MapUtils.java 文件源码

public static <K, V> Multimap<K, V> combine(Iterable<Multimap<K, V>> maps) {
    Multimap<K, V> singleton = null;
    ImmutableMultimap.Builder<K, V> builder = null;
    for(Multimap<K, V> map : maps) {
        if(!map.isEmpty()) {
            if(singleton == null) {
                singleton = map;
            } else {
                if(builder == null) {
                    builder = ImmutableMultimap.builder();
                }
                builder.putAll(singleton);
                builder.putAll(map);
            }
        }
    }

    if(builder != null) {
        return builder.build();
    } else if(singleton != null) {
        return singleton;
    } else {
        return ImmutableMultimap.of();
    }
}