How to get the first non-null value in Java

Question

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

User · Answer

How about using suppliers when you want to avoid evaluating some expensive method?

Like this:

public static <T> T coalesce(Supplier<T>... items) {
for (Supplier<T> item : items) {
    T value = item.get();
    if (value != null) {
        return value;
    }
    return null;
}

And then using it like this:

Double amount = coalesce(order::firstAmount, order::secondAmount, order::thirdAmount)

You can also use overloaded methods for the calls with two, three or four arguments.

In addition, you could also use streams with something like this:

public static <T> T coalesce2(Supplier<T>... s) {
    return Arrays.stream(s).map(Supplier::get).filter(Objects::nonNull).findFirst().orElse(null);
}

User · Answer

Object coalesce Object    objects        for Object o   object          if o    null              return o      return null

User · Answer

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  lt T gt  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     hled  me neNULLov   hodnoty      written in Czech  but I hope that code examples are understandable for everyone

User · Answer

Following on from LES2 s answer  you can eliminate some repetition in the efficient version  by calling the overloaded function   public static  lt T gt  T coalesce T a  T b        return a    null   a   b    public static  lt T gt  T coalesce T a  T b  T c        return a    null   a   coalesce b c     public static  lt T gt  T coalesce T a  T b  T c  T d        return a    null   a   coalesce b c d     public static  lt T gt  T coalesce T a  T b  T c  T d  T e        return a    null   a   coalesce b c d e

User · Answer

With Guava you can do   Optional fromNullable a  or b     which doesn t throw NPE if both a and b are null   EDIT  I was wrong  it does throw NPE  The correct way as commented by Michal Cizmazia is   Optional fromNullable a  or Optional fromNullable b   orNull

User · Answer

If there are only two variables to check and you re using Guava  you can use MoreObjects firstNonNull T first  T second

User · Answer

If there are only two references to test and you are using Java 8  you could use  Object o   null  Object p    p   Object r   Optional ofNullable  o   orElse  p    System out println  r         p   If you import static Optional the expression is not too bad   Unfortunately your case with  several variables  is not possible with an Optional-method  Instead you could use   Object o   null  Object p   null  Object q    p    Optional lt Object gt  r   Stream of  o  p  q   filter  Objects  nonNull   findFirst    System out println  r orElse null          p

User · Answer

How about   firstNonNull   FluentIterable from      Lists newArrayList  a  b  c                   firstMatch  Predicates notNull                  or  someKnownNonNullDefault      Java ArrayList conveniently allows null entries and this expression is consistent regardless of the number of objects to be considered   In this form  all the objects considered need to be of the same type

User · Answer

No  there isn t   The closest you can get is   public static  lt T gt  T coalesce T    items        for T i   items  if i    null  return i      return null      For efficient reasons  you can handle the common cases as follows   public static  lt T gt  T coalesce T a  T b        return a    null   b   a    public static  lt T gt  T coalesce T a  T b  T c        return a    null   a    b    null   b   c     public static  lt T gt  T coalesce T a  T b  T c  T d        return

User · Answer

You can try this   public static  lt T gt  T coalesce T    t        return Stream of t  filter Objects  nonNull  findFirst   orElse null       Based on this response

User · Answer

Just for completness  the  several variables  case is indeed possible  though not elegant at all   For example  for variables o  p  and q   Optional ofNullable  o   orElseGet   - gt  Optional ofNullable  p   orElseGet   - gt  q       Please note the use of orElseGet   attending to the case that o  p  and q are not variables but expressions either expensive or with undesired side-effects   In the most general case coalesce e 1  e 2  e 3      e N    coalesce-expression i      e i   when i   N coalesce-expression i      Optional ofNullable  e i    orElseGet   - gt  coalesce-expression i 1     when i  lt  N   This can generate expressions excessively long   However  if we are trying to move to a world without null  then v i  are most probably already of type Optional lt String gt   as opposed to simply String   In this case    result  o orElse p orElse q get         or in the case of expressions   result  o orElseGet   - gt  p orElseGet   - gt  q get           Furthermore  if you are also moving to a functional-declarative style  o  p  and q should be of type Supplier lt String gt  like in   Supplier lt String gt  q    - gt  q-expr   Supplier lt String gt  p    - gt  Optional ofNullable p-expr  orElseGet  q     Supplier lt String gt  o    - gt  Optional ofNullable o-expr  orElseGet  p       And then the whole coalesce reduces simply to o get     For a more concrete example   Supplier lt Integer gt  hardcodedDefaultAge    - gt  99   Supplier lt Integer gt  defaultAge    - gt  defaultAgeFromDatabase   orElseGet  hardcodedDefaultAge     Supplier lt Integer gt  ageInStore    - gt  ageFromDatabase memberId  orElseGet  defaultAge     Supplier lt Integer gt  effectiveAge    - gt  ageFromInput   orElseGet  ageInStore       defaultAgeFromDatabase    ageFromDatabase    and ageFromInput   would already return Optional lt Integer gt   naturally   And then the coalesce becomes effectiveAge get   or simply effectiveAge if we are happy with a Supplier lt Integer gt    IMHO  with Java 8 we will see more and more code structured like this  as it s extremely self-explainatory and efficient at the same time  especially in more complex cases   I do miss a class Lazy lt T gt  that invokes a Supplier lt T gt  only one time  but lazily  as well as consistency in the definition of Optional lt T gt   i e  Optional lt T gt -Optional lt T gt  operators  or even Supplier lt Optional lt T gt  gt

User · Answer

Apache Commons Lang 3  ObjectUtils firstNonNull T      Java 8 Stream  Stream of T     filter Objects  nonNull  findFirst   orElse null

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

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