TreeMap以及自定義排序的Comparable和Comparator的實現

      TreeMap和HashMap實現了同樣的接口Map，因此，用法基本么有區別，但是hashMap的效率高於TreeMap，在需要排序的Map才選用TreeMap。TreeMap是紅黑二叉樹實現的，打開源碼會看到核心代碼：

private transient Entry<K,V> root;

root用來存儲整個樹的根結點。root是Entry<k,v>類型，接下來看看Entry。

   static final class Entry<K,V> implements Map.Entry<K,V> {
        K key;
        V value;
        Entry<K,V> left;
        Entry<K,V> right;
        Entry<K,V> parent;
        boolean color = BLACK;

    
        Entry(K key, V value, Entry<K,V> parent) {
            this.key = key;
            this.value = value;
            this.parent = parent;
        }

Entry是TreeMap的內部類，可以看到里面存儲的本身數據，左右結點，父節點以及節點的顏色。

public class TestTreeMap {
    public staticvoid main(String[] args) {
        TreeMap<Integer,Integer> treeMap = new TreeMap<>();
        treeMap.put(2,3);
        treeMap.put(3,4);
        treeMap.put(1,5);
        treeMap.put(7,9);
        for (Integer key:treeMap.keySet()
             ) {
            System.out.println(key+":"+treeMap.get(key));
        }
    }
}

 由代碼發現treeMap,是按照key自增的方式排列。

 

如果想要上面的數據實現從大到小的順序排列：

 

 

 

 

當key為自定義的類型時，便需要實現Comparable接口，實現ComparaTo方法。有三個返回值，分別是負數，零，正數。

例如下面的例子對學生Student類進行score屬性進行從小到大的順序進行排序。

import java.util.TreeMap;

public class TestTreeMap {
    public static void main(String[] args) {
        TreeMap<Integer, Integer> treeMap = new TreeMap<>();
        treeMap.put(2, 3);
        treeMap.put(3, 4);
        treeMap.put(1, 5);
        treeMap.put(7, 9);
        for (Integer key : treeMap.keySet()
                ) {
            System.out.println(key + ":" + treeMap.get(key));
        }

        TreeMap<Student, Integer> treeMap1 = new TreeMap<>();
        treeMap1.put(new Student(1001,18,95),1001);
        treeMap1.put(new Student(1002,18,87),1002);
        treeMap1.put(new Student(1003,18,85),1003);
        treeMap1.put(new Student(1004,18,97),1004);

        for (Student key : treeMap1.keySet()
                ) {
            System.out.println(key + ":" + treeMap1.get(key));
        }
    }
}
    class Student implements Comparable<Student>{
        private int age;
        private int id;
        private int score;

        public Student(int id, int age, int score) {
            this.age = age;
            this.id = id;
            this.score = score;
        }

        @Override
        public String toString() {
            return "Student{" +
                    "age=" + age +
                    ", id=" + id +
                    ", score=" + score +
                    '}';
        }

        @Override
        public int compareTo(Student o) {
            if(this.score < o.score){
                return -1;
            }else if(this.score == o.score){
                return 0;
            }else {
                return 1;
            }
        }
}

 當然還想實現別的屬性排序，只需要在comparaTo里面實現即可。