Generics Module 2Generics ModuleGenerics Module 2

Generics Module 2Generics Module 2

1. Generics

2. Contents
•Introduction
•Benefits of Generics
•Generic Classes and Interfaces
•Generic Methods
•Wildcard Generic Types
•Restrictions on Generics

3. Introduction
•Enables to create classes, interfaces, and methods in which the type of data upon
which they operate is specified as a parameter.
•Introduced in Java by jdk1.5.
•Generics means parameterized types.
•Generics add the type-safety.
•Generics add stability to your code by making more of your bugs detectable at
compile time.

4. Why Generics?
•The functionality of Gen class can be achieved without generics by specifying
Object type and using proper casting whenever required.
• Then why we use Generics?
•Java compiler does not have knowledge about the type of data actually stored in
NonGen. So:
•Explicit casts must be employed to retrieve the stored data.
•Several type mismatch errors cannot be found until run time.

5. Why Generics?
•Stronger type checks at compile time
•Elimination of casts
ArrayListlist = new ArrayList();
list.add("hello");
String s = (String) list.get(0);
•Using generics:
List<String> list = new ArrayList<String>();
list.add("hello");
String s = list.get(0); // no cast
•Enabling programmers to implement generic algorithms.
We can implement generic algorithms that work on collections of different types, can
be customized, and are type safe and easier to read.

6. Example

7. Generics Work Only with Objects
•When declaring an instance of a generic type, the type argument passed to the
type parameter must be a class type.
• Gen<int> strOb= new Gen<int>(53);
•The above declaration is an error.
•A reference of one specific version of a generic type is not type compatible with
another version of the same generic type.
• iOb= strOb; // Wrong!

8. Generic Class

9. General Form of Generic Class
•The generics syntax for declaring a generic class:
class class-name<type-param-list>
{ // ... }
•The syntax for declaring a reference to a generic class:
class-name<type-arg-list> var-name =
new class-name<type-arg-list>(cons-arg-list);

10. Generic Class with Multiple Type
Parameters

11. Generic Class with Multiple Type
Parameters

12. Problem
•Create a generic class that contains a method that returns the average of an array
of numbers of any type, including integers, floats, and doubles.

13. Possible Solution

14. Why Error?
•The compiler has no way to know that you are intending to create Gen class
objects using only numeric types.
•When we try to compile Gen class, an error is reported that indicates that the
doubleValue( ) method is unknown.
•We need some way to tell the compiler that we intend to pass only numeric types
to T.

15. Bounded Types
•Used to limit the types that can be passed to a type parameter.
•When specifying a type parameter, we can create an upper bound that declares
the super-class from which all type arguments must be derived.
<T extends superclass>
•A bound can include both a class type and one or more interfaces.
class Gen<T extends MyClass& MyInterface>
•Any type argument passed to T must be a subclass of MyClassand implement
MyInterface.

16. Solution

17. WILD CARD Generics

18. Example (comparing object values)
method compare(Stats<Integer>) in the type
Gen<Integer> is not applicable for the arguments
(Gen<Double>)

19. Gen<Integer> obj1
Memory representation of
objects
Gen<Double> obj2
Object of Gen<Integer>
required

20. wildcard generic (i.e. it accepts any type)
Solution

21. Problem
•Create a generic class Stats that contains a method sameAvg( ) that determines
whether two different Stats class objects contain arrays that yield the same
average or not; no matter what type of numeric data each object holds.
•For example, if one object contains the double values 1.0, 2.0, and 3.0, and the
other object contains the integer values 2, 1, and 3, then the averages will be the
same.

22. Possible Solution
method same_Avg(Stats<Integer>) in the type Stats<Integer> is not
applicable for the arguments (Stats<Double>)

23. Why Error?
•It will work only with the objects of same type.
•if the invoking object is of type Stats<Integer>, then the parameter obmust also be
of type Stats<Integer>.

24. Wildcard Argument
•The wildcard simply matches the validity of object.
•The wildcard argument is specified by the ?, and it represents an unknown type.
booleansame_Avg(Stats<?> ob)
{
if(average() == ob.average())
return true;
return false;
}
•Important: It is important to understand that the wildcard does not affect what type of
Stats class objects can be created. This is governed by the extends clause in the Stats
class declaration. The wildcard simply matches any valid Stats class object.

