『Javaによる関数型プログラミング - Java 8ラムダ式とStream』例6-12 の処理内容と同等のものを Stream API, Eclipse Collections (逐次実行/遅延実行) で比較

EclipseCollectionsLazyTest.java

package com.nashcft;

import org.eclipse.collections.api.list.MutableList;
import org.eclipse.collections.impl.factory.Lists;
import org.junit.Test;

public class EclipseCollectionsLazyTest {

    private MutableList<String> list = Lists.mutable.of("a", "bb", "ccc", "dd", "e", "ff", "ggg", "hh", "i");

    @Test
    public void seq() {
        String result = list
            .select(s -> predicate(s))
            .collect(s -> toUpper(s))
            .getFirst();
        System.out.println("result: " + result);
        // =>
        // length: 1
        // length: 2
        // length: 3
        // length: 2
        // length: 1
        // length: 2
        // length: 3
        // length: 2
        // length: 1
        // toUpper: CCC
        // toUpper: GGG
        // result: CCC
    }

    @Test
    public void lazy() {
        String result = list.asLazy()
            .select(s -> predicate(s))
            .collect(s -> toUpper(s))
            .getFirst();
        System.out.println("result: " + result);
        // =>
        // length: 1
        // length: 2
        // length: 3
        // length: 1   <= ???
        // length: 2   <= ???
        // length: 3   <= ???
        // toUpper: CCC
        // result: CCC
    }
    
    @Test
    public void streamApi() {
        String result = list.stream()
                .filter(s -> predicate(s))
                .map(s -> toUpper(s))
                .findFirst().orElse("none");
        System.out.println("result: " + result);
        // =>
        // length: 1
        // length: 2
        // length: 3
        // toUpper: CCC
        // result: CCC
    }

    private boolean predicate(String s) {
        System.out.println("length: " + s.length());
        return s.length() == 3;
    }

    private String toUpper(String s) {
        System.out.println("toUpper: " + s.toUpperCase());
        return s.toUpperCase();
    }
}