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I have seen various codes of my seniors and they have used this at many places.What effect does it have? why cant they include anything != null. This is true for below thing too

"true".equals(x).

Why is this so?

Chetan Kinger
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Nilesh
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6 Answers6

9

anything != null is exactly equivalent to null != anything.

On the other hand, "true".equals(x) avoids the need to check if x is null when doing x.equals("true").

M A
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The reason for including constants on the left hand side of an equals expression is to avoid NullPointerException even when the variable is null. This also improves the readability of the expression.

That being said, null!=something is a personal choice and I would prefer using something!=null.

Chetan Kinger
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2

These are two separate practices.

Using null on the left side when comparing to null ensures that you won't, by mistake, use = instead of ==. However, this is a practice taken from languages other than Java, as in Java the expression

if ( something = null )

is going to fail as it does not resolve to a boolean value. In other languages it may be permissible to use a condition whose value is actually a pointer.

Thus this practice has no meaning in Java.

The second practice, as you were told, has to do with preventing a NullPointerException. It should be noted that this practice is controversial, because it allows a null to propagate to a later point in the program, and that usually means that it will be harder to find when it causes a bug.

RealSkeptic
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That check is so the program ensures the variable in question has been initialized before reading it. If a program tries to access a null variable, it will crash (specifically in Java it will throw a NullPointerException). It is a critical aspect of defensive programming.

As far as ordering, the evaluation of whether something is null or not is commutative. Meaning:

something != null

Is the same as

null != something

It is all a matter of personal preference. And as @manouti explains above, "true".equals(x) avoids the need to check if x is null when doing x.equals("true"), so you save a line of code.

Lawrence Aiello
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  • *"That check is so the program ensures the variable in question has been initialized before reading it."* How does the variable automatically get initialized just by changing the order of the operands of `equals`? And no, I did not downvote your answer in case you are wondering. – Chetan Kinger Jun 02 '15 at 18:07
  • It looks like I misunderstood the question. I thought he was wondering what null was.... – Lawrence Aiello Jun 02 '15 at 18:11
  • @Lawrenece Programmers are not always on the same page. You can always edit your answer. – Chetan Kinger Jun 02 '15 at 18:13
  • I'm always doubtful of doing that because I don't want it to look like I'm stealing answers. Thanks :) – Lawrence Aiello Jun 02 '15 at 18:19
  • It's not stealing if you know the answer but just misinterpreted the question. Besides, some other user might land up on your answer some other day and decide to downvote it. Loss of hard earned rep.. – Chetan Kinger Jun 02 '15 at 18:22
1

This is called a Yoda condition. Among other things, it helps prevent an accidental assignment due to a missing = or !, as in foo = null. The reverse, null = foo, would not even compile.

That said, it is purely a stylistic preference. foo != null is logically equivalent to null != foo.

elixenide
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Seems that some people prefer null != anything, because anything might be quite long expression, so writing null first makes it visible in the narrow window and simplifies code reading ("ok, we just check if something is not null"). But it's totally a matter of style.

As for "true".equals(x), it's well explained by @manouti.

Tagir Valeev
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