为什么不建议你用去 “! = null” 做判空？

...if (someobject != null) {
    someobject.doCalc();}...

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();}

foo.equals("bar")