Java Does Not Equal (!=) Not Working? [duplicate]

Question

Here is my code snippet:

public void joinRoom(String room) throws MulticasterJoinException {
  String statusCheck = this.transmit("room", "join", room + "," + this.groupMax + "," + this.uniqueID);

  if (statusCheck != "success") {
    throw new MulticasterJoinException(statusCheck, this.PAppletRef);
  }
}

However for some reason, if (statusCheck != "success") is returning false, and thereby throwing the MulticasterJoinException.

leepoint.net/notes-java/data/strings/12stringcomparison.html — Pavan Kumar
– Pavan Kumar, Commented Dec 13, 2011 at 5:22
SO should start detecting this question and answering it on its own. — Paul Bellora
– Paul Bellora, Commented Dec 13, 2011 at 5:24
Didn't see an answer to this question in the suggestions as I entered in the title, so I figured it was unique. Guess not... — Oliver Spryn
– Oliver Spryn, Commented Dec 13, 2011 at 5:28
@KublaiKhan, I ask this question (String equality) during interviews. It's not often someone can answer when == works and when it doesn't (and how to make it work again). — Paul
– Paul, Commented Dec 13, 2011 at 5:29

勿绮语 · Accepted Answer · 2012-02-13 04:24:19Z

241

if (!"success".equals(statusCheck))

edited Feb 13, 2012 at 4:24

answered Dec 13, 2011 at 5:23

勿绮语

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2 Comments

Alvin Over a year ago

note that how Peter uses !"success".equals(statusCheck) rather than !statusCheck.equals("success"). This way you can avoid NullPointerException if statusCheck is null.

Malakai Over a year ago

I'd suggest you also take a look on "*.equalsIgnoreCase(string)", just to make sure everything is fine.

Jens Schauder · Accepted Answer · 2016-11-22 15:20:10Z

44

== and != work on object identity. While the two Strings have the same value, they are actually two different objects.

use !"success".equals(statusCheck) instead.

edited Nov 22, 2016 at 15:20

answered Dec 13, 2011 at 5:25

Jens Schauder

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1 Comment

Jonathan M Over a year ago

This is better than the accepted answer because it gives explanation.

Paul · Accepted Answer · 2018-04-11 13:23:05Z

35

Sure, you can use equals if you want to go along with the crowd, but if you really want to amaze your fellow programmers check for inequality like this:

if ("success" != statusCheck.intern())

intern method is part of standard Java String API.

edited Apr 11, 2018 at 13:23

answered Dec 13, 2011 at 5:31

Paul

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4 Comments

Peter Štibraný Over a year ago

+1 from me. Although I'd prefer not to see this idiom in codebase that I have to maintain, it sure is useful to know what intern() is and how it works. (Using Google Guava's Interner makes this stuff more explicit)

Paul Over a year ago

Four years later I realize I should have said that I'd never use that in "real" code and I don't recommend anyone else do so either. There's probably some edge case where interning a string provides a slight performance improvement but the point of my answer was to educate and amuse.

Tasgall Over a year ago

Even assuming someone reading the code knows what "Intern()" does, they'll still be confused at why it's there, because they'll be trying to figure out why it was used.

Paul Over a year ago

@Tasgall, I agree - if there's ever a need for intern() then it's safe to say that the code should be well documented.

subodh · Accepted Answer · 2011-12-13 05:26:56Z

9

do the one of these.

   if(!statusCheck.equals("success"))
    {
        //do something
    }

      or

    if(!"success".equals(statusCheck))
    {
        //do something
    }

answered Dec 13, 2011 at 5:26

subodh

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2 Comments

Alican Balik Over a year ago

what is the difference between those 2 conditions?

pawpaw Over a year ago

Wrong! Don't you ever do that! (I mean, the first one). Why? if statusCheck==null, then you gonna get NPE in your face. In the second example you only get false is statusCheck is null. @AlicanBalik

lobster1234 · Accepted Answer · 2011-12-13 05:22:53Z

4

Please use !statusCheck.equals("success") instead of !=.

Here are more details.

answered Dec 13, 2011 at 5:22

lobster1234

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1 Comment

pawpaw Over a year ago

Actually you both @anubhava and SubOP are wrong. Never put a nullable object first in an equals(Object) method if you can. Should be !"success".equals(statusCheck). Otherwise you can get a NPE if statusCheck == null

Matt Lacey · Accepted Answer · 2016-11-17 22:10:36Z

4

You need to use the method equals() when comparing a string, otherwise you're just comparing the object references to each other, so in your case you want:

if (!statusCheck.equals("success")) {

edited Nov 17, 2016 at 22:10

answered Dec 13, 2011 at 5:23

Matt Lacey

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2 Comments

Jon Skeet Over a year ago

equals, not Equals.

Jose Rui Santos Over a year ago

even better is to check for !"success".equals(statusCheck) in case statusCheck is null, will not throw a NullPointerException

zhaoyw · Accepted Answer · 2011-12-14 04:56:28Z

2

you can use equals() method to statisfy your demands. == in java programming language has a different meaning!

answered Dec 14, 2011 at 4:56

zhaoyw

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