Mega Code Archive
0114 Generic Bounded Wildcards
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Wildcard arguments can be bounded in the same way that a type parameter can be bounded. A bounded wildcard is important when creating a generic type that will operate on a class hierarchy.
A bounded wildcard specifies either an upper bound or a lower bound for the type argument.
class TwoD {
int x, y;
TwoD(int a, int b) {
x = a;
y = b;
}
}
// Three-dimensional coordinates.
class ThreeD extends TwoD {
int z;
ThreeD(int a, int b, int c) {
super(a, b);
z = c;
}
}
class Map<T extends TwoD> {
T[] coords;
Map(T[] o) {
coords = o;
}
}
public class Main {
static void showXY(Map<?> c) {
for (int i = 0; i < c.coords.length; i++){
System.out.println(c.coords[i].x + " " + c.coords[i].y);
}
}
static void showXYZ(Map<? extends ThreeD> c) {
for (int i = 0; i < c.coords.length; i++){
System.out.println(c.coords[i].x + " " + c.coords[i].y + " "
+ c.coords[i].z);
}
}
public static void main(String args[]) {
TwoD td[] = { new TwoD(0, 0), new TwoD(-1, -2) };
Map<TwoD> map = new Map<TwoD>(td);
System.out.println("Contents of tdlocs.");
showXY(map);
}
}
In general, to establish an upper bound for a wildcard, use the following type of wildcard expression:
<? extends superclass>
superclass is the name of the class that serves as the upper bound. This is an inclusive clause.
You can specify a lower bound for a wildcard by adding a super clause to a wildcard declaration.
<? super subclass>
In this case, only classes that are superclasses of subclass are acceptable arguments. This is an exclusive clause, because it will not match the class specified by subclass
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