Java流创建对象并进行比较

Question

I'm pretty new to Java streams. I've to split a string returned by filter in stream, create a new object with the strings in the split and compare each object with a predefined object. Stream looks like this (I know this is incorrect, just a representation of what I am trying to do):

xmlstream.stream()
         .filter(xml->xml.getName()) //returns a string
         .map(returnedString -> split("__"))
         .map(eachStringInList -> new TestObj(returnedStr[0], returnedStr[1]))
         .map(eachTestObj -> eachTestObj.compareTo(givenObj))
         .max(Comparing.compare(returnedObj :: aProperty))

How do I achieve this? Basically map each string in list to create an object, compare that to a fix object and return max based on one of the properties. Thanks.

Answer 1

You could use reduce like so:

TestObj predefined = ...

TestObj max = 
       xmlstream.stream()
                .map(xml -> xml.getName()) //returns a string
                .map(s -> s.split("__"))
                .map(a -> new TestObj(a[0], a[1]))
                .reduce(predifined, (e, a) -> 
                      e.aProperty().compareTo(a.aProperty()) >= 0 ? e : a);

A more efficient version of the above would be:

TestObj predefined = ...
TestObj max =
        xmlstream.stream()
                 .map(xml -> xml.getName()) //returns a string
                 .map(s -> s.split("__"))
                 .map(a -> new TestObj(a[0], a[1]))
                 .filter(e -> e.aProperty().compareTo(predefined.aProperty()) > 0)
                 .findFirst()
                 .orElse(predefined);

Update:

if you want to retrieve the max object by a given property from all the TestObj objects less than the predefined TestObj , then you can proceed as follows:

TestObj predefined = ...
Optional<TestObj> max =
             xmlstream.stream()
                      .map(xml -> xml.getName()) 
                      .map(s -> s.split("_"))
                      .map(a -> new TestObj(a[0], a[1]))
                      .filter(e -> e.aProperty().compareTo(predefined.aProperty()) < 0)
                      .max(Comparator.comparing(TestObj::aProperty));

max returns an Optional<T> ; if you're unfamiliar with it then consult the documentation here to familiarise you're with the different ways to unwrap an Optional<T> object.