如何在ArrayList中打印键和HashMap中的平均值

Question

我已经能够写代码，但我不能打印出钥匙和相应的平均数。

这是我的代码：

Map<String,List<Double>> stdMap = new HashMap<>();

//Create arraylist of integers to store student's marks

//Create and initialise at creation

ArrayList<Double> Mark1 = new ArrayList<>(Arrays.asList(78.0,68.0,82.0));
ArrayList<Double> Mark2 = new ArrayList<>(Arrays.asList(80.0,73.0));
ArrayList<Double> Mark3 = new ArrayList<>(Arrays.asList(72.0,81.0,75.0));
ArrayList<Double> Mark4 = new ArrayList<>(Arrays.asList(83.0,76.0,65.0,93.0));
ArrayList<Double> Mark5 = new ArrayList<>(Arrays.asList(85.0,78.0,77.0));

//Add content to HashMap
stdMap.put("Nii", Mark1);
stdMap.put("Felicity", Mark2);
stdMap.put("Evelyn", Mark3);
stdMap.put("Samuelis", Mark4);
stdMap.put("Bertina", Mark5);  //(1a)Print out Each student's content from hashmap
     System.out.println("\tStudent Results: \n");
     for(Map.Entry<String, ArrayList<Double>>stdMapEntry: stdMap.entrySet()){
        System.out.println("\t\t Student Name:" + stdMapEntry.getKey());
        System.out.println("\t\t Student Marks:" + stdMapEntry.getValue()+"\n");
    }
//(1b) Finding the Average Mark of Each Student
     System.out.println("\tAverage Mark: \n");


for (Map.Entry<String, ArrayList<Double>> stdMapEntry: stdMap.entrySet()) {
        double averageNumber;
        double sum = 0;

    String key = stdMapEntry.getKey();           
    for(int i=0; i<stdMapEntry.getValue().size(); i++){
        sum += stdMapEntry.getValue().get(i);

    }   
    averageNumber = sum / stdMapEntry.getValue().size();
    System.out.println("\nThis is the averaged map: ");
    System.out.println(stdMap.put(key, averageNumber)); // won't work
}

Answer 1

您正在计算平均值，但是您试图将平均值插入到Hashmap中。由于hashmap有一个类型为值的ArrayList，但是您要在其中插入一个double，这将给出编译时错误。

HashMap是无序的，所以它不一定按照数据插入的相同顺序给出entrySet (就像ArrayList)。因此，您获得的entrySet的顺序可能是不同的。如果结果的顺序无关紧要，则只需打印键和平均值，而不必将其插入Hashmap中。

您可以这样修改代码：

​

for (Map.Entry<String, ArrayList<Double>> stdMapEntry: stdMap.entrySet()) {
            double averageNumber;
            double sum = 0;

            System.out.println("key: " + stdMapEntry.getKey());

            String key = stdMapEntry.getKey();
            for(int i=0; i<stdMapEntry.getValue().size(); i++){
                sum += stdMapEntry.getValue().get(i);

            }
            averageNumber = sum / stdMapEntry.getValue().size();
            System.out.println("average value:" + averageNumber);
        }

​