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Wildcards can be used in three ways −
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Upper bound Wildcard − ? extends Type.
-
Lower bound Wildcard − ? super Type.
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Unbounded Wildcard − ?
In order to decide which type of wildcard best suits the condition, let”s first classify the type of parameters passed to a method as in and out parameter.
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in variable − An in variable provides data to the code. For example, copy(src, dest). Here src acts as in variable being data to be copied.
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out variable − An out variable holds data updated by the code. For example, copy(src, dest). Here dest acts as in variable having copied data.
Guidelines for Wildcards.
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Upper bound wildcard − If a variable is of in category, use extends keyword with wildcard.
-
Lower bound wildcard − If a variable is of out category, use super keyword with wildcard.
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Unbounded wildcard − If a variable can be accessed using Object class method then use an unbound wildcard.
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No wildcard − If code is accessing variable in both in and out category then do not use wildcards.
Example
Following example illustrates the above mentioned concepts.
package com.tutorialspoint;
import java.util.ArrayList;
import java.util.List;
public class GenericsTester {
//Upper bound wildcard
//in category
public static void deleteCat(List<? extends Cat> catList, Cat cat) {
catList.remove(cat);
System.out.println("Cat Removed");
}
//Lower bound wildcard
//out category
public static void addCat(List<? super RedCat> catList) {
catList.add(new RedCat("Red Cat"));
System.out.println("Cat Added");
}
//Unbounded wildcard
//Using Object method toString()
public static void printAll(List<?> list) {
for (Object item : list)
System.out.println(item + " ");
}
public static void main(String[] args) {
List<Animal> animalList= new ArrayList<Animal>();
List<RedCat> redCatList= new ArrayList<RedCat>();
//add list of super class Animal of Cat class
addCat(animalList);
//add list of Cat class
addCat(redCatList);
addCat(redCatList);
//print all animals
printAll(animalList);
printAll(redCatList);
Cat cat = redCatList.get(0);
//delete cat
deleteCat(redCatList, cat);
printAll(redCatList);
}
}
class Animal {
String name;
Animal(String name) {
this.name = name;
}
public String toString() {
return name;
}
}
class Cat extends Animal {
Cat(String name) {
super(name);
}
}
class RedCat extends Cat {
RedCat(String name) {
super(name);
}
}
class Dog extends Animal {
Dog(String name) {
super(name);
}
}
This will produce the following result −
Cat Added Cat Added Cat Added Red Cat Red Cat Red Cat Cat Removed Red Cat
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