groupingby-and-mapping-example Map in andorid

 

https://www.mkyong.com/java8/java-8-collectors-groupingby-and-mapping-example/

Map<String, List<PieDataChart>> map = new HashMap<String, List<PieDataChart>>();



for (PieDataChart student : lstItemArry) {

    String key  = String.valueOf(student.getProduct_id());

    if(map.containsKey(key)){

        List<PieDataChart> list = map.get(key);

        list.add(student);



    }else{

        List<PieDataChart> list = new ArrayList<PieDataChart>();

        list.add(student);

        map.put(key, list);

    }



}

for (Map.Entry<String, List<PieDataChart>> entry : map.entrySet()) {

   //  System.out.println("Key : " + entry.getKey() + " value : " + entry.getValue());



    // for (int i=0 ; i< entry.getKey().length(); i++){

     int sum = 0, value;

         for (int j = 0; j < entry.getValue().size(); j++){



             value = Integer.valueOf(String.valueOf(entry.getValue().get(j)));

             //System.out.println(j+ "value = " + value);

             sum += value;



           //  System.out.println(j+"chgevalue = " + sum);

         }

// }

     System.out.println( "Key : " + entry.getKey() + "qty = " + sum);



 }

 

2.2 Group by the name + Count or Sum the Qty.

Java8Examples3.java

package com.mkyong.java8;

import java.math.BigDecimal;

import java.util.Arrays;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

public class Java8Examples3 {

    public static void main(String[] args) {

        //3 apple, 2 banana, others 1

        List<Item> items = Arrays.asList(

                new Item("apple", 10, new BigDecimal("9.99")),

                new Item("banana", 20, new BigDecimal("19.99")),

                new Item("orang", 10, new BigDecimal("29.99")),

                new Item("watermelon", 10, new BigDecimal("29.99")),

                new Item("papaya", 20, new BigDecimal("9.99")),

                new Item("apple", 10, new BigDecimal("9.99")),

                new Item("banana", 10, new BigDecimal("19.99")),

                new Item("apple", 20, new BigDecimal("9.99"))

        );

        Map<String, Long> counting = items.stream().collect(

                Collectors.groupingBy(Item::getName, Collectors.counting()));

        System.out.println(counting);

        Map<String, Integer> sum = items.stream().collect(

                Collectors.groupingBy(Item::getName, Collectors.summingInt(Item::getQty)));

        System.out.println(sum);

    }

}

Copy

output

//Group by + Count

{

        papaya=1, banana=2, apple=3, orang=1, watermelon=1

}

//Group by + Sum qty

{

        papaya=20, banana=30, apple=40, orang=10, watermelon=10

}

 

 

https://www.codingame.com/playgrounds/6162/6-ways-to-iterate-or-loop-a-map-in-java

Using entrySet()

// { autofold

import java.util.HashMap;

import java.util.Iterator;

import java.util.Map;

public class Main {

public static void main(String[] args) {

// }

    Map<Integer, String> customers = new HashMap<>();

              customers.put(1, "Jhon");

              customers.put(2, "Smith");

              customers.put(3, "Sally");

    for (Map.Entry<Integer, String> entry : customers.entrySet()) {

      System.out.println("Key : " + entry.getKey() + " value : " + entry.getValue());

    }

https://www.baeldung.com/java-array-sum-average

2.1. Sum Using a For Loop

In order to find the sum of all elements in an array, we can simply iterate the array and add each element to a sum accumulating variable.

This very simply starts with a sum of 0 and add each item in the array as we go:

1 2 3 4 5 6 7

public static int findSumWithoutUsingStream(int[] array) {     int sum = 0;     for (int value : array) {         sum += value;     }     return sum; }