Lambda化的一个例子

计算用户购买专辑的一些有用属性，如音乐家人数、曲目和专辑时长等

命令式的实现：

public long countRunningTime() {
    long count = 0;
    for (Album album : albums) {
        for (Track track : album.getTracks()) {
            count += track.getLength();
        }
    }
    return count;
}

public long countMusicians() {
    long count = 0;
    for (Album album : albums) {
        count += album.getMusicians().size();
    }
    return count;
}

public long countTracks() {
    long count = 0;
    for (Album album : albums) {
        count += album.getTracks().size();
    }
    return count;
}

没有重用公有的概念，写出了更多代码需要测试和维护

使用流重构命令式的代码：

public long countRunningTime() {
    return albums.stream()
            .mapToLong(album -> album.getTracks()
                    .stream().mapToLong(track -> track.getLength()).sum())
            .sum();
}

public long countMusicians() {
    return albums.stream()
            .mapToLong(album -> album.getMusicians().size())
            .sum();
}

public long countTracks() {
    return albums.stream()
            .mapToLong(album -> album.getTracks().size())
            .sum();
}

这段代码仍有重用可读性的问题，因为有一些抽象和共性只能使用领域内的只是来表达

使用领域方法重构：

public long countFeature(ToLongFunction<Album> function) {
    return albums.stream()
            .mapToLong(function).sum();
}

public long countRunningTime() {
    return countFeature(album -> album.getTracks().stream()
            .mapToLong(track -> track.getLength()).sum());
}

public long countMusicians() {
    return countFeature(album -> album.getMusicians().size());
}

public long countTracks() {
    return countFeature(album -> album.getTracks().size());
}