How to implement a list fold in Java

Question

I have a List and want to reduce it to a single value (functional programming term "fold", Ruby term inject), like

Arrays.asList("a", "b", "c") ... fold ... "a,b,c" 

As I am infected with functional programming ideas (Scala), I am looking for an easier/shorter way to code it than

sb = new StringBuilder for ... {   append ... } sb.toString 

Answer 1

To answer your original question:

public static <A, B> A fold(F<A, F<B, A>> f, A z, Iterable<B> xs) { A p = z;   for (B x : xs)     p = f.f(p).f(x);   return p; } 

Where F looks like this:

public interface F<A, B> { public B f(A a); } 

As dfa suggested, Functional Java has this implemented, and more.

Example 1:

import fj.F; import static fj.data.List.list; import static fj.pre.Monoid.stringMonoid; import static fj.Function.flip; import static fj.Function.compose;  F<String, F<String, String>> sum = stringMonoid.sum(); String abc = list("a", "b", "c").foldLeft1(compose(sum, flip(sum).f(","))); 

Example 2:

import static fj.data.List.list; import static fj.pre.Monoid.stringMonoid; ... String abc = stringMonoid.join(list("a", "b", "c"), ","); 

Example 3:

import static fj.data.Stream.fromString; import static fj.data.Stream.asString; ... String abc = asString(fromString("abc").intersperse(',')); 

Answer 2

Given

public static <T,Y> Y fold(Collection<? extends T> list, Injector<T,Y> filter){   for (T item : list){     filter.accept(item);   }   return filter.getResult(); }  public interface Injector<T,Y>{   public void accept(T item);   public Y getResult(); } 

Then usage just looks like

fold(myArray, new Injector<String,String>(){   private StringBuilder sb = new StringBuilder();   public void Accept(String item){ sb.append(item); }   public String getResult() { return sb.toString(); } } );