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(xiàn)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(jié)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(fā)現(xiàn)這個方法其實調(diào)用的是NavigableMap里面的comparator

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

查看官網(wǎng)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(diào)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(wǎng)對reeSet的無參構(gòu)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(xiàn)Comparable這個接口，而剛剛的元素是String，查看String的代碼發(fā)現(xiàn):

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

確實實現(xiàn)了，再測試一個沒有實現(xiàn)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(jié)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(xiàn)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(xiàn)Comparable接口并且重寫了compareTo方法的

則是通過comparator并且重寫compare方法