受到@skiwi的启发，我用下面的方法解决了这个问题:

public static <T> T toSingleton(Stream<T> stream) {
    List<T> list = stream.limit(1).collect(Collectors.toList());
    if (list.isEmpty()) {
        return null;
    } else {
        return list.get(0);
    }
}

然后:

User user = toSingleton(users.stream().filter(...).map(...));

使用减少

这是我发现的更简单灵活的方法(基于@prunge的答案)

Optional<User> user = users.stream()
        .filter(user -> user.getId() == 1)
        .reduce((a, b) -> {
            throw new IllegalStateException("Multiple elements: " + a + ", " + b);
        })

这样你就可以得到:

Optional -和你的对象一样，如果不存在则使用Optional.empty() 如果有多个元素，则使用Exception(最终使用YOUR自定义类型/消息)

我自己尝试了一个示例代码，这里是解决方案。

User user = Stream.of(new User(2), new User(2), new User(1), new User(2))
            .filter(u -> u.getAge() == 2).findFirst().get();

和用户类

class User {
    private int age;

public User(int age) {
    this.age = age;
}

public int getAge() {
    return age;
}

public void setAge(int age) {
    this.age = age;
 }
}

public List<state> getAllActiveState() {
    List<Master> master = masterRepository.getActiveExamMasters();
    Master activeMaster = new Master();
    try {
        activeMaster = master.stream().filter(status -> status.getStatus() == true).reduce((u, v) -> {
            throw new IllegalStateException();
        }).get();
        return stateRepository.getAllStateActiveId(activeMaster.getId());
    } catch (IllegalStateException e) {
        logger.info(":More than one status found TRUE in Master");
        return null;
    }
}

In this above code, As per the condition if its find more than one true in the list then it will through the exception. When it through the error will showing custom message because it easy maintain the logs on server side. From Nth number of element present in list just want only one element have true condition if in list there are more than one elements having true status at that moment it will through an exception. after getting all the this we using get(); to taking that one element from list and stored it into another object. If you want you added optional like Optional<activeMaster > = master.stream().filter(status -> status.getStatus() == true).reduce((u, v) -> {throw new IllegalStateException();}).get();

受到@skiwi的启发，我用下面的方法解决了这个问题:

public static <T> T toSingleton(Stream<T> stream) {
    List<T> list = stream.limit(1).collect(Collectors.toList());
    if (list.isEmpty()) {
        return null;
    } else {
        return list.get(0);
    }
}

然后:

User user = toSingleton(users.stream().filter(...).map(...));

其他涉及编写自定义Collector的答案可能更有效(如Louis Wasserman的+1)，但如果你想要简洁，我建议如下:

List<User> result = users.stream()
    .filter(user -> user.getId() == 1)
    .limit(2)
    .collect(Collectors.toList());

然后验证结果列表的大小。

if (result.size() != 1) {
  throw new IllegalStateException("Expected exactly one user but got " + result);
User user = result.get(0);
}