java 了解 Optionals.orElse
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Understanding Optionals.orElse
提问by silentsudo
I am trying to learn Java 8 feature Optional. I am confused about how Optional.orElse is working. Here is what i have tried:
我正在尝试学习Java 8 功能 Optional。我对 Optional.orElse 的工作方式感到困惑。这是我尝试过的：
public class OptionalsExample {
public static void main(String[] args) {
Name userName = new Name();
userName.setName("John Doe");
Optional<Name> optionalName = Optional.ofNullable(userName);
optionalName.ifPresent(
(Name value) -> {
Optional<String> optionalNameString = Optional.ofNullable(value.getName());
optionalNameString.ifPresent(name -> System.err.println("Name is: " + optionalNameString.get()));
}
);
}
private static void printError() {
System.err.println("No Name present");
}
}
My Concern is that if name is set everything works fine but when no name is set i want to execute orElse. I want to do something when i comment
我担心的是，如果设置了 name 一切正常，但是当没有设置 name 时，我想执行orElse。我评论的时候想做点什么
userName.setName("John Doe");
like printing No Name Found
喜欢打印 No Name Found
How can i do that?
我怎样才能做到这一点？
TIA
TIA
回答by Holger
Your usage of nested ifPresentcalls is very similar to the (nested) forEachcalls on streams, seen in a lot of other questions.
您对嵌套ifPresent调用的使用与对流的（嵌套）forEach调用非常相似，可以在许多其他问题中看到。
They are a clear sign that you should try better on functional thinking.
它们清楚地表明你应该更好地尝试功能性思维。
What you want to do is
你想做的是
System.out.println(
Optional.ofNullable(userName) // will be an empty optional if userName is null
.map(Name::getName) // will turn to empty optional if getName returns null
.map("Name is: "::concat) // prepend "Name is: " (only when we have a name)
.orElse("No Name Found") // get the result string or the alternative
);
The important point to understand, is, that the mapping steps applied to an empty Optionaldon't do anything but return an empty Optionalagain. Since you want to print in either case, the print statement is not passed as consumer, but written as stand-alone statement, printing the value returned by the orElseinvocation, which will be either, the non-nullresult of the processing steps or the alternative String.
要理解的重要一点是，map应用于空的ping 步骤Optional除了Optional再次返回空之外什么都不做。由于您想在任何一种情况下都打印，因此打印语句不是作为消费者传递的，而是作为独立语句编写的，打印orElse调用返回的值，这将是null处理步骤的非结果或替代String.
回答by explv
The orElse method allows you to return an alternative value when there isn't one in the Optional.
当 Optional 中没有替代值时，orElse 方法允许您返回一个替代值。
If you just want to print out an alternative String you can use the orElse method to return an alternative String:
如果您只想打印一个替代字符串，您可以使用 orElse 方法返回一个替代字符串：
Optional<String> optionalNameString = Optional.ofNullable(value.getName());
System.out.println(optionalNameString.orElse("Name is not present"));
Otherwise you can just check if the value is present using isPresent()
否则，您可以使用 isPresent() 检查该值是否存在
Optional<String> optionalNameString = Optional.ofNullable(value.getName());
if(optionalNameString.isPresent()) {
// do something
} else {
//do something else
}
