23
有关于Java Map，应该掌握的8个问题
source link: http://developer.51cto.com/art/202002/609899.htm
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前言
最近几天看了几篇有关于Java Map的外国博文，写得非常不错，所以整理了Java map 应该掌握的8个问题，都是日常开发司空见惯的问题，希望对大家有帮助;如果有不正确的地方，欢迎提出，万分感谢哈~
本章节所有代码demo已上传github
1、如何把一个Map转化为List
日常开发中，我们经常遇到这种场景，把一个Map转化为List。map转List有以下三种转化方式：
- 把map的键key转化为list
- 把map的值value转化为list
- 把map的键值key-value转化为list
伪代码如下：
// key list List keyList = new ArrayList(map.keySet()); // value list List valueList = new ArrayList(map.values()); // key-value list List entryList = new ArrayList(map.entrySet());
示例代码：
public class Test {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(2, "jay");
map.put(1, "whx");
map.put(3, "huaxiao");
//把一个map的键转化为list
List<Integer> keyList = new ArrayList<>(map.keySet());
System.out.println(keyList);
//把map的值转化为list
List<String> valueList = new ArrayList<>(map.values());
System.out.println(valueList);
把map的键值转化为list
List entryList = new ArrayList(map.entrySet());
System.out.println(entryList);
}
}
运行结果：
[1, 2, 3] [whx, jay, huaxiao] [1=whx, 2=jay, 3=huaxiao]
2、如何遍历一个Map
我们经常需要遍历一个map，可以有以下两种方式实现：
通过entrySet+for实现遍历
for(Entry entry: map.entrySet()) {
// get key
K key = entry.getKey();
// get value
V value = entry.getValue();
}
实例代码：
public class EntryMapTest {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(2, "jay");
map.put(1, "whx");
map.put(3, "huaxiao");
for(Map.Entry entry: map.entrySet()) {
// get key
Integer key = (Integer) entry.getKey();
// get value
String value = (String) entry.getValue();
System.out.println("key:"+key+",value:"+value);
}
}
}
通过Iterator+while实现遍历
Iterator itr = map.entrySet().iterator();
while(itr.hasNext()) {
Entry entry = itr.next();
// get key
K key = entry.getKey();
// get value
V value = entry.getValue();
}
实例代码：
public class IteratorMapTest {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(2, "jay");
map.put(1, "whx");
map.put(3, "huaxiao");
Iterator itr = map.entrySet().iterator();
while(itr.hasNext()) {
Map.Entry entry = (Map.Entry) itr.next();
// get key
Integer key = (Integer) entry.getKey();
// get value
String value = (String) entry.getValue();
System.out.println("key:"+key+",value:"+value);
}
}
}
运行结果：
key:1,value:whx key:2,value:jay key:3,value:huaxiao
3、如何根据Map的keys进行排序
对Map的keys进行排序，在日常开发很常见，主要有以下两种方式实现。
把Map.Entry放进list，再用Comparator对list进行排序
List list = new ArrayList(map.entrySet());
Collections.sort(list, (Entry e1, Entry e2)-> {
return e1.getKey().compareTo(e2.getKey());
});
实例代码：
public class SortKeysMapTest {
public static void main(String[] args) {
Map<String, String> map = new HashMap<>();
map.put("2010", "jay");
map.put("1999", "whx");
map.put("3010", "huaxiao");
List<Map.Entry<String,String>> list = new ArrayList<>(map.entrySet());
Collections.sort(list, (Map.Entry e1, Map.Entry e2)-> {
return e1.getKey().toString().compareTo(e2.getKey().toString());
});
for (Map.Entry entry : list) {
System.out.println("key:" + entry.getKey() + ",value:" + entry.getValue());
}
}
}
使用SortedMap+TreeMap+Comparator实现
1. SortedMap sortedMap = new TreeMap(new Comparator() {
2. @Override
3. public int compare(K k1, K k2) {
4. return k1.compareTo(k2);
5. }
6. });
7. sortedMap.putAll(map);
实例代码：
public class SortKeys2MapTest {
