java-design-patterns/dependency-injection/src/main/java/com/iluwatar/dependency/injection/App.java

package com.iluwatar.dependency.injection;

import com.google.inject.Guice;
import com.google.inject.Injector;

/**
 * 
 * Dependency Injection pattern deals with how objects handle their dependencies. The pattern
 * implements so called inversion of control principle. Inversion of control has two specific rules:
 * - High-level modules should not depend on low-level modules. Both should depend on abstractions.
 * - Abstractions should not depend on details. Details should depend on abstractions.
 * <p>
 * In this example we show you three different wizards. The first one ({@link SimpleWizard}) is a
 * naive implementation violating the inversion of control principle. It depends directly on a
 * concrete implementation which cannot be changed.
 * <p>
 * The second wizard ({@link AdvancedWizard}) is more flexible. It does not depend on any concrete
 * implementation but abstraction. It utilizes Dependency Injection pattern allowing its
 * {@link Tobacco} dependency to be injected through its constructor. This way, handling the
 * dependency is no longer the wizard's responsibility. It is resolved outside the wizard class.
 * <p>
 * The third example takes the pattern a step further. It uses Guice framework for Dependency
 * Injection. {@link TobaccoModule} binds a concrete implementation to abstraction. Injector is then
 * used to create {@link GuiceWizard} object with correct dependencies.
 *
 */
public class App {

  /**
   * Program entry point
   * 
   * @param args command line args
   */
  public static void main(String[] args) {
    SimpleWizard simpleWizard = new SimpleWizard();
    simpleWizard.smoke();

    AdvancedWizard advancedWizard = new AdvancedWizard(new SecondBreakfastTobacco());
    advancedWizard.smoke();

    Injector injector = Guice.createInjector(new TobaccoModule());
    GuiceWizard guiceWizard = injector.getInstance(GuiceWizard.class);
    guiceWizard.smoke();
  }
}