状态模式（State DP）允许对象在内部状态改变时改变行为，就好像换了一个类一样（原文：Allow an object to alter its behavior when its internal state changes. The object will appear to change its class）。
说起状态模式，就不得不提策略模式。两者都是在运行时可以改变行为，不同的是，策略模式是用户自己临时选择，而状态模式是用户之前定义好状态的转换，运行时根据既定的状态机来执行转换。当然，如果非得手动控制状态，那这两个模式确实很接近。

代码：
State接口：

/**
 * 
 * State interface.
 * 
 */
public interface State {

  void onEnterState();

  void observe();

}

猛犸：

/**
 * 
 * Mammoth has internal state that defines its behavior.
 * 
 */
public class Mammoth {

  private State state;

  public Mammoth() {
    state = new PeacefulState(this);
  }

  /**
   * Makes time pass for the mammoth
   */
  public void timePasses() {
    if (state.getClass().equals(PeacefulState.class)) {
      changeStateTo(new AngryState(this));
    } else {
      changeStateTo(new PeacefulState(this));
    }
  }

  private void changeStateTo(State newState) {
    this.state = newState;
    this.state.onEnterState();
  }

  @Override
  public String toString() {
    return "The mammoth";
  }

  public void observe() {
    this.state.observe();
  }
}

愤怒模式：

/**
 * 
 * Angry state.
 *
 */
public class AngryState implements State {

  private static final Logger LOGGER = LoggerFactory.getLogger(AngryState.class);

  private Mammoth mammoth;

  public AngryState(Mammoth mammoth) {
    this.mammoth = mammoth;
  }

  @Override
  public void observe() {
    LOGGER.info("{} is furious!", mammoth);
  }

  @Override
  public void onEnterState() {
    LOGGER.info("{} gets angry!", mammoth);
  }

}

平静模式：

/**
 * 
 * Peaceful state.
 *
 */
public class PeacefulState implements State {

  private static final Logger LOGGER = LoggerFactory.getLogger(PeacefulState.class);

  private Mammoth mammoth;

  public PeacefulState(Mammoth mammoth) {
    this.mammoth = mammoth;
  }

  @Override
  public void observe() {
    LOGGER.info("{} is calm and peaceful.", mammoth);
  }

  @Override
  public void onEnterState() {
    LOGGER.info("{} calms down.", mammoth);
  }

}

测试：

public static void main(String[] args) {

    Mammoth mammoth = new Mammoth();
    mammoth.observe();
    mammoth.timePasses();
    mammoth.observe();
    mammoth.timePasses();
    mammoth.observe();

  }