Generic Bounded Wildcards

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.");


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.

  Java Book 
    Language Basics  

  1. Generic Class
  2. Generic Bounded Types
  3. Generic Wildcard Arguments
  4. Generic Bounded Wildcards
  5. Generic Method
  6. Generic Constructors
  7. Generic Interfaces
  8. Raw Types and Legacy Code
  9. Generic Class Hierarchies
  10. Run-Time Type Comparisons Within a Generic Hierarchy
  11. Overriding Methods in a Generic Class
  12. Generic Restrictions
  13. Generic Array Restrictions