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How to get the first non-null value in Java?

Asked 2010-05-04T18:44:13.767
222

Is there a Java equivalent of SQL's COALESCE function? That is, is there any way to return the first non-null value of several variables?

e.g.

Double a = null;
Double b = 4.4;
Double c = null;

I want to somehow have a statement that will return the first non-null value of a, b, and c - in this case, it would return b, or 4.4. (Something like the sql method - return COALESCE(a,b,c)). I know that I can do it explicitly with something like:

return a != null ? a : (b != null ? b : c)

But I wondered if there was any built-in, accepted function to accomplish this.

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1 Answer

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This situation calls for some preprocessor. Because if you write a function (static method) which picks the first not null value, it evaluates all items. It is problem if some items are method calls (may be time expensive method calls). And this methods are called even if any item before them is not null.

Some function like this

public static <T> T coalesce(T ...items) …

should be used but before compiling into byte code there should be a preprocessor which find usages of this „coalesce function“ and replaces it with construction like

a != null ? a : (b != null ? b : c)

Update 2014-09-02:

Thanks to Java 8 and Lambdas there is possibility to have true coalesce in Java! Including the crucial feature: particular expressions are evaluated only when needed – if earlier one is not null, then following ones are not evaluated (methods are not called, computation or disk/network operations are not done).

I wrote an article about it Java 8: coalesce – hledáme neNULLové hodnoty – (written in Czech, but I hope that code examples are understandable for everyone).

answered 2012-02-19T18:45:24.863

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