What's the best way to check for not null values in java.
String query ="abcd";
query != null vs !query.equals(null).
which is better?why?
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Rohit Jain
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yonikawa
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.equals defaults to == as long as it's not explicitly overridden. – Jainathan Leung Feb 13 '13 at 21:07
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@JaynathanLeung. Are you talking about `String` objects too? – Rohit Jain Feb 13 '13 at 21:08
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@RohitJain are there any types of `String` which aren't objects? There is no primitive equivalent as far as I know... – Jesse Webb Feb 13 '13 at 21:10
4 Answers
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1st one is better (and the only option), because 2nd one will throw NPE, when your value is actually null. As simple as that.
Try this out:
String str = null;
str.equals(null); // will throw `NPE`.
So basically, the test which you wanted to perform itself triggers a NullPointerException in the 2nd case. So, it is no choice.
Rohit Jain
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This is wrong.`String.equals` will not throw a NPE.Since it checks for `instanceof` which does not throw exception for `null` – Cratylus Feb 13 '13 at 21:09
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"Better" is an extreme understatement here. Second doesn't do at all what you expect. – djechlin Feb 13 '13 at 21:11
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2@RohitJain:I did:`String test = "aa"; System.out.println(test.equals(null));`.Prints `false` – Cratylus Feb 13 '13 at 21:11
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1@Cratylus - now try it with `String test = null;` and see what happens... – Jesse Webb Feb 13 '13 at 21:12
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1@Cratylus what rohit meant was to try `String s= null; s.equals(null)` – PermGenError Feb 13 '13 at 21:12
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1@Cratylus. Why would he test for `non-null` if there is no possibility of `null` value at all? And if there is a possibility of `null` value, then there is `NPE` in 2nd case. – Rohit Jain Feb 13 '13 at 21:16
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!str.equals(null) will
- always return false if it does not throw an exception, and
- throw an exception if str is null
The point of a null check is to make sure the reference, in this code str, actually refers to an object. If it is null, you can't call any methods on it, including equals, without throwing a NullPointerException... which the point of null checks is to avoid happening.
So !str.equals(null) causes the very problem null checks are meant to prevent and doesn't do what you think at all. Never do this.
djechlin
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+1, i made the same point about `str.equals(null)` would always return false, but my answer not clear about the concrete question.. :) – PermGenError Feb 13 '13 at 21:11
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query != null better as !query.equals(null) will throw an exception when query is actually null. Also query != null is easily readable
Syst3m_NZ
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