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Generics in Java:
We can make generic
class, method and constructor, by creating a generic data type which we can use
for all type of data (int, float, double, etc.). Generics are used for strict type checking introduced in jdk 1.5 version.
We cannot create a
reference of one generic object of particular type to another generic object of
other type.
Create a java file GenericsDemo.java inside dev21century.generics package
package
dev21century.generics;
class
Gen<T>
{
T
obj; //declaring object of type T
Gen(T
o)
{
obj = o;
}
T
getOb()
{
return obj;
}
void
showType()
{
System.out.println("Type
of T is " + obj.getClass().getName());
}
}
public class
GenericsDemo {
public static void
main(String[] args) {
Gen<Integer>
iob = new Gen<Integer>(80);
iob.showType();
int y =
iob.getOb();
System.out.println("value
: " + y);
Gen<String>
strOb = new Gen<String>("Generic Test");
strOb.showType();
String
str = strOb.getOb();
System.out.println("value
= " + str);
}
}
Bounds in generics:
In
below case, any class must be a subclass of Number.
Example:
Create a java file BoundsDemo.java
inside dev21century.generics package
package dev21century.generics;
class Stats<T extends Number>
{
T[] nums;
Stats(T[] o)
{
nums = o;
}
double average()
{
double sum = 0.0;
for(int i=0;i<nums.length;i++)
sum += nums[i].doubleValue();
return (sum/nums.length);
}
}
public class BoundsDemo {
public static void main(String args[])
{
Integer inums[] = {1,2,3,4,5};
Stats<Integer> tab = new Stats<Integer>(inums);
double y = tab.average();
System.out.println("Avegare is: " + y);
Double dnums[] = {1.1,1.2,1.3,1.4};
Stats<Double> dob = new Stats<Double>(dnums);
double w = dob.average();
System.out.println("double average
is: " + w);
}
}
Output:
Stats<String> sob =
new Stats<String>(inums);
Above
statement will give compiler error “Bound mismatch: The type String is not a valid substitute for the
bounded parameter <T extends Number> of the type Stats<T>” this
is because String class is not a child of Number.
Generic Arrays:
Example:
Create a java file GenArrays.java
inside dev21century.generics package
package
dev21century.generics;
class Gen<T extends Number>
{
T
obj;
T[]
vals;
Gen(T
o, T[] nums)
{
obj = o;
vals = nums;
}
}
public class GenArrays {
public static void main(String[] args) {
Integer
n[] = {1,2,3,4,5};
Gen<Integer>
iob = new Gen<Integer>(50, n);
/*can't create an
array of type specific
generic
reference, following statement is not allowed
*/
//Gen<Integer>
gen[] = new Gen<Integer>[10];
//instead,
following is allowed
Gen<?>
gens[] = new Gen<?>[10];
gens[0]
= new
Gen<Integer>(10,n);
Double[]
d2 = new
Double[]{1.2,2.3,4.1};
gens[1]
= new
Gen<Double>(2.4, d2);
System.out.println("Integer
values: ");
for(int i =0;i<gens[0].vals.length;i++)
{
System.out.print(gens[0].vals[i] + " ");
}
System.out.println();
System.out.println("Double
values: ");
for(int i =0;i<gens[1].vals.length;i++)
{
System.out.print(gens[1].vals[i] + " ");
}
System.out.println();
}
}
Output:
Local argument as
generic - constructor:
Example:
Create a java file GenConsDemo.java
inside dev21century.generics package
package
dev21century.generics;
class GenCons
{
private double val;
<T
extends Number> GenCons(T
arg)
{
val = arg.doubleValue();
}
void showVal()
{
System.out.println("val = " + val);
}
}
public class GenConsDemo {
public static void main(String[] args)
{
//GenCons test1 =
new GenCons('d');
//above line will
give Bounds mismatch error
GenCons
test = new GenCons(100);
GenCons
test2 = new GenCons(123.5);
test.showVal();test2.showVal();
}
}
Output:
Generic method
arguments – generic method to which we can supply any
type of arguments.
Example:
Create a java file GenConsDemo.java
inside dev21century.generics package
package
dev21century.generics;
public class GenMethDemo {
//determine if an
object is in an array
static <T, V extends T> boolean isIn(T x, V[] y)
{
for(int i=0;i<y.length;i++)
if(x.equals(y[i]))
return true;
return false;
}
static <T extends Number> double add(T x, T y)
{
double d = x.doubleValue()
+ y.doubleValue();
return d;
}
public static void main(String[] args)
{
//use isIn() method
on integer
Integer
nums[] = {1,2,3,4,5,6};
if(isIn(2,nums))
System.out.println("2 is in
nums");
if(!isIn(9,nums))
System.out.println("9 is not in
nums");
//using isIn method
on String
String
strs[] = {"one", "two", "three", "four"};
if(isIn("two", strs))
System.out.println("two is in
nums");
if(!isIn("seven", strs))
System.out.println("seven is not
in nums");
System.out.println("10 + 20 =
" +
add(10,20));
System.out.println("10.5d + 20.5d
= "
+ add(10.5d,20.6d));
}
}
Output:
Generic
& Non Generic relationships:
Example:
Create a java file GenHireDemo.java inside dev21century.generics package
package
dev21century.generics;
class NonGen
{
int num;
NonGen(int i)
{
num = i;
}
int getNum()
{
return num;
}
}
//a generic class
class GenClass<T> extends NonGen
{
T
ob;//declare object of
type T
//pass the
constructor a reference
//to an object of
type T
GenClass(T
o, int i)
{
super(i);
ob = o;
}
T
getOb()
{
return ob;
}
}
public class GenHireDemo {
public static void main(String args[])
{
//create a generic
object for String
GenClass<String>
w = new
GenClass<String>("Hello", 47);
System.out.println(w.getOb()+ " ");
System.out.println(w.getNum());
}
}
Output:
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