【Java】对ArrayList排序

java如何对ArrayList中对象按照该对象某属性排序

（从小到大）

两种方法：

方法一：<br>Comparator<KNNNode> comparator = new Comparator<KNNNode>() {
 
        @Override
        public int compare(KNNNode o1, KNNNode o2) {
            System.out.println("进入 compare function");
            // TODO Auto-generated method stub
            if (o1.getDistance() <= o2.getDistance()) {//o1属性小于等于o2属性时，返回1
                return 1;
            } else {
                return -1;
            }
        }
 
    };

调用方法：

List<KNNNode> nodelist = new ArrayList<KNNNode>();
//此处需要填充nodelist.add();
Collections.sort(nodelist, comparator);// 调用

方法二：<br>class SortBy implements Comparator {
        public int compare(Object obj1, Object obj2) {
            KNNNode point1 = (KNNNode) obj1;
            KNNNode point2 = (KNNNode) obj2;
            if (point1.getDistance() <= point2.getDistance()) {
                return 1;
            } else {
                return 0;
            }
        }
    }

调用方法：

1	`Collections.sort(nodelist,` `new` `SortBy());`

KNNNode结点内的属性：

package newKNN;
 
/**
 * KNN结点类，用来存储最近邻的k个元组相关的信息
 */
public class KNNNode {
    private int index; // 元组标号
    private double distance; // 与测试元组的距离
    private String seq;//滑动窗口motif序列片段
     
    public KNNNode(int index, double distance, String seq) {
        super();
        this.index = index;
        this.distance = distance;
        this.seq = seq;
         
    }
    public int getIndex() {
        return index;
    }
    public void setIndex(int index) {
        this.index = index;
    }
    public double getDistance() {
        return distance;
    }
    public void setDistance(double distance) {
        this.distance = distance;
    }
    public String getSeq() {
        return seq;
    }
    public void setSeq(String seq) {
        this.seq = seq;
    }
     
}

我想要按照KNNNode的distance属性排序。

若要按照从大到小的顺序排序，如果只是将

>=返回值改为-1

<返回值改为1 会出现异常。

Comparison method violates its general contract!

原因是是实现接口Comparator 的类有问题，在compare中没有处理好两个比较的对象相等的情况

public int compare(KNNNode o1, KNNNode o2）;

即 o1.getDistance() == o2.getDistance() 时，返回值不是0

我最终的实现代码是：（从大到小排序）

Comparator<KNNNode> comparator = new Comparator<KNNNode>() {
 
        @Override
        public int compare(KNNNode o1, KNNNode o2) {
            // TODO Auto-generated method stub
            if (o1.getDistance() < o2.getDistance()) {
                return 1;
            } else if(o1.getDistance() > o2.getDistance()){
                return -1;
            }else{
                return 0;
            }
        }
 
    };