你可以参考下面的代码，你还应该检查colorsList是否为null

List<List<String>> colorsList = List.of(
        List.of("red","maroon"),
        List.of("blue", "skyblue"),
        List.of("pink"));

int var1 = 5;
List<String> listReturn = new ArrayList<>();

if (5 == var1) {
    colorsList.stream()
            .filter(innerList -> innerList.contains("blue"))
            .forEach(listReturn::addAll);
} else if (10 == var1) {
    colorsList.stream()
            .filter(innerList -> innerList.contains("pink") || innerList.contains("red"))
            .forEach(listReturn::addAll);
}

如果我想写这段代码，我将首先创建一个colorsList流，然后过滤它。如果筛选条件为true，则该项将添加到输出中。接下来，我们使用flatMap（）将嵌套列表扁平化为单个字符串流。

List<String> listReturn = colorsList.stream()
        .filter(innerList -> var1 == 5 && innerList.contains("blue")
                || var1 == 10 && (innerList.contains("pink") || innerList.contains("red")))
        .flatMap(Collection::stream)
        .collect(Collectors.toList());

你可以分解出 predicate ，这样你就不必重复检查和过滤列表的代码：

List<List<String>> colorsList = List.of(
        List.of("red","maroon"),
        List.of("blue, skyblue"),
        List.of("pink"));

int var1 = 5;

// switch expression
Predicate<List<String>> predicate = switch (var1) {
    case 5 -> innerList -> innerList.contains("blue");
    case 10 -> innerList -> innerList.contains("pink") || innerList.contains("red");
    default -> innerList -> true; // if something else than 5 or 10 accept all
};

List<String> listReturn = colorsList.stream()
               .filter(predicate).flatMap(List::stream).toList();