在Java代码中避免 "!=null " 判断

1. 就约定来说是一个合理的响应
2. 不是合理的响应  

assert <condition>

assert <condition> : <object>

public interface Action {
  void doSomething();
}

public interface Parser {
  Action findAction(String userInput);
}

public class MyParser implements Parser {
  private static Action DO_NOTHING = new Action() {
    public void doSomething() { /* do nothing */ }
  };

  public Action findAction(String userInput) {
    // ...
    if ( /* we can‘t find any actions */ ) {
      return DO_NOTHING;
    }
  }
}

Parser parser = ParserFactory.getParser();
if (parser == null) {
  // now what?
  // this would be an example of where null isn‘t (or shouldn‘t be) a valid response
}
Action action = parser.findAction(someInput);
if (action == null) {
  // do nothing
} else {
  action.doSomething();
}

ParserFactory.getParser().findAction(someInput).doSomething();

try {
    ParserFactory.getParser().findAction(someInput).doSomething();
} catch(ActionNotFoundException anfe) {
    userConsole.err(anfe.getMessage());
}

public Action findAction(final String userInput) {
    /* Code to return requested Action if found */
    return new Action() {
        public void doSomething() {
            userConsole.err("Action not found: " + userInput);
        }
    }
}