对 java 中的 ArrayList<Integer> 的数组进行排序

Question

我想按 ArrayLists 元素的第一个 int 对 ArrayList 的 Array 进行排序。我试图覆盖 Comparator class 的比较方法，但它抛出：

Exception in thread "main" java.lang.NullPointerException
at BikeGA.compare(BikeGA.java:515)
at BikeGA.compare(BikeGA.java:1)
at java.util.TimSort.countRunAndMakeAscending(TimSort.java:351)
at java.util.TimSort.sort(TimSort.java:230)
at java.util.Arrays.sort(Arrays.java:1438)
at BikeGA.main(BikeGA.java:512)

密码是：

int max_generations = 20;
static ArrayList<Integer>[] population = new ArrayList[max_generations];

Arrays.sort(population, new Comparator<ArrayList<Integer>>(){
        @Override
        public int compare(final ArrayList<Integer> entry1, final ArrayList<Integer> entry2){
            return entry1.get(0).compareTo(entry2.get(0));
        }
    });

有人可以帮助我吗？谢谢

Answer 1

您正在比较 2 个数组，但没有检查比较对象是否存在...

int max_generations = 20;
static ArrayList<Integer>[] population = new ArrayList[max_generations];

Arrays.sort(population, new Comparator<ArrayList<Integer>>(){
        @Override
        public int compare(final ArrayList<Integer> entry1, final ArrayList<Integer> entry2){
            Integer value1 = entry1.get(0) == null : -1 ? entry1.get(0);
            Integer value2 = entry2.get(0) == null : -1 ? entry2.get(0);

            return value1.compareTo(value2);
        }
    });

Answer 2

您的比较器应该处理 null 或空的 ArrayLists，以便它可以处理您放入数组中的任何数据：

    @Override
    public int compare(final ArrayList<Integer> entry1, final ArrayList<Integer> entry2){
        if (entry1 == null && entry2 == null)
            return 0;
        if (entry1 == null)
            return 1;
        if (entry2 == null)
            return -1;
        if (entry1.isEmpty() && entry2.isEmpty())
            return 0;
        if (entry1.isEmpty())
            return 1;
        if (entry2.isEmpty())
            return -1;
        return entry1.get(0).compareTo(entry2.get(0));
    }

这会将空元素放在数组的末尾，空列表放在它们之前。

Answer 3

在我的案例中，对象是：

public class Edge {
    private Integer weight;
    public Edge(Integer weight) {
        this.weight = weight;
    }
    public Integer getWeight() {
        return weight;
    }
}

然后我从其他 class:

调用 sort for (List edges = new ArrayList<>();)

Collections.sort(edges,(edge1,edge2) -> edge1.getWeight().compareTo(edge2.getWeight()));

结果如下： Exception img

解决方案是初始化对象的值：

public class Edge {
    private Integer weight = 0;
    public Edge(Integer weight) {
        this.weight = weight;
    }
    public Integer getWeight() {
        return weight;
    }
}

而且有效！！！ code 0

对 java 中的 ArrayList<Integer> 的数组进行排序

Sort an Array of ArrayList<Integer> in java

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arraylist