TreeSet和TreeMap的一点总结

摘要：若是通过进行排序的话当前集合采用的。最后附上一个标准的使用的方法自然排序是实现接口并且重写了方法的另一个则是通过并且重写方法

首先简单介绍下TreeSet和TreeMap的两种排序：

自然排序

通过comparator排序

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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();
    }

运行之后的结果如下:

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a
b
d
comparator1是空

这段代码里面获取的comparator是空的，Debug一遍，发现这个方法其实调用的是NavigableMap里面的comparator

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

查看官网上对其的介绍：

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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

在调用这个方法的时候若是自然排序，那么会返回一个null。若是通过comparator进行排序的话当前集合采用的comparator。
查看官网对reeSet的无参构造器的解释：

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/**

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 * 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的时候，插入的元素需要实现Comparable这个接口，而刚刚的元素是String，查看String的代码发现:

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

确实实现了，再测试一个没有实现的元素：

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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());
        }
    }

运行结果如下:

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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)

很明显，所以放在TreeSet里面的元素要么是实现Comparable了的自然排序，要么是通过comparator来进行排序的。

最后附上一个标准的使用Comparator的方法：

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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());
        }

是实现Comparable接口并且重写了compareTo方法的

则是通过comparator并且重写compare方法