StreamStudy.java

package java8;

import java.util.Arrays;
import java.util.EnumSet;
import java.util.List;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;
import java.util.stream.Stream;

/**
 * Java8のCollectorをfor文と比較しながら一から作って理解する
 * @author civic
 */
public class StreamStudy {
    
    public static void main(String[] args) {

        //forでの実装
        {
            List<String> list = Arrays.asList("Hello", "World", "Java8", "Stream", "API");
            StringBuilder buff = new StringBuilder();
            for (String s: list){
                if (buff.length() != 0){
                    buff.append(", ");
                }
                buff.append(s);
            }
            String commaJoinedString = buff.toString();
            System.out.println(commaJoinedString);
        }


        //collectorでの実装
        {
            Stream<String> stream = Arrays.asList("Hello", "World", "Java8", "Stream", "API").stream();
            
            String commaJoinedString = stream.parallel().collect(new MyStringJoinCollector());
            System.out.println(commaJoinedString);
        }

    }

    private static class MyStringJoinCollector implements Collector<String, StringBuilder, String>{

        @Override
        public Supplier<StringBuilder> supplier() {
            return () -> new StringBuilder();
        }

        @Override
        public BiConsumer<StringBuilder, String> accumulator() {
            return (sb, s) -> {
                if (sb.length() != 0){
                    sb.append(", ");
                }
                sb.append(s);
            };
        }

        @Override
        public BinaryOperator<StringBuilder> combiner() {
            return (sb1, sb2) ->{
                if (sb1.length() != 0){
                    sb1.append(", ");
                }
                sb1.append(sb2);
                return sb1;
            };
        }

        @Override
        public Function<StringBuilder, String> finisher() {
            return sb -> sb.toString();
        }

        @Override
        public Set<Characteristics> characteristics() {
            return EnumSet.noneOf(Characteristics.class);
        }
    }
}