Java 8 可选：ifPresent 返回对象 orElseThrow 异常
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Java 8 optional: ifPresent return object orElseThrow exception
提问by RichardK
I'm trying to make something like this:
我正在尝试做这样的事情：
private String getStringIfObjectIsPresent(Optional<Object> object){
object.ifPresent(() ->{
String result = "result";
//some logic with result and return it
return result;
}).orElseThrow(MyCustomException::new);
}
This won't work, because ifPresent takes Consumer functional interface as parameter, which has void accept(T t). It cannot return any value. Is there any other way to do it ?
这是行不通的，因为 ifPresent 将 Consumer 功能接口作为参数，该接口具有 void accept(T t)。它不能返回任何值。有没有其他方法可以做到？
采纳答案by Roland
Actually what you are searching is: Optional.map. Your code would then look like:
实际上你正在搜索的是：Optional.map。您的代码将如下所示：
object.map(o -> "result" /* or your function */)
.orElseThrow(MyCustomException::new);
I would rather omit passing the Optionalif you can. In the end you gain nothing using an Optionalhere. A slightly other variant:
Optional如果可以的话，我宁愿省略传递。最后，使用Optionalhere 将一无所获。一个稍微不同的变体：
public String getString(Object yourObject) {
if (Objects.isNull(yourObject)) { // or use requireNonNull instead if NullPointerException suffices
throw new MyCustomException();
}
String result = ...
// your string mapping function
return result;
}
If you already have the Optional-object due to another call, I would still recommend you to use the map-method, instead of isPresent, etc. for the single reason, that I find it more readable (clearly a subjective decision ;-)).
如果Optional由于另一个调用您已经有了-object，我仍然建议您使用map-method，而不是isPresent等，原因只有一个，我觉得它更具可读性（显然是主观决定；-)）。
回答by marstran
Use the map-function instead. It transforms the value inside the optional.
改用map- 函数。它转换可选中的值。
Like this:
像这样：
private String getStringIfObjectIsPresent(Optional<Object> object) {
return object.map(() -> {
String result = "result";
//some logic with result and return it
return result;
}).orElseThrow(MyCustomException::new);
}
回答by Vlad Bochenin
Two options here:
这里有两个选项：
Replace ifPresentwith mapand use Functioninstead of Consumer
替换ifPresent为map并使用Function代替Consumer
private String getStringIfObjectIsPresent(Optional<Object> object) {
return object
.map(obj -> {
String result = "result";
//some logic with result and return it
return result;
})
.orElseThrow(MyCustomException::new);
}
Use isPresent:
使用isPresent：
private String getStringIfObjectIsPresent(Optional<Object> object) {
if (object.isPresent()) {
String result = "result";
//some logic with result and return it
return result;
} else {
throw new MyCustomException();
}
}
回答by iamiddy
I'd prefer mapping after making sure the value is available
在确保该值可用后，我更喜欢映射
private String getStringIfObjectIsPresent(Optional<Object> object) {
Object ob = object.orElseThrow(MyCustomException::new);
// do your mapping with ob
String result = your-map-function(ob);
return result;
}
or one liner
或一个班轮
private String getStringIfObjectIsPresent(Optional<Object> object) {
return your-map-function(object.orElseThrow(MyCustomException::new));
}
