Java 8 lambda filtering based on condition as well as order

Question

I was trying to filter a list based on multiple conditions, sorting.

class Student{
        private int Age;
        private String className;
        privat         


        

Answer 1

Use the toMap collector:

Collection values = students.stream()
                .collect(toMap(Student::getName,
                        Function.identity(),
                        BinaryOperator.maxBy(Comparator.comparingInt(Student::getAge))))
                .values();

Explanation

We're using this overload of toMap:

toMap​(Function keyMapper,
      Function valueMapper,
      BinaryOperator mergeFunction)

• Student::getName above is the keyMapper function used to extract the values for the map keys.
• Function.identity() above is the valueMapper function used to extract the values for the map values where Function.identity() simply returns the elements in the source them selves i.e. the Student objects.
• BinaryOperator.maxBy(Comparator.comparingInt(Student::getAge)) above is the merge function used to "decide which Student object to return in the case of a key collission i.e. when two given students have the same name" in this case taking the oldest Student .
• Finally, invoking values() returns us a collection of students.

The equivalent C# code being:

var values = students.GroupBy(s => s.Name, v => v,
                          (a, b) => b.OrderByDescending(e => e.Age).Take(1))
                      .SelectMany(x => x);

Explanation (for those unfamiliar with .NET)

We're using this extension method of GroupBy:

System.Collections.Generic.IEnumerable GroupBy 
       (this System.Collections.Generic.IEnumerable source, 
         Func keySelector, 
         Func elementSelector, 
     Func,TResult> resultSelector);

• s => s.Name above is the keySelector function used to extract the value to group by.
• v => v above is the elementSelector function used to extract the values i.e. the Student objects them selves.
• b.OrderByDescending(e => e.Age).Take(1) above is the resultSelector which given an IEnumerable represented as b takes the oldest student.
• Finally, we apply .SelectMany(x => x); to collapse the resulting IEnumerable> into a IEnumerable.