OnJava8-Examples/generics/BasicBounds.java

// generics/BasicBounds.java
// (c)2021 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

interface HasColor { java.awt.Color getColor(); }

class WithColor<T extends HasColor> {
  T item;
  WithColor(T item) { this.item = item; }
  T getItem() { return item; }
  // The bound allows you to call a method:
  java.awt.Color color() { return item.getColor(); }
}

class Coord { public int x, y, z; }

// This fails. Class must be first, then interfaces:
// class WithColorCoord<T extends HasColor & Coord> {

// Multiple bounds:
class WithColorCoord<T extends Coord & HasColor> {
  T item;
  WithColorCoord(T item) { this.item = item; }
  T getItem() { return item; }
  java.awt.Color color() { return item.getColor(); }
  int getX() { return item.x; }
  int getY() { return item.y; }
  int getZ() { return item.z; }
}

interface Weight { int weight(); }

// As with inheritance, you can have only one
// concrete class but multiple interfaces:
class Solid<T extends Coord & HasColor & Weight> {
  T item;
  Solid(T item) { this.item = item; }
  T getItem() { return item; }
  java.awt.Color color() { return item.getColor(); }
  int getX() { return item.x; }
  int getY() { return item.y; }
  int getZ() { return item.z; }
  int weight() { return item.weight(); }
}

class Bounded
extends Coord implements HasColor, Weight {
  @Override
  public java.awt.Color getColor() { return null; }
  @Override public int weight() { return 0; }
}

public class BasicBounds {
  public static void main(String[] args) {
    Solid<Bounded> solid =
      new Solid<>(new Bounded());
    solid.color();
    solid.getY();
    solid.weight();
  }
}