name and the parenthesis:

Box<Integer> integerBox = new Box<Integer>();

Box<Integer> integerBox = new Box<>();

Type Parameter and Type Argument:
• Many developers use the terms "type parameter" and "type
argument" interchangeably, but these terms are not the same.
When coding, one provides type arguments in order to create a
parameterized type. Therefore, the T in Box<T> is a type
parameter and the String in Box<Integer> is a type argument. This
lesson observes this definition when using these terms.

Multiple Type Parameters
• As mentioned previously, a generic class can have multiple type
parameters. For example, the generic OrderedPair class, which
implements the generic Pair interface:

public interface Pair<K, V> {
public K getKey();
public V getValue();
}

public class OrderedPair<K, V> implements Pair<K, V> {
private K key;
private V value;
public OrderedPair(K key, V value) {
this.key = key;
this.value = value;
}
public K getKey() {
return key;
}

public V getValue() {
return value;
}
}

Multiple Type Parameters
The following statements create two instantiations of the OrderedPair class:
Pair<String, Integer> p1 = new OrderedPair<String, Integer>("Even", 8);
Pair<String, String> p2 = new OrderedPair<String, String>("hello", "world");
The code, new OrderedPair<String, Integer>, instantiates K as a String and V as an
Integer. Therefore, the parameter types of OrderedPair's constructor are String and
Integer, respectively. Due to autoboxing, is it valid to pass a String and an int to the class.
You can also substitute a type parameter (i.e., K or V) with a parameterized type (i.e.,
Box<Integer>). For example, using the OrderedPair<K, V> example:
OrderedPair<String, Box<Integer>> p = new OrderedPair<>(“numbers", new Box<Integer>());

Raw Types
• A raw type is the name of a generic class or interface without any type
arguments.
• To create a parameterized type of Box<T>, you supply an actual type
argument for the formal type parameter T:
Box<Integer> intBox = new Box<>();

• If the actual type argument is omitted, you create a raw type of Box<T>:
Box rawBox = new Box();

• Therefore, Box is the raw type of the generic type Box<T>. However, a
non-generic class or interface type is not a raw type.

Raw Types
• Raw types show up in legacy code because lots of API classes (such
as the Collections classes) were not generic prior to JDK 5.0.
• When using raw types, you essentially get pre-generics behavior —
a Box gives you Objects. For backward compatibility, assigning a
parameterized type to its raw type is allowed:
Box<String> stringBox = new Box<>();
Box rawBox = stringBox;

// OK

Raw Types
• But if you assign a raw type to a parameterized type, you get a
warning:
Box rawBox = new Box();
Box<Integer> intBox = rawBox;

// rawBox is a raw type of Box<T>
// warning: unchecked conversion

• You also get a warning if you use a raw type to invoke generic methods
defined in the corresponding generic type:
Box<String> stringBox = new Box<>();
Box rawBox = stringBox;
rawBox.set(8); // warning: unchecked invocation to set(T)

• The warning shows that raw types bypass generic type checks,
deferring the catch of unsafe code to runtime. Therefore, you should
avoid using raw types.

Unchecked Error Messages
• As mentioned previously, when mixing legacy code with generic
code, you may encounter warning messages similar to the
following:
Note: Example.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

Generic Methods
• Generic methods are methods that introduce their own type
parameters.
• This is similar to declaring a generic type, but the type
parameter's scope is limited to the method where it is declared.
• Static and non-static generic methods are allowed, as well as
generic class constructors.

Generic Methods
• The syntax for a generic method includes a type parameter, inside
angle brackets, and appears before the method's return type. For
static generic methods, the type parameter section must appear
before the method's return type.
public static <T> void print(T t){
return t;

}

Bounded Type Parameters
• There may be times when you want to restrict the types that can
be used as type arguments in a parameterized type.
• 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, which in this example is Number.
• Note that, in this context, extends is used in a general sense to mean
either "extends" (as in classes) or "implements" (as in interfaces).

public <T extends Number> void inspect(T t){
System.out.println(“Type: " + t.getClass().getName());
}

• In addition to limiting the types you can use to instantiate a generic type,
bounded type parameters allow you to invoke methods defined in the bounds:
public class NaturalNumber<T extends Integer> {
private T n;
public NaturalNumber(T n) { this.n = n; }
public boolean isEven() {
return n.intValue() % 2 == 0;
}

}

• The isEven method invokes the intValue method defined in the Integer class
through n.

Refferences & Further Readings
• http://docs.oracle.com/javase/tutorial/java/generics/

Generics

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Generics