How does Java order items in a HashMap or a HashTable?_问答_开发者

I was wondering how Java orders items in the Map (HashMap or Hashtable) when they are added. Are开发者_JAVA技巧 the keys ordered by the hashcode, memory reference or by allocation precedence...?

It's because I've noticed same pairs in the Map are not always in the same order

java.util.HashMap is unordered; you can't and shouldn't assume anything beyond that.

This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

java.util.LinkedHashMap uses insertion-order.

This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order).

java.util.TreeMap, a SortedMap, uses either natural or custom ordering of the keys.

The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used.

First of all: HashMap specifically doesn't provide a stable and/or defined ordering. So anything you observe is simply an implementation detail and you must not depend on it in any way.

Since it is sometimes useful to know the reason for the seemingly random ordering, here's the basic idea:

A HashMap has number of buckets (implemented as an array) in which to store entries.

When an item is added to the map, it is assigned to a buckets based on a value derived of its hashCode and the bucket size of the HashMap. (Note that it's possible that the bucket is already occupied, which is called a collision. That's handled gracefully and correctly, but I'll ignore that handling for the description because it doesn't change the concept).

The perceived ordering of the entires (such as returned by iterating over the Map) depends on the order of the entries in those buckets.

Whenever the size is rehashed (because the map exceeded its fullness threshold), then the number of buckets changes, which means that the position of each element might change, since the bucket position is derived from the number of buckets as well.

HashMap does not sort at all. For a map that sorts by key values you should use TreeMap instead.

From the JavaDocs for TreeMap:

Red-Black tree based implementation of the SortedMap interface. This class guarantees that the map will be in ascending key order, sorted according to the natural order for the key's class (see Comparable), or by the comparator provided at creation time, depending on which constructor is used.

From the documentation of HashMap:

This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

A Map is not an ordered data structure - you should not rely on entries in a HashMap being in a certain order. Some Map implementations such as LinkedHashMap and TreeMap do guarantee a certain order, but HashMap does not.

If you really want to know what happens internally, lookup the source code of HashMap - you can find it in src.zip which should be in your JDK installation directory.

A HashMap has a number of "buckets" in which it stores its entries. Which bucket an entry is stored in is determined by the hash code of the key of the entry. The order in which you see the entries in the HashMap depends on the hash codes of the keys. But don't write programs that rely on entries being in a certain order in a HashMap - the implementation might change in a future version of Java and your program then would not work anymore.

hashmap has a not defined order of the elements

There is no defined ordering in a hash table. Keys are placed into a slot, based on the hash code, but even that isn't a trivial order-by-hash-code.

HashMap stores the values using the unique hash-value generated using a part of the key. This hash-value maps to the address where it is going to be stored. This is how it ensures an access O(1).

LinkedHashmap on the other hand preserves the order in which you added to the map.