为什么列表中的最后一个元素显示为 null

Question

The code:

public class main {
public static void countFrequencies(ArrayList<String> list,int n)
{
    int k=0;
    Map<String, Integer> hm = new HashMap<String, Integer>();

    for (String i : list) {
        Integer j = hm.get(i);
        hm.put(i, (j == null) ? 1 : j + 1);
    }

    for (Map.Entry<String, Integer> val : hm.entrySet()) {
        System.out.println("Element " + val.getKey() + " "
                + "occurs"
                + ": " + val.getValue() + " times");
        k++;
        if(k==n)
            break;
    }
}

public static void main(String[] args)  {
    String nr1;
    ArrayList<String> nr = new ArrayList<String>();
    Scanner obj= new Scanner(System.in);
    System.out.println("Input n");
    int n= obj.nextInt();

    System.out.println("Input k ");
    int k= obj.nextInt();
    System.out.println("Input n elemente: ");
    for(int i=1;i<=n;i++)
    {
            nr1=obj.nextLine();
            System.out.println("Element " + (i));
            nr.add(nr1);
    }
    countFrequencies(nr,k);


}

}

If I run the code everything goes ok until I input the last element in the list, if do It appears as null, it simply doesn't let me input it, also when I print the frequencies list I need it to show the top k elements after the number of frequencies, in the descending order.

Answer 1

   public static void countFrequencies(ArrayList<String> list,int n)
{
    int k=0;
    Map<String, Integer> hm = new HashMap<String, Integer>();
    Map<String, Integer> hm2 = new LinkedHashMap<>();

    for (String i : list) {
        Integer j = hm.get(i);
        hm.put(i, (j == null) ? 1 : j + 1);
    }

    hm.entrySet().stream()
            .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
            .forEachOrdered(i -> hm2.put(i.getKey(),i.getValue()));

    for (Map.Entry<String, Integer> val : hm2.entrySet()) {
        System.out.println("Element " + val.getKey() + " "
                + "occurs"
                + ": " + val.getValue() + " times");
        k++;
        if(k==n)
            break;
    }
}
public static void main(String[] args) {
    String nr1;
    ArrayList<String> nr = new ArrayList<String>();
    Scanner obj= new Scanner(System.in);
    System.out.println("Input n");
    int n= obj.nextInt();

    System.out.println("Input k ");
    int k= obj.nextInt();
    System.out.println("Input n elemente: ");
    obj.nextLine(); // Consuming the space
    for(int i=1;i<=n;i++)
    {
        nr1=obj.nextLine();
        System.out.println("Element " + (i));
        nr.add(nr1);
    }
    countFrequencies(nr,k);
}

1. You are getting last element as null because your code considering the spaces as input. Hence I consumed the extra space.
2. And to print it in decreasing order you need another LinkedHashMap which actually maintains the insertion order, and I've used stream to sorted the HashMap by values in reverse Order.

Answer 2

The code:

public class main {
public static void countFrequencies(ArrayList<String> list,int n)
{
    int k=0;
    Map<String, Integer> hm = new HashMap<String, Integer>();

    for (String i : list) {
        Integer j = hm.get(i);
        hm.put(i, (j == null) ? 1 : j + 1);
    }

    for (Map.Entry<String, Integer> val : hm.entrySet()) {
        System.out.println("Element " + val.getKey() + " "
                + "occurs"
                + ": " + val.getValue() + " times");
        k++;
        if(k==n)
            break;
    }
}

public static void main(String[] args)  {
    String nr1;
    ArrayList<String> nr = new ArrayList<String>();
    Scanner obj= new Scanner(System.in);
    System.out.println("Input n");
    int n= obj.nextInt();

    System.out.println("Input k ");
    int k= obj.nextInt();
    System.out.println("Input n elemente: ");
    for(int i=1;i<=n;i++)
    {
            nr1=obj.nextLine();
            System.out.println("Element " + (i));
            nr.add(nr1);
    }
    countFrequencies(nr,k);


}

}

If I run the code everything goes ok until I input the last element in the list, if do It appears as null, it simply doesn't let me input it, also when I print the frequencies list I need it to show the top k elements after the number of frequencies, in the descending order.