JAVA - Storing data from result set to hashmap and aggregating them

Question

as the title, I'd like to store data from result set to hash map and then use them for further processing (max, min, avg, grouping).

So far, I achieved this by using a proper hash map and implementing each operation from scratch - iterating over the hash map (key, value) pairs.

My question is: does it exist a library that performs such operations? For example, a method that computes the maximum value over a List or a method that, given two same-size arrays, performs a "index-to-index" difference.

Thanks in advance.

Answer 1

Well there is the Collection class for instance. There is a bunch of useful static methods but you'll have to read and choose the one you need. Here is the documentation:

https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html

This class consists exclusively of static methods that operate on or return collections.

Example:

        List<Integer> list = new ArrayList<>();
        List<String> stringList = new ArrayList<>();

        // Populate the lists
        for(int i=0; i<=10; ++i){
            list.add(i);
            String newString = "String " + i;
            stringList.add(newString);
        }
        // add another negative value to the integer list
        list.add(-1939);

        // Print the min value from integer list and max value form the string list.

        System.out.println("Max value: " + Collections.min(list));
        System.out.println("Max value: " + Collections.max(stringList));

The output will be:

run:
Max value: -1939
Max value: String 9
BUILD SUCCESSFUL (total time: 0 seconds)

Similar question, however, was answered before for example here: how to get maximum value from the List/ArrayList

Answer 2

There are some usefull functions in Collections API already.

For example max or min

Collections.max(arrayList);

Please investigate collections documentation to see if there is a function that you need. Probably there woulde be.

Answer 3

You can use java 8 streams for this.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Testing {
    public static void main(String[] args) {

        //List of integers
    List<Integer> list = new ArrayList<>();
    list.add(7);
    list.add(5);
    list.add(4);
    list.add(6);
    list.add(9);
    list.add(11);
    list.add(12);

    //get sorted list using streams
    System.out.println(list.stream().sorted().collect(Collectors.toList()));

        //find min value in list
        System.out.println(list.stream().min(Integer::compareTo).get());

        //find max value in list
        System.out.println(list.stream().max(Integer::compareTo).get());

        //find average of list
        System.out.println(list.stream().mapToInt(val->val).average().getAsDouble());


        //Map of integers
        Map<Integer,Integer> map = new HashMap<>();

        map.put(1, 10);
        map.put(2, 12);
        map.put(3, 15);

        //find max value in map
        System.out.println(map.entrySet().stream().max((entry1,entry2) -> entry1.getValue() > entry2.getValue() ? 1: -1).get().getValue());

        //find key of max value in map
        System.out.println(map.entrySet().stream().max((entry1,entry2) -> entry1.getValue() > entry2.getValue() ? 1: -1).get().getKey());

        //find min value in map
        System.out.println(map.entrySet().stream().min((entry1,entry2) -> entry1.getValue() > entry2.getValue() ? 1: -1).get().getValue());

        //find key of max value in map
        System.out.println(map.entrySet().stream().min((entry1,entry2) -> entry1.getValue() > entry2.getValue() ? 1: -1).get().getKey());

        //find average of values in map
        System.out.println(map.entrySet().stream().map(Map.Entry::getValue).mapToInt(val ->val).average().getAsDouble());
    }



}

Keep in mind that it will only work if your system has jdk 1.8 .For lower version of jdk streams are not supported.

Answer 4

In Java8 there are IntSummaryStatistics , LongSummaryStatistics , DoubleSummaryStatistics to calculate max , min , count , average and sum

public static void main(String[] args) {
    List<Employee> resultSet = ...
    Map<String, DoubleSummaryStatistics> stats = resultSet.stream().collect(Collectors.groupingBy(Employee::getName, Collectors.summarizingDouble(Employee::getSalary)));
    stats.forEach((n, stat) -> System.out.println("Name " + n + " Average " + stat.getAverage() + " Max " + stat.getMax())); // min, sum, count can also be taken from stat
}

static class Employee {
    String name;
    Double salary;

    public String getName() {
        return name;
    }

    public Double getSalary() {
        return salary;
    }
}

Answer 5

对于max，min，avg，您可以使用Java 8及其流处理。