Return first non-null value

Question

I have a number of functions:

String first(){}
String second(){}
...
String default(){}

Each can return a null value, except the default. <

Answer 1

String s = Stream.>of(this::first, this::second /*, ... */)
                 .map(Supplier::get)
                 .filter(Objects::nonNull)
                 .findFirst()
                 .orElseGet(this::defaultOne);

It stops on the first non-null value or else sets the value which is returned from defaultOne. As long as you stay sequential, you are safe. Of course this requires Java 8 or later.

The reason why it stops on the first occurrence of a non-null value is due how the Stream handles each step. The map is an intermediate operation, so is filter. The findFirst on the other side is a short-circuiting terminal operation. So it continues with the next element until one matches the filter. If no element matches an empty optional is returned and so the orElseGet-supplier is called.

this::first, etc. are just method references. If they are static replace it with YourClassName::first, etc.

Here is an example if the signature of your methods would differ:

String s = Stream.>of(() -> first("takesOneArgument"),
                                       () -> second("takes", 3, "arguments")
                                   /*, ... */)
                 .map(Supplier::get)
                 .filter(Objects::nonNull)
                 .findFirst()
                 .orElseGet(this::defaultOne);

Note that the Supplier is only evaluated when you call get on it. That way you get your lazy evaluation behaviour. The method-parameters within your supplier-lambda-expression must be final or effectively final.