如何用Java 8 Stream根据条件为对象列表设置值（含if-else逻辑）

Let's break down the issues with your current code first, then fix it properly:

What's wrong with your existing code? #

1. Chained filters make the second condition unreachable: After filtering for User::isActive, your stream only contains active users. The subsequent filter(user -> !user.isActive()) will have zero elements to process, so that peek call never runs.
2. No terminal operation: Stream intermediate operations (like filter, peek) don't execute until a terminal operation (like forEach, collect, count) is called. Your code doesn't trigger the stream at all, so none of the setText calls actually modify your users list.

Correct implementations #

Since you're modifying the existing User objects in your list, here are the simplest, most readable approaches:

Option 1: Use forEach with inline if-else (most straightforward)

List<User> users = userDao.getAllByCompanyId(companyId);
users.stream()
    .filter(Objects::nonNull) // Guard against null entries
    .forEach(user -> {
        if (user.isActive()) {
            user.setText("ABC");
        } else {
            user.setText("XYZ");
        }
    });

forEach is a terminal operation that triggers the stream execution, and putting the if-else logic in one lambda handles both cases without splitting your stream.

Option 2: Use peek with a terminal operation (if you prefer peek syntax)

If you want to stick with peek, you just need to add a terminal operation to trigger the stream:

users.stream()
    .filter(Objects::nonNull)
    .peek(user -> {
        user.setText(user.isActive() ? "ABC" : "XYZ");
    })
    .count(); // Terminal operation to run the stream

We use count() here because we don't need the result—it just forces the stream to process all elements.

Bonus: Create new User objects (instead of modifying originals)

If you want to keep your original list unchanged and create a new list of updated users, use map:

List<User> updatedUsers = users.stream()
    .filter(Objects::nonNull)
    .map(user -> {
        // Create a new User instance to avoid modifying the original
        User updatedUser = new User();
        updatedUser.setId(user.getId());
        updatedUser.setName(user.getName());
        updatedUser.setActive(user.isActive());
        updatedUser.setText(user.isActive() ? "ABC" : "XYZ");
        return updatedUser;
    })
    .collect(Collectors.toList());

内容的提问来源于stack exchange，提问作者Newbie