如何计算ArrayList中的重复元素？ [英] How to count duplicate elements in ArrayList?

问题描述

我需要分开并计算arraylist中有多少个值是相同的，并根据出现次数打印出来。

I need to separate and count how many values in arraylist are the same and print them according to the number of occurrences.

我有一个名为digits的arraylist ：

I've got an arraylist called digits :

 [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765]

我创建了一个方法分隔每个值并将其保存到新数组。

I created a method which separates each value and saves it to a new array.

public static ArrayList<Integer> myNumbers(int z) {

    ArrayList<Integer> digits = new ArrayList<Integer>();
    String number = String.valueOf(z);
    for (int a = 0; a < number.length(); a++) {
        int j = Character.digit(number.charAt(a), 10);
        digits.add(j);
    }
    return digits;

}

在此之后，我有了一个名为numbers的新数组。我在这个数组上使用sort

After this I've got a new array called numbers. I'm using sort on this array

Collections.sort(numbers);

我的ArrayList如下所示：

and my ArrayList looks like this:

[0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 6, 6, 6, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 9, 9, 9]

它有：

2 times 0; 
9 times 1;
4 times 2;
6 times 3;
5 times 4;
6 times 5;
5 times 6;
5 times 7;
5 times 8;
3 times 9;

我需要打印出数字字符串取决于它们的数量是多少
所以它假设看起来像这样：
1354678290

I need to print out the string of numbers depend on how many are they So it suppose to look like this : 1354678290

推荐答案

例如，使用Stream API。

By using the Stream API for example.

package tests;

import org.junit.Assert;
import org.junit.Test;

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Duplicates {

    @Test
    public void duplicates() throws Exception {
        List<Integer> items = Arrays.asList(1, 1, 2, 2, 2, 2);

        Map<Integer, Long> result = items.stream()
                .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

        Assert.assertEquals(Long.valueOf(2), result.get(1));
        Assert.assertEquals(Long.valueOf(4), result.get(2));
    }
}

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