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What are lower-bounded wildcard w.r.t Generics method in Java?
Generics is a concept in Java where you can enable a class, interface and, method, accept all (reference) types as parameters. In other words it is the concept which enables the users to choose the reference type that a method, constructor of a class accepts, dynamically. By defining a class as generic you are making it type-safe i.e. it can act up on any datatype.
To define a generic class you need to specify the type parameter you are using in the angle brackets “<>” after the class name and you can treat this as datatype of the instance variable an proceed with the code.
Example
class Student<T>{
T age;
Student(T age){
this.age = age;
}
public void display() {
System.out.println("Value: "+this.age);
}
}
public class GenericsExample {
public static void main(String args[]) {
Student<Float> std1 = new Student<Float>(25.5f);
std1.display();
Student<String> std2 = new Student<String>("25");
std2.display();
Student<Integer> std3 = new Student<Integer>(25);
std3.display();
}
}Output
Value: 25.5 Value: 25 Value: 25
Wildcards
Instead of the typed parameter in generics (T) you can also use “?”, representing an unknown type. You can use a wild card as a −
- Type of parameter.
- Field.
- Local field.
The only restriction on wilds cards is that you cannot it as a type argument of a generic method while invoking it.
Java provides 3 types of wild cards namely upper-bounded, lower-bounded, un-bounded.
Lower-Bounded wildcards
Upper-bounded wildcard enables the usage of all the subtypes of a particular class as a typed parameter.
Similarly, if we use the lower-bounded wildcards you can restrict the type of the “?” to a particular type or a super type of it.
For example, if want to accept a Collection object as a parameter of a method with the typed parameter as a super class of the Integer class, you just need to declare a wildcard with the Integer class as lower bound.
To create/declare a lower-bounded wildcard, you just need to specify the super keyword after the “?” followed by the class name.
Example
Following Java example demonstrates the creation of the Lower-bounded wildcard.
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.Iterator;
public class LowerBoundExample {
public static void sampleMethod(Collection<? super Integer> col){
Iterator it = col.iterator();
while (it.hasNext()) {
System.out.print(it.next()+" ");
}
System.out.println("");
}
public static void main(String args[]) {
ArrayList<Integer> col1 = new ArrayList<Integer>();
col1.add(24);
col1.add(56);
col1.add(89);
col1.add(75);
col1.add(36);
sampleMethod(col1);
List<Object> col2 = Arrays.asList(22.1f, 3.32f, 51.4f, 82.7f, 95.4f, 625.f);
sampleMethod(col2);
}
}Output
24 56 89 75 36 22.1 3.32 51.4 82.7 95.4 625.0
If you pass a collection object other of type other than Integer and its super type as a parameter to the sampleMethod() of the above program a compile time error will be generated.
Example
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.Iterator;
import java.util.HashSet;
public class LowerBoundExample {
public static void sampleMethod(Collection<? super Integer> col){
Iterator it = col.iterator();
while (it.hasNext()) {
System.out.print(it.next()+" ");
}
System.out.println("");
}
public static void main(String args[]) {
ArrayList<Integer> col1 = new ArrayList<Integer>();
col1.add(24);
col1.add(56);
col1.add(89);
col1.add(75);
col1.add(36);
sampleMethod(col1);
List<Object> col2 = Arrays.asList(22.1f, 3.32f, 51.4f, 82.7f, 95.4f, 625.f);
sampleMethod(col2);
HashSet<Double> col3 = new HashSet<Double>();
col3.add(25.225d);
col3.add(554.32d);
col3.add(2254.22d);
col3.add(445.21d);
sampleMethod(col3);
}
}Compile time error
LowerBoundExample.java:34: error: incompatible types: HashSet<Double> cannot be converted to Collection<? super Integer> sampleMethod(col3); ^ Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output 1 error
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