Java 如何正确返回方法的 Optional<>？
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How to properly return Optional<> of a method?
提问by Nimrod
I have read a lot of Java 8 Optional and I do understand the concept, but still get difficulties when trying to implement it myself in my code.
我已经阅读了很多 Java 8 Optional 并且我确实理解这个概念，但是在尝试在我的代码中自己实现它时仍然遇到困难。
Although I have serached the web for good examples, I didn't found one with good explanation.
虽然我已经在网上搜索了很好的例子，但我没有找到一个很好的解释。
I have the next method:
我有下一个方法：
public static String getFileMd5(String filePath) throws NoSuchAlgorithmException, IOException {
AutomationLogger.getLog().info("Trying getting MD5 hash from file: " + filePath);
MessageDigest md = MessageDigest.getInstance("MD5");
InputStream inputStream;
try {
inputStream = Files.newInputStream(Paths.get(filePath));
} catch (NoSuchFileException e) {
AutomationLogger.getLog().error("No such file path: " + filePath, e);
return null;
}
DigestInputStream dis = new DigestInputStream(inputStream, md);
byte[] buffer = new byte[8 * 1024];
while (dis.read(buffer) != -1);
dis.close();
inputStream.close();
byte[] output = md.digest();
BigInteger bi = new BigInteger(1, output);
String hashText = bi.toString(16);
return hashText;
}
This simple method returns the md5 of a file, by passing it the file path. As you can notice, if the file path doesn't exists (or wrong typed) a NoSuchFileExceptionget thrown and the method return Null.
这个简单的方法通过将文件路径传递给文件来返回文件的 md5。如您所见，如果文件路径不存在（或输入错误），则会抛出NoSuchFileException并且该方法返回Null。
Instead of returning null, I want to use Optional, so my method should return Optional <String>, right?
我想使用 Optional 而不是返回 null，所以我的方法应该返回Optional <String>，对吗？
- What is the proper way of doing it right?
- If the returned String is null - can I use here
orElse(), or this kind of method should be used by the client side?
- 正确的做法是什么？
- 如果返回的字符串为空 - 我可以在这里使用
orElse()，还是客户端应该使用这种方法？
采纳答案by Joop Eggen
Right.
对。
public static Optional<String> getFileMd5(String filePath)
throws NoSuchAlgorithmException, IOException {
return Optional.empty(); // I.o. null
return Optional.of(nonnullString);
}
Usage:
用法：
getFileMd5(filePath).ifPresent((s) -> { ... });
or (less nice as undoing the Optional)
或（不如撤消 Optional 好）
String s = getFileMd5(filePath).orElse("" /* null */);
