在Java流中，map適用于一對一轉(zhuǎn)換，而flatMap用于一對多轉(zhuǎn)換或扁平化嵌套結(jié)構(gòu)。例如，使用map將字符串列表轉(zhuǎn)為大寫，每個(gè)元素生成一個(gè)結(jié)果；而flatMap可將嵌套列表展開，如將List>轉(zhuǎn)為單一流，也可處理Optional值。關(guān)鍵區(qū)別在于：map將每個(gè)元素轉(zhuǎn)為新元素，而flatMap將每個(gè)元素轉(zhuǎn)為流后再合并為一個(gè)流。常見誤區(qū)包括誤用map導(dǎo)致出現(xiàn)嵌套流，或混淆返回類型，此時(shí)編譯器報(bào)錯(cuò)提示應(yīng)改用flatMap。

The difference between map and flatMap in Java Streams comes down to how they transform elements in a stream, especially when dealing with nested collections or streams.

When to Use map

Use map when you want to apply a function to each element of a stream and get a new element for each. It's straightforward: one input item becomes one transformed output item.

For example, if you have a list of strings and you want to convert them all to uppercase:

List<String> upperCase = list.stream()
                                .map(String::toUpperCase)
                                .toList();

Each string is processed individually, and the result is a stream of the same number of items — just transformed.

When to Use flatMap

Use flatMap when your transformation results in multiple elements per original element — like when you're dealing with nested lists or optional values.

A common scenario is flattening a list of lists into a single list. For instance, say you have this:

List<List<String>> lists = List.of(
    List.of("a", "b"), 
    List.of("c"), 
    List.of("d", "e")
);

To turn that into a single list of strings (["a", "b", "c", "d", "e"]), you'd do:

List<String> flattened = lists.stream()
                                 .flatMap(List::stream)
                                 .toList();

Here, flatMap takes each inner list and pulls its elements out into the top-level stream.

Another use case is working with Optional. If you have a stream of objects and a method that returns an Optional, using flatMap allows you to include the value only if it exists.

Key Differences Summary

• map transforms each element into a single new element.
• flatMap transforms each element into a stream (or something that can be turned into a stream), then flattens all those streams into one.

So:

• Use map for 1-to-1 transformations.
• Use flatMap for 1-to-many transformations or flattening nested structures.

Common Pitfalls

• Using map instead of flatMap when you expect to flatten — this leads to a Stream<stream>></stream>, which isn't usually what you want.
• Misunderstanding return types: map expects a direct object, while flatMap expects something that can become a stream (like another stream, an optional, or even a collection).

You’ll often find yourself switching from map to flatMap when the compiler complains about nested streams — that’s a good hint you need to flatten things.

基本上就這些.

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