25. Bounded Wildcards
•A bounded wildcard is especially important when you are creating a generic type
that will operate on a class hierarchy.
// Two-dimensional coordinates.
class TwoD{
int x, y;
TwoD(int a, int b) {
x = a;
y = b;
}
}

26. Bounded Wildcards
// Three-dimensional coordinates.
class ThreeDextends TwoD{
int z;
ThreeD(int a, int b, int c) {
super(a, b);
z = c; }
}
// Four-dimensional coordinates.
class FourDextends ThreeD{
int t;
FourD(int a, int b, int c, int d) {
super(a, b, c);
t = d; }
}

27. Bounded Wildcards
// This class holds an array of coordinate objects.
class Coords<T extends TwoD>
{
T[] coords;
Coords(T[] o) { coords = o; }
}
Notice that Coords specifies a type parameter bounded by TwoD.
static void showXY(Coords<?> c)
{
System.out.println("X Y Coordinates:");
for(int i=0; i< c.coords.length; i++)
System.out.println(c.coords[i].x + " " + c.coords[i].y);
System.out.println();
}

28. Bounded Wildcards
Problem Starts if I want to create a methods showXYZ() for ThreeDand FourDObject.
static void showXYZ(Coords<? extends ThreeD> c)
{
System.out.println("X Y Z Coordinates:");
for(int i=0; i< c.coords.length; i++)
System.out.println(c.coords[i].x + " " + c.coords[i].y + " " + c.coords[i].z);
System.out.println();
}

29. Generic Method

30. Generic Method
•It is possible to declare a generic method that uses one or more type parameters.
•Methods inside a generic class are automatically generic relative to the type
parameters.
•It is possible to create a generic method that is enclosed within a non-generic
class.

31. Generic Method
•The type parameters are declared before the return type of the method.
•Generic methods can be either static or non-static.
<type-param-list> ret-type method-name(param-list) {…}
•Example:
static <T, V extends T> booleanisIn(T x, V[] y)
•This ability to enforce type safety is one of the most important advantages of
generic methods.

32. Example

34. Generic Interfaces

35. Generic Interfaces
•Generic interfaces are specified just like generic classes.

36. Generic Interfaces
•Generic interfaces are specified just like generic classes.
•Example:
interface MinMax<T extends Comparable<T>>
{ T min(); T max(); }
•The implementing class must specify the same bound.
•Once the bound has been established, it need not to be specified again in the
implements clause.

37. Generic Interfaces
class MyClass<T extends Comparable<T>> implements MinMax<T extends
Comparable<T>> { //Wrong
•In general, if a class implements a generic interface, then that class must also be
generic, at least to the extent that it takes a type parameter that is passed to the
interface. For example, the following attempt to declare MyClassis in error:
class MyClassimplements MinMax<T> { // Wrong!
•Because MyClassdoes not declare a type parameter, there is no way to pass one to
MinMax. In this case, the identifier T is simply unknown, and the compiler reports an
error. Of course, if a class implements a specific type of generic interface, such as
shown here:
class MyClassimplements MinMax<Integer> { // OK

38. Generic Interfaces
interface MinMax<T extendsComparable<T>>
{
T min();
T max();
}
class Myclass<T extendsComparable<T>> implementsMinMax<T>
{
…
}

39. Example

40. Generic Constructors

42. Generic Constructors
It is possible for constructors to be generic, even if their class is not.
Example:
class GenCons
{
private double val;
<T extendsNumber> GenCons(T arg)
{
val= arg.doubleValue();
}
void show_val() { System.out.println("val: " + val);
}
}

43. Erasure
// Here, T is bound by Object by default.
class Gen<T> {
T ob; // here, T will be replaced by Object
Gen(T o) { ob = o; }
// Return ob.
T getob() { return ob; }
}
// Here, T is bound by String.
class GenStr<T extends String> {
T str; // here, T will be replaced by String
GenStr(T o) { str = o; }
T getstr() { return str; }
}

44. erasureclass Gen extends java.lang.Object{
java.lang.Object ob;
Gen(java.lang.Object);
java.lang.Object getob();
}
class GenStr extends java.lang.Object{
java.lang.String str;
GenStr(java.lang.String);
java.lang.String getstr();
}

45. Some Generic Restrictions
Type Parameters Can’t Be Instantiated
// Can't create an instance of T.
class Gen<T>
{
T ob;
Gen()
{
ob= new T(); // Illegal!!!
}
}

46. Some Generic Restrictions
Restrictions on Static Members
class Wrong<T>
{
// Wrong, no static variables of type T.
static T ob;
// Wrong, no static method can use T.
static T getob() {
return ob;
}

47. Some Generic Restrictions
•Generic Array Restrictions
•// vals = new T[10]; // can't create an array of T
•// Gen<Integer> gens[] = new Gen<Integer>[10]; // Wrong!
•it’s valid to declare a reference to an array of type.
•cannot create generic exception classes.
•Cannot Instantiate Generic Types with Primitive Types
•Pair<int, char> p = new Pair<>(8, 'a');
•Cannot Create, Catch, or Throw Objects of Parameterized Types