Java 8 lambda predicate chaining?

Question

I can\'t get it to compile, is it even possible to chain predicate lambdas?

Arrays.asList(\"1\",\"2\",\"3\").stream().filter( (e -> e==\"1\" ).or(e-> e         


        

Answer 1

I was working on a similar problem of predicate chaining and came up with the following

public static <T> Predicate<T> chain (Function<T,Predicate<T>> mapFn, T[]args) {
    return Arrays.asList(args)
        .stream()
        .map(x->mapFn.apply(x))
        .reduce(p->false, Predicate::or);
}

The first parameter to chain is a lambda (Function) that returns a lambda (Predicate) so it needs a couple of arrows

public static void yourExample() {
    String[] filterVals = { "1", "2" };

    Arrays.asList("1","2","3")
        .stream()
        .filter(chain(x-> (y-> y.equals(x)), filterVals))
        .count();
}

For comparison, here is what I was trying to achieve...

public static void myExample() {
    String[] suffixes = { ".png", ".bmp" };
    Predicate<String> p = chain (x-> y-> y.endsWith(x), suffixes);
    File[] graphics = new File("D:/TEMP").listFiles((dir,name)->p.test(name));
    Arrays.asList(graphics).forEach(System.out::println);
}

Answer 2

You can use:

((Predicate<String>) e -> e.equals("1")).or(e -> e.equals("2"))

but it's not very elegant. If you're specifying the conditions in-line, just use one lambda:

e -> e.equals("1") || e.equals("2")

Answer 3

Arrays.asList("1","2","3").stream().filter( Arrays.asList("1", "2")::contains).count();

and yes method "either" is good idea

    public static void main(String[] args) {
        long count = Arrays.asList("1","2","3").stream().filter(either("1"::equals).or("2"::equals)).count();
        System.out.println(count);
    }

    private static <T> Predicate<T> either(Predicate<T> predicate) {
        return predicate;
    }

or you can use import static java.util.function.Predicate.isEqual; and write isEqual("1").or(isEqual("2"))

Answer 4

You can use with magic method $:

<T> Predicate<T> $(Predicate<T> p) {
     return p;
}

Arrays.asList("1", "2", "3").stream().filter( $(e -> e=="1").or(e -> e=="2") ).count();

Answer 5

Predicate<String> pred1 =  "1"::equals;
Predicate<String> pred2 =  "2"::equals;

public void tester(){
      Arrays.asList("1","2","3").stream().filter(pred1.or(pred2)).count();
}

You can move your separate your conditions to make it possible to recombine them other ways.

Answer 6

This is just a small addition to @Chris Jester-Young answer. It is possible to make the expression shorter by using method reference:

((Predicate<String>) "1"::equals).or("2"::equals)