如何遍历有序的TreeMap

[haorengoodman]

	public static void main(String[] args) {
		TreeMap<String, String> sPara = new TreeMap<String, String>();
		sPara.put("abc", "abc");
		sPara.put("cds", "cds");
		sPara.put("ads","ads");
		sPara.put("fff", "fff");
		sPara.put("ccc", "ccc");
		sPara.put("ddd", "ddd");
		sPara.put("qqqqq", "qqqqq");
		sPara.put("knns", "knns");
		sPara.put("ioio", "ioio");
		Set<String> keys = sPara.keySet();
		for (String key : keys){
			System.out.println(key);
		}
	}

结果：
abc
ads
ccc
cds
ddd
fff
ioio
knns
qqqqq
问题：
TreeMap 默认按照key的自然顺序排序，
但是遍历的时候key被放入到一个Set集合中，Set集合本身是无序的，为什么结果仍然有序？

[tony4geek]

/** * Returns a {@link Set} view of the keys contained in this map. * The set's iterator returns the keys in ascending order. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. If the map is modified * while an iteration over the set is in progress (except through * the iterator's own <tt>remove</tt> operation), the results of * the iteration are undefined. The set supports element removal, * which removes the corresponding mapping from the map, via the * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> * operations. It does not support the <tt>add</tt> or <tt>addAll</tt> * operations. */ public Set<K> keySet() { return navigableKeySet(); }

[tony4geek]

里面是有序的。

[a137655624]

TreeSet 底层是通过 TreeMap 来实现的 TreeSet set = TreeMap.getKeySet(); 这样 set 就是默认有序的

[zy_think123]

String类型以及基本数据类型都会给你排好序的，但是自定义类型就是无序了，需要自己实现比较器，可以考虑实现comparable或者compartor接口

[xiaofeifeiloving]

Map map=new TreeMap();//创建集合对象 Iterator ir=map.keySet().iterator();//获取hashMap的键值，并进行遍历 while(ir.hasNext()){ Object key= ir.next(); System.out.println("键为"+key+"所对应的值为"+map.get(key)); } 即是通过TreeMap中指针的移动，实现对TreeMap的遍历

[小绵羊]

没有谁说过Set是无序的啊。 Set只保证元素的唯一性。

[haorengoodman]

引用 1 楼 rui888 的回复:

/** * Returns a {@link Set} view of the keys contained in this map. * The set's iterator returns the keys in ascending order. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. If the map is modified * while an iteration over the set is in progress (except through * the iterator's own <tt>remove</tt> operation), the results of * the iteration are undefined. The set supports element removal, * which removes the corresponding mapping from the map, via the * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> * operations. It does not support the <tt>add</tt> or <tt>addAll</tt> * operations. */ public Set<K> keySet() { return navigableKeySet(); }

谢啦，看了一下源码，navigableKeySet 类型是 KeySet ，实现了SortedSet接口，所以最后拿到的set集合就是有序的了

static final class KeySet<E> extends AbstractSet<E> implements NavigableSet<E>{......}
public interface NavigableSet<E> extends SortedSet<E>{......}