千萬不要濫用Stream.toList（），有坑！

2024-03-07碼農

作者：下次用金塊
連結：https://juejin.cn/post/7325717778597904420

Stream toList()返回的是唯讀List原則上不可修改，collect(Collectors.toList())預設返回的是ArrayList,可以增刪改查

1. 背景

在公司看到開發環境突然發現了UnsupportedOperationException 報錯，想到了不是自己throw的應該就是操作collection不當。

發現的確是同事使用了類似stringList.stream().filter(number -> Long.parseLong(number) > 1).toList() 以stream.toList()作為返回，後繼續使用了返回值做add操作，導致報錯

2. Stream toList()和 collect(Collectors.toList())的區別

JDK version: 21

IDE: IDEA

從Java16開始，Stream有了直接toList方法， java8時候常用的方法是 stringList.stream().filter(number -> Long.parseLong(number) > 1).collect(Collectors.toList())。

Stream toList()

/** * Accumulates the elements of this stream into a {@code List}. The elements in * the list will be in this stream's encounter order, if one exists. The returned List * is unmodifiable; calls to any mutator method will always cause * {@code UnsupportedOperationException} to be thrown. There are no * guarantees on the implementation type or serializability of the returned List. * * <p>The returned instance may be <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>. * Callers should make no assumptions about the identity of the returned instances. * Identity-sensitive operations on these instances (reference equality ({@code ==}), * identity hash code, and synchronization) are unreliable and should be avoided. * * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>. * * @apiNote If more control over the returned object is required, use * {@link Collectors#toCollection(Supplier)}. * * @implSpec The implementation in this interface returns a List produced as if by the following: * <pre>{@code * Collections.unmodifiableList(new ArrayList<>(Arrays.asList(this.toArray()))) * }</pre> * * @implNote Most instances of Stream will override this method and provide an implementation * that is highly optimized compared to the implementation in this interface. * * @return a List containing the stream elements * * @since 16 */@SuppressWarnings("unchecked")default List<T> toList() {return (List<T>) Collections.unmodifiableList(new ArrayList<>(Arrays.asList(this.toArray()))); }

檢視源碼 Stream toList呼叫的是Collections.unmodifiableList 而在unmodifiableList(List<? extends T> list)實作中，都會返回一個不可修改的List，所以不能使用set/add/remove等改變list陣列的方法。

return (listinstanceof RandomAccess ?new UnmodifiableRandomAccessList<>(list) :new UnmodifiableList<>(list));

但其實也可以修改List的元素的某些內容，例如

List<String> stringList = List.of("1", "2", "3");List<String> largeNumberList = stringList.stream().filter(number -> Long.parseLong(number) > 1).toList();List<String> largeNumberList2 = stringList.stream().filter(number -> Long.parseLong(number) > 1).collect(Collectors.toList());// largeNumberList.add("4"); // java.lang.UnsupportedOperationException largeNumberList2.add("4"); //success// modify custom object attribute User userZhang = new User("ZhangSan"); User userLi = new User("LiSi");List<User> userList = List.of(userZhang, userLi);List<User> filterList = userList.stream().filter(user -> "LiSi".equals(user.name)).toList(); User first = filterList.getFirst();//java 21 first.name = "WangWu"; filterList.forEach(u -> System.out.println(u.name));//List.of返回的也是不能修改的List userList.forEach(u -> System.out.print(u.name));

輸出結果是：

WangWu

ZhangSanWangWu

Stream collect(Collectors.toList())

返回一個ArrayList 如果沒有在toList()方法入參中傳入指定Supplier的話，可以做ArrayList的任何操作

/** * Returns a {@code Collector} that accumulates the input elements into a * new {@code List}. There are no guarantees on the type, mutability, * serializability, or thread-safety of the {@code List} returned; if more * control over the returned {@code List} is required, use {@link #toCollection(Supplier)}. * * @param <T> the type of the input elements * @return a {@code Collector} which collects all the input elements into a * {@code List}, in encounter order */publicstatic <T> Collector<T, ?, List<T>> toList() {returnnew CollectorImpl<>(ArrayList::new, List::add, (left, right) -> { left.addAll(right); return left; }, CH_ID); }