public static void main(String[] args) {
Map<String, String> map = new HashMap<>();
map.put("2010", "jay");
map.put("1999", "whx");
map.put("3010", "huaxiao");
SortedMap sortedMap = new TreeMap(new Comparator<String>() {
@Override
public int compare(String k1, String k2) {
return k1.compareTo(k2);
}
});
sortedMap.putAll(map);
Iterator itr = sortedMap.entrySet().iterator();
while(itr.hasNext()) {
Map.Entry entry = (Map.Entry) itr.next();
// get key
String key = (String) entry.getKey();
// get value
String value = (String) entry.getValue();
System.out.println("key:"+key+",value:"+value);
}
}
}
运行结果：
key:1999,value:whx key:2010,value:jay key:3010,value:huaxiao
4、如何对Map的values进行排序
List list = new ArrayList(map.entrySet());
Collections.sort(list, (Entry e1, Entry e2) ->{
return e1.getValue().compareTo(e2.getValue());
});
实例代码：
public class SortValuesMapTest {
public static void main(String[] args) {
Map<String, String> map = new HashMap<>();
map.put("2010", "jay");
map.put("1999", "whx");
map.put("3010", "huaxiao");
List <Map.Entry<String,String>>list = new ArrayList<>(map.entrySet());
Collections.sort(list, (Map.Entry e1, Map.Entry e2)-> {
return e1.getValue().toString().compareTo(e2.getValue().toString());
}
);
for (Map.Entry entry : list) {
System.out.println("key:" + entry.getKey() + ",value:" + entry.getValue());
}
}
}
运行结果：
key:3010,value:huaxiao key:2010,value:jay key:1999,value:whx
5、如何初始化一个静态/不可变的Map
初始化一个静态不可变的map，单单static final+static代码块还是不行的，如下：
public class Test1 {
private static final Map <Integer,String>map;
static {
map = new HashMap<Integer, String>();
map.put(1, "one");
map.put(2, "two");
}
public static void main(String[] args) {
map.put(3, "three");
Iterator itr = map.entrySet().iterator();
while(itr.hasNext()) {
Map.Entry entry = (Map.Entry) itr.next();
// get key
Integer key = (Integer) entry.getKey();
// get value
String value = (String) entry.getValue();
System.out.println("key:"+key+",value:"+value);
}
}
}
这里面，map继续添加元素(3,"three")，发现是OK的，运行结果如下：
key:1,value:one key:2,value:two key:3,value:three
真正实现一个静态不可变的map，需要Collections.unmodifiableMap，代码如下：
public class Test2 {
private static final Map<Integer, String> map;
static {
Map<Integer,String> aMap = new HashMap<>();
aMap.put(1, "one");
aMap.put(2, "two");
map = Collections.unmodifiableMap(aMap);
}
public static void main(String[] args) {
map.put(3, "3");
Iterator itr = map.entrySet().iterator();
while(itr.hasNext()) {
Map.Entry entry = (Map.Entry) itr.next();
// get key
Integer key = (Integer) entry.getKey();
// get value
String value = (String) entry.getValue();
System.out.println("key:"+key+",value:"+value);
}
}
}
运行结果如下：
可以发现，继续往map添加元素是会报错的，实现真正不可变的map。
6、HashMap, TreeMap, and Hashtable，ConcurrentHashMap的区别
7、如何创建一个空map
如果map是不可变的，可以这样创建：
Map map=Collections.emptyMap()；
or
Map<String,String> map=Collections.<String, String>emptyMap();
//map1.put("1", "1"); 运行出错
如果你希望你的空map可以添加元素的，可以这样创建
Map map = new HashMap();
8、有关于map的复制
有关于hashmap的复制，在日常开发中，使用也比较多。主要有 =，clone，putAll，但是他们都是浅复制，使用的时候注意啦，可以看一下以下例子：
例子一，使用=复制一个map：
public class CopyMapAssignTest {
public static void main(String[] args) {
Map<Integer, User> userMap = new HashMap<>();
userMap.put(1, new User("jay", 26));
userMap.put(2, new User("fany", 25));
//Shallow clone
Map<Integer, User> clonedMap = userMap;
//Same as userMap
System.out.println(clonedMap);
System.out.println("\nChanges reflect in both maps \n");
//Change a value is clonedMap
clonedMap.get(1).setName("test");
//Verify content of both maps
System.out.println(userMap);
System.out.println(clonedMap);
}
}
运行结果：
{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}
Changes reflect in both maps
{1=User{name='test', age=26}, 2=User{name='fany', age=25}}
{1=User{name='test', age=26}, 2=User{name='fany', age=25}}
从运行结果看出，对cloneMap修改，两个map都改变了，所以=是浅复制。
例子二，使用hashmap的clone复制：
{
public static void main(String[] args) {
HashMap<Integer, User> userMap = new HashMap<>();
userMap.put(1, new User("jay", 26));
userMap.put(2, new User("fany", 25));
//Shallow clone
HashMap<Integer, User> clonedMap = (HashMap<Integer, User>) userMap.clone();
//Same as userMap
System.out.println(clonedMap);
System.out.println("\nChanges reflect in both maps \n");
//Change a value is clonedMap
clonedMap.get(1).setName("test");
//Verify content of both maps
System.out.println(userMap);
System.out.println(clonedMap);
}
}
运行结果：
{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}
Changes reflect in both maps
{1=User{name='test', age=26}, 2=User{name='fany', age=25}}
{1=User{name='test', age=26}, 2=User{name='fany', age=25}}
从运行结果看出，对cloneMap修改，两个map都改变了，所以hashmap的clone也是浅复制。
例子三，通过putAll操作
public class CopyPutAllMapTest {
public static void main(String[] args) {
HashMap<Integer, User> userMap = new HashMap<>();
userMap.put(1, new User("jay", 26));
userMap.put(2, new User("fany", 25));
//Shallow clone
HashMap<Integer, User> clonedMap = new HashMap<>();
clonedMap.putAll(userMap);
//Same as userMap
System.out.println(clonedMap);
System.out.println("\nChanges reflect in both maps \n");
//Change a value is clonedMap
clonedMap.get(1).setName("test");
//Verify content of both maps
System.out.println(userMap);
System.out.println(clonedMap);
}
}
运行结果：
{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}
Changes reflect in both maps
{1=User{name='test', age=26}, 2=User{name='fany', age=25}}
{1=User{name='test', age=26}, 2=User{name='fany', age=25}}
从运行结果看出，对cloneMap修改，两个map都改变了，所以putAll还是浅复制。
那么，如何实现深度复制呢?
可以使用序列化实现，如下为谷歌Gson序列化HashMap，实现深度复制的例子：
public class CopyDeepMapTest {
public static void main(String[] args) {
HashMap<Integer, User> userMap = new HashMap<>();
userMap.put(1, new User("jay", 26));
userMap.put(2, new User("fany", 25));
//Shallow clone
Gson gson = new Gson();
String jsonString = gson.toJson(userMap);
Type type = new TypeToken<HashMap<Integer, User>>(){}.getType();
HashMap<Integer, User> clonedMap = gson.fromJson(jsonString, type);
//Same as userMap
System.out.println(clonedMap);
System.out.println("\nChanges reflect in only one map \n");
//Change a value is clonedMap
clonedMap.get(1).setName("test");
//Verify content of both maps
System.out.println(userMap);
System.out.println(clonedMap);
}
}
运行结果：
{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}
Changes reflect in only one map
{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}
{1=User{name='test', age=26}, 2=User{name='fany', age=25}}
从运行结果看出，对cloneMap修改，userMap没有被改变，所以是深度复制。
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