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java-design-patterns/double-dispatch/src/main/java/com/iluwatar/doubledispatch/App.java

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package com.iluwatar.doubledispatch;
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import java.util.ArrayList;
import java.util.List;
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/**
*
* When a message with a parameter is sent to an object, the resultant behaviour is defined by the
* implementation of that method in the receiver. Sometimes the behaviour must also be determined by
* the type of the parameter.
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* <p>
* One way to implement this would be to create multiple instanceof-checks for the methods
* parameter. However, this creates a maintenance issue. When new types are added we would also need
* to change the method's implementation and add a new instanceof-check. This violates the single
* responsibility principle - a class should have only one reason to change.
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* <p>
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* Instead of the instanceof-checks a better way is to make another virtual call on the parameter
* object. This way new functionality can be easily added without the need to modify existing
* implementation (open-closed principle).
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* <p>
* In this example we have hierarchy of objects ({@link GameObject}) that can collide to each other.
* Each object has its own coordinates which are checked against the other objects' coordinates. If
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* there is an overlap, then the objects collide utilizing the Double Dispatch pattern.
*
*/
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public class App {
/**
* Program entry point
*
* @param args command line args
*/
public static void main(String[] args) {
// initialize game objects and print their status
List<GameObject> objects = new ArrayList<>();
objects.add(new FlamingAsteroid(0, 0, 5, 5));
objects.add(new SpaceStationMir(1, 1, 2, 2));
objects.add(new Meteoroid(10, 10, 15, 15));
objects.add(new SpaceStationIss(12, 12, 14, 14));
objects.stream().forEach(o -> System.out.println(o));
System.out.println("");
// collision check
objects.stream().forEach(o1 -> objects.stream().forEach(o2 -> {
if (o1 != o2 && o1.intersectsWith(o2))
o1.collision(o2);
}));
System.out.println("");
// output eventual object statuses
objects.stream().forEach(o -> System.out.println(o));
System.out.println("");
}
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}