Java春招面试复习：有关于Java Map，应该掌握的8个问题

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Java春招面试复习：有关于Java Map，应该掌握的8个问题

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listList<Integer> keyList = new ArrayList<>(map.keySet());System.out.println(keyList);//把map的值转化为listList<String> valueList = new ArrayList<>(map.values());System.out.println(valueList);把map的键值转化为listList 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 keyK key = entry.getKey();// get valueV 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 keyInteger key = (Integer) entry.getKey();// get valueString 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 keyK key = entry.getKey();// get valueV 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 keyInteger key = (Integer) entry.getKey();// get valueString 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()pareTo(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()pareTo(e2.getKey().toString());});for (Map.Entry entry : list) {System.out.println("key:" + entry.getKey() + ",value:" + entry.getValue());}}
}

使用SortedMap+TreeMap+Comparator实现

SortedMap sortedMap = new TreeMap(new Comparator() {@Overridepublic int compare(K k1, K k2) {return k1pareTo(k2);}
});
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>() {@Overridepublic int compare(String k1, String k2) {return k1pareTo(k2);}});sortedMap.putAll(map);Iterator itr = sortedMap.entrySet().iterator();while(itr.hasNext()) {Map.Entry entry = (Map.Entry) itr.next();// get keyString key = (String) entry.getKey();// get valueString 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()pareTo(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()pareTo(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 keyInteger key = (Integer) entry.getKey();// get valueString 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 keyInteger key = (Integer) entry.getKey();// get valueString 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java=，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 cloneMap<Integer, User> clonedMap = userMap;//Same as userMapSystem.out.println(clonedMap);System.out.println("\nChanges reflect in both maps \n");//Change a value is clonedMapclonedMap.get(1).setName("test");//Verify content of both mapsSystem.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 class CopyCloneMapTest {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 cloneHashMap<Integer, User> clonedMap = (HashMap<Integer, User>) userMap.clone();//Same as userMapSystem.out.println(clonedMap);System.out.println("\nChanges reflect in both maps \n");//Change a value is clonedMapclonedMap.get(1).setName("test");//Verify content of both mapsSystem.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 cloneHashMap<Integer, User> clonedMap = new HashMap<>();clonedMap.putAll(userMap);//Same as userMapSystem.out.println(clonedMap);System.out.println("\nChanges reflect in both maps \n");//Change a value is clonedMapclonedMap.get(1).setName("test");//Verify content of both mapsSystem.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 cloneGson 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 userMapSystem.out.println(clonedMap);System.out.println("\nChanges DO NOT reflect in other map \n");//Change a value is clonedMapclonedMap.get(1).setName("test");//Verify content of both mapsSystem.