Generic wildcards
In generics, ? (question mark) represents wildcard, means unknown type. Wildcard can be applied in different situations as the parameter, field, or local variable, or even as a return type to a method. We can't use wildcards as type argument for a generic method invocation, a generic class instance creation, or a super type.
There can be three types of Generic wildcards
- Upper bound wildcards
- Unbounded wildcards
- Lower wild cards
Upper Bound Wildcards
We can reduce/relax the restrictions on a variable using upper bound wildcards. We use keyword extends like <? extends MyClass> to apply Upper bound wildcards.
Example:
'
Upper bounded wildcards
/**********************************************************************/
class GenRectangle<T>
{
T h;
T w;
public GenRectangle(T h, T w) {
super();
this.h = h;
this.w = w;
}
}
class Utilnew
{
public static <T> void getArea(GenRectangle<? extends Number> r)//upper bound wildcard
{
System.out.println("Area:" + r.h.floatValue() * r.w.floatValue());
}
}
public class WildcardDemo {
public static void main(String[] args) {
GenRectangle<Float> g=new GenRectangle(12.3f, 15.7f);
Utilnew.getArea(g);
}
}
Unbound Wildcards
When, we are not relaxing or applying any restriction over the type, then we can simply use unbounded wildcards.
Example:
Unbounded wildcards
/***********************************************************/
class GenRectangle<T>//generic type
{
T h,w;
public GenRectangle(T h, T w) {//generic
constructor
super();
this.h = h;
this.w = w;
}
}
class Utilnew
{//generic method
public static <T> void getDimensions(GenRectangle<?> r)//unbounded wildcard
{
System.out.println("Rectangle h :" + r.h+ "
Rectangle w :"+r.w);
}
}
public class WildcardDemo {
public static void main(String[] args) {
GenRectangle<Float> g=new GenRectangle(12.3f, 15.7f);
Utilnew.getDimensions(g);
}
}
Lower Bound Wildcards
Lower bounded wildcard restricts the unknown type to be a specific type or a super type of that type. We can use <? super MyClass> to apply lower bound wildcards. We can apply either upper bound on a wildcard or a lower bound. we can not apply both of them together.
Example:
Lower bounded wild cards
/*******************************************************************************/
class GenRectangle<T>//generic type
{
T h,w;
public GenRectangle(T h, T w) {//generic
constructor
super();
this.h = h;
this.w = w;
}
}
class Utilnew
{//generic method
public static <T> void getDimensions(GenRectangle<? super T> r)//unbounded wildcard
{
System.out.println("Rectangle h :" + r.h+ "
Rectangle w :"+r.w);
}
}
public class WildcardDemo {
public static void main(String[] args) {
GenRectangle<Float> g=new GenRectangle(12.3f, 15.7f);
Utilnew.<Float>getDimensions(g);
}
}
/**********************************************************************/
Video Tutorial:
