可能有人請解釋Date maxDate = list.stream（）。map（u - > u.date）.max（Date :: compareTo）.get（）;

Question

可能有人請解釋這一點：

Date maxDate = list.stream().map(u -> u.date).max(Date::compareTo).get(); 

Answer 1

理解將是分解成小塊，最簡單的。想象一個持有Date object的類：

static class Holder { 
    public Date date; 

    public Holder(Date d) { 
     super(); 
    } 
} 

List<Holder> list= Arrays.asList(new Holder(new Date())); 

// creates a Stream<Holder> having the list as the source 
Stream<Holder> s1 = list.stream(); 

// creates a Stream<Date> by taking the previous elements from the Stream 
// and mapping those to `Date date` 
Stream<Date> s2 = s1.map(u -> u.date); 

// consumes the stream by invoking max using the compareTo method 
// two Date objects are Comparable by invoking compareTo 
Optional<Date> optionalDate = s2.max(Date::compareTo); 

// gets the maxValue from that Optional 
// if the initial list is empty, your last get will throw a NoSuchElementException 
Date maxDate = optionalDate.get(); 

Answer 2

Date maxDate = 
    list.stream() // create a Stream<TheElementTypeOfTheList> 
     .map(u -> u.date) // map each element of a Date, thus creating a Stream<Date> 
     .max(Date::compareTo) // find the max Date of the Stream 
     .get(); // return that max Date (will throw an exception if the 
       // list is empty)