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Bridge

April 14, 2023 4 min read Structural Gang of Four
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Also known as

Handle/Body

Intent

Decouple an abstraction from its implementation so that the two can vary independently.

Explanation

Real-world example

Consider you have a weapon with different enchantments, and you are supposed to allow mixing different weapons with different enchantments. What would you do? Create multiple copies of each of the weapons for each of the enchantments or would you just create separate enchantment and set it for the weapon as needed? Bridge pattern allows you to do the second.

In Plain Words

Bridge pattern is about preferring composition over inheritance. Implementation details are pushed from a hierarchy to another object with a separate hierarchy.

Wikipedia says

The bridge pattern is a design pattern used in software engineering that is meant to “decouple an abstraction from its implementation so that the two can vary independently”

Programmatic Example

Translating our weapon example from above. Here we have the Weapon hierarchy:

 1public interface Weapon {
 2  void wield();
 3  void swing();
 4  void unwield();
 5  Enchantment getEnchantment();
 6}
 7
 8public class Sword implements Weapon {
 9
10  private final Enchantment enchantment;
11
12  public Sword(Enchantment enchantment) {
13    this.enchantment = enchantment;
14  }
15
16  @Override
17  public void wield() {
18    LOGGER.info("The sword is wielded.");
19    enchantment.onActivate();
20  }
21
22  @Override
23  public void swing() {
24    LOGGER.info("The sword is swinged.");
25    enchantment.apply();
26  }
27
28  @Override
29  public void unwield() {
30    LOGGER.info("The sword is unwielded.");
31    enchantment.onDeactivate();
32  }
33
34  @Override
35  public Enchantment getEnchantment() {
36    return enchantment;
37  }
38}
39
40public class Hammer implements Weapon {
41
42  private final Enchantment enchantment;
43
44  public Hammer(Enchantment enchantment) {
45    this.enchantment = enchantment;
46  }
47
48  @Override
49  public void wield() {
50    LOGGER.info("The hammer is wielded.");
51    enchantment.onActivate();
52  }
53
54  @Override
55  public void swing() {
56    LOGGER.info("The hammer is swinged.");
57    enchantment.apply();
58  }
59
60  @Override
61  public void unwield() {
62    LOGGER.info("The hammer is unwielded.");
63    enchantment.onDeactivate();
64  }
65
66  @Override
67  public Enchantment getEnchantment() {
68    return enchantment;
69  }
70}

Here’s the separate enchantment hierarchy:

 1public interface Enchantment {
 2  void onActivate();
 3  void apply();
 4  void onDeactivate();
 5}
 6
 7public class FlyingEnchantment implements Enchantment {
 8
 9  @Override
10  public void onActivate() {
11    LOGGER.info("The item begins to glow faintly.");
12  }
13
14  @Override
15  public void apply() {
16    LOGGER.info("The item flies and strikes the enemies finally returning to owner's hand.");
17  }
18
19  @Override
20  public void onDeactivate() {
21    LOGGER.info("The item's glow fades.");
22  }
23}
24
25public class SoulEatingEnchantment implements Enchantment {
26
27  @Override
28  public void onActivate() {
29    LOGGER.info("The item spreads bloodlust.");
30  }
31
32  @Override
33  public void apply() {
34    LOGGER.info("The item eats the soul of enemies.");
35  }
36
37  @Override
38  public void onDeactivate() {
39    LOGGER.info("Bloodlust slowly disappears.");
40  }
41}

Here are both hierarchies in action:

 1LOGGER.info("The knight receives an enchanted sword.");
 2var enchantedSword = new Sword(new SoulEatingEnchantment());
 3enchantedSword.wield();
 4enchantedSword.swing();
 5enchantedSword.unwield();
 6
 7LOGGER.info("The valkyrie receives an enchanted hammer.");
 8var hammer = new Hammer(new FlyingEnchantment());
 9hammer.wield();
10hammer.swing();
11hammer.unwield();

Here’s the console output.

The knight receives an enchanted sword.
The sword is wielded.
The item spreads bloodlust.
The sword is swung.
The item eats the soul of enemies.
The sword is unwielded.
Bloodlust slowly disappears.
The valkyrie receives an enchanted hammer.
The hammer is wielded.
The item begins to glow faintly.
The hammer is swung.
The item flies and strikes the enemies finally returning to owner's hand.
The hammer is unwielded.
The item's glow fades.

Class diagram

alt text

Applicability

Use the Bridge pattern when

  • You want to avoid a permanent binding between an abstraction and its implementation. This might be the case, for example, when the implementation must be selected or switched at run-time.
  • Both the abstractions and their implementations should be extensible by subclassing. In this case, the Bridge pattern lets you combine the different abstractions and implementations and extend them independently.
  • Changes in the implementation of an abstraction should have no impact on clients; that is, their code should not have to be recompiled.
  • You have a proliferation of classes. Such a class hierarchy indicates the need for splitting an object into two parts. Rumbaugh uses the term “nested generalizations” to refer to such class hierarchies.
  • You want to share an implementation among multiple objects (perhaps using reference counting), and this fact should be hidden from the client. A simple example is Coplien’s String class, in which multiple objects can share the same string representation.

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