根据数组列表中的百分比计算记录数

Question

I have an Arraylist which contains a list of records and in each single record I have percentage field.

I am trying to show the count of students who have percentage in between

40% to 50% , 51% to 60% , 61% to 70% , 71% to 80% , 81% to 90% , 91% to 100% .

For Example, Suppose there are 5 students in between 61% to 70% I want to get the count as 5. How can I get count of all students with specified percentages dynamically . Can anyone please suggest me the approach to follow in this regard.

Answer 1

Don't count Students, count their percentages, like this:

int count = list.stream()
    .map(Student::getPercentage)
    .filter(n -> n >= low && n <= high)
    .count();

Being a Collection, an ArrayList lets us stream the Student objects, which are then converted to a stream of percentages, which then is filtered to keep only those percentages in the desired range, which are then counted.

---

To collect and iterate though ranges of 10 point brackets:

new TreeMap<>(list.stream()
    .collect(Collectors.groupingBy(s -> s.getPercentage() / 10, Collectors.counting()))
.forEach((r, n) -> System.out.print(r + "0% to " + ++r + "0% had " + n));

This groups by the result of integer division by 10 (add a cast to the percentage to int if it's not an integer), adds a downstream counting operation, then processes each one.

The TreeMap wrapper puts the percentage ranges in order.

Answer 2

I would take the stream-approach a bit further:

Map<PercentageRange, Integer> counts =
    list.stream()
        .collect(Collectors.groupingBy( 
             student -> rangeOf(student.getPercentage())), 
             Collectors.counting()));

Answer 3

You should use a lambda expression. This will get you a map with all specified ranges and the students.

ArrayList<Student> students = new ArrayList<>();

//Each element will have the range as key and all students who fit in the range.
Map<MinMax, List<Student>> map = new HashMap();

//Add all ranges you want to filter your students-list by.
ArrayList<MinMax> minMaxes = new ArrayList<>();
minMaxes.add(new MinMax(20,40);
minMaxes.add(new MinMax(40,60);
minMaxes.add(new MinMax(60,80);

for (MinMax minMax : minMaxes) {
  map.put(minMax, students.stream().filter(u -> u.percentage < minMax.min && u.percentage > minMax.max).collect(Collectors.toList()));
}

class MinMax {
  private int min;
  private int max;


  public MinMax(int min, int max) {
    this.min = min;
    this.max = max;
  }

  public int getMin() {
    return min;
  }
  public void setMin(int min) {
    this.min = min;
  }
  public int getMax() {
    return max;
  }
  public void setMax(int max) {
    this.max = max;
  }

}