HashMap, LinkedHashMap和TreeMap在Java中的区别是什么?
我在输出中没有看到任何不同,因为这三个都有keySet和values。什么是哈希表?
Map m1 = new HashMap();
m1.put("map", "HashMap");
m1.put("schildt", "java2");
m1.put("mathew", "Hyden");
m1.put("schildt", "java2s");
print(m1.keySet());
print(m1.values());
SortedMap sm = new TreeMap();
sm.put("map", "TreeMap");
sm.put("schildt", "java2");
sm.put("mathew", "Hyden");
sm.put("schildt", "java2s");
print(sm.keySet());
print(sm.values());
LinkedHashMap lm = new LinkedHashMap();
lm.put("map", "LinkedHashMap");
lm.put("schildt", "java2");
lm.put("mathew", "Hyden");
lm.put("schildt", "java2s");
print(lm.keySet());
print(lm.values());
虽然这里有很多很好的答案,但我想给出我自己的表,描述与Java 11绑定的各种Map实现。
我们可以在图表中看到这些差异:
HashMap is the general-purpose Map commonly used when you have no special needs.
LinkedHashMap extends HashMap, adding this behavior: Maintains an order, the order in which the entries were originally added. Altering the value for key-value entry does not alter its place in the order.
TreeMap too maintains an order, but uses either (a) the “natural” order, meaning the value of the compareTo method on the key objects defined on the Comparable interface, or (b) invokes a Comparator implementation you provide.
TreeMap implements both the SortedMap interface, and its successor, the NavigableMap interface.
NULLs: TreeMap does not allow a NULL as the key, while HashMap & LinkedHashMap do.
All three allow NULL as the value.
HashTable is legacy, from Java 1. Supplanted by the ConcurrentHashMap class. Quoting the Javadoc: ConcurrentHashMap obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable.
它们都提供一个key->值映射和一种遍历键的方法。最重要的区别
这些类是时间保证和键的顺序。
HashMap offers 0(1) lookup and insertion. If you iterate through the keys, though, the ordering of the
keys is essentially arbitrary. It is implemented by an array of linked lists.
TreeMap offers O(log N) lookup and insertion. Keys are ordered, so if you need to iterate through
the keys in sorted order, you can. This means that keys must implement the Comparable interface.TreeMap is implemented by a Red-Black Tree.
LinkedHashMap offers 0(1) lookup and insertion. Keys are ordered by their insertion order. It is
implemented by doubly-linked buckets.
假设你将一个空的TreeMap, HashMap和LinkedHashMap传递给下面的函数:
void insertAndPrint(AbstractMap<Integer, String> map) {
int[] array= {1, -1, 0};
for (int x : array) {
map.put(x, Integer.toString(x));
}
for (int k: map.keySet()) {
System.out.print(k + ", ");
}
}
它们的输出如下所示。
对于HashMap,在我自己的测试中,输出是{0,1,-1},但它可以是任何顺序。没有任何保证
排序。
Treemap,输出为{- 1,0,1}
LinkedList,输出为{1,-1,0}
HashMap
可以包含一个空键。
HashMap没有秩序。
TreeMap
TreeMap不能包含任何空键。
TreeMap保持升序。
LinkedHashMap
LinkedHashMap可用于维护插入顺序,即键被插入到Map中,也可用于维护访问顺序,即键被访问。
例子::
1) HashMap map = new HashMap();
map.put(null, "Kamran");
map.put(2, "Ali");
map.put(5, "From");
map.put(4, "Dir");`enter code here`
map.put(3, "Lower");
for (Map.Entry m : map.entrySet()) {
System.out.println(m.getKey() + " " + m.getValue());
}
2) TreeMap map = new TreeMap();
map.put(1, "Kamran");
map.put(2, "Ali");
map.put(5, "From");
map.put(4, "Dir");
map.put(3, "Lower");
for (Map.Entry m : map.entrySet()) {
System.out.println(m.getKey() + " " + m.getValue());
}
3) LinkedHashMap map = new LinkedHashMap();
map.put(1, "Kamran");
map.put(2, "Ali");
map.put(5, "From");
map.put(4, "Dir");
map.put(3, "Lower");
for (Map.Entry m : map.entrySet()) {
System.out.println(m.getKey() + " " + m.getValue());
}
这三个都表示从唯一键到值的映射,因此实现了Map接口。
HashMap is a map based on hashing of the keys. It supports O(1) get/put operations. Keys must have consistent implementations of hashCode() and equals() for this to work.
LinkedHashMap is very similar to HashMap, but it adds awareness to the order at which items are added (or accessed), so the iteration order is the same as insertion order (or access order, depending on construction parameters).
TreeMap is a tree based mapping. Its put/get operations take O(log n) time. It requires items to have some comparison mechanism, either with Comparable or Comparator. The iteration order is determined by this mechanism.