摘要：若是通過進行排序的話當前集合采用的。最后附上一個標準的使用的方法自然排序是實現接口并且重寫了方法的另一個則是通過并且重寫方法

首先簡單介紹下TreeSet和TreeMap的兩種排序：

自然排序

通過comparator排序

private static void compareWithCpmparator(){
        TreeSet treeSet =new TreeSet<>();
        List list =new ArrayList<>();
        list.add("a");
        list.add("d");
        list.add("b");
        treeSet.addAll(list);
        Iterator iterator =treeSet.iterator();
        while (iterator.hasNext()){
            System.out.println(iterator.next());
        }
        Comparator  comparator1 = (Comparator) treeSet.comparator();
        if (comparator1 == null){
            System.out.println("comparator1是空");
        }else {
            System.out.println("comparator1不是空");
        }
    }

    public static void main(String[] args) {
        compareWithCpmparator();
    }

運行之后的結果如下:

a
b
d
comparator1是空

這段代碼里面獲取的comparator是空的，Debug一遍，發現這個方法其實調用的是NavigableMap里面的comparator

    public Comparator comparator() {
        return m.comparator();
    }

查看官網上對其的介紹：

Comparator comparator()
Returns the comparator used to order the keys in this map, or null if this map uses the natural ordering of its keys.
Returns:
the comparator used to order the keys in this map, or null if this map uses the natural ordering of its keys

在調用這個方法的時候若是自然排序，那么會返回一個null。若是通過comparator進行排序的話當前集合采用的comparator。
查看官網對reeSet的無參構造器的解釋：

/**

 * Constructs a new, empty tree set, sorted according to the
 * natural ordering of its elements.  All elements inserted into
 * the set must implement the {@link Comparable} interface.
 * Furthermore, all such elements must be mutually
 * comparable: {@code e1.compareTo(e2)} must not throw a
 * {@code ClassCastException} for any elements {@code e1} and
 * {@code e2} in the set.  If the user attempts to add an element
 * to the set that violates this constraint (for example, the user
 * attempts to add a string element to a set whose elements are
 * integers), the {@code add} call will throw a
 * {@code ClassCastException}.

在使用TreeSet的時候，插入的元素需要實現Comparable這個接口，而剛剛的元素是String，查看String的代碼發現:

public final class String implements java.io.Serializable, Comparable, CharSequence {

確實實現了，再測試一個沒有實現的元素：

public class PojoTest {
    private int id;
    private String name;

    public PojoTest() {
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public PojoTest(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

 private static void  com(){
        TreeSet treeSet =new TreeSet<>();
        treeSet.add(new PojoTest(1,"a"));
        treeSet.add(new PojoTest(2,"b"));
        treeSet.add(new PojoTest(3,"c"));
        Iterator iterator =treeSet.iterator();
        while (iterator.hasNext()){
            System.out.println(iterator.next().getName());
        }
    }

運行結果如下:

Exception in thread "main" java.lang.ClassCastException: SetAndMap.TreeSetAndTreeMap.PojoTest cannot be cast to java.lang.Comparable
    at java.util.TreeMap.compare(TreeMap.java:1294)
    at java.util.TreeMap.put(TreeMap.java:538)
    at java.util.TreeSet.add(TreeSet.java:255)
    at SetAndMap.TreeSetAndTreeMap.TestTreeSet.com(TestTreeSet.java:77)
    at SetAndMap.TreeSetAndTreeMap.TestTreeSet.main(TestTreeSet.java:88)

很明顯，所以放在TreeSet里面的元素要么是實現Comparable了的自然排序，要么是通過comparator來進行排序的。

最后附上一個標準的使用Comparator的方法：

private static void  construct(){
        Comparator  comparator =new Comparator() {
            @Override
            public int compare(String o1, String o2) {
                if(o1.toCharArray()[0] >o2.toCharArray()[0]){
                    return -1;
                }else if(o1.toCharArray()[0] == o2.toCharArray()[0]){
                    return 0;
                }else{
                    return 1;
               }
            }
        };
        TreeSet treeSet =new TreeSet<>(comparator);
        List list =new ArrayList<>();
        list.add("a");
        list.add("d");
        list.add("b");
        treeSet.addAll(list);
        Iterator iterator =treeSet.iterator();
        while (iterator.hasNext()){
            System.out.println(iterator.next());
        }
        Comparator  comparator1 = (Comparator) treeSet.comparator();
        TreeSet treeSet1 =new TreeSet<>(comparator1);
        treeSet1.add("c");
        treeSet1.add("g");
        treeSet1.add("a");
        Iterator iterator1 =treeSet1.iterator();
        while (iterator1.hasNext()){
            System.out.println(iterator1.next());
        }

是實現Comparable接口并且重寫了compareTo方法的

則是通過comparator并且重寫compare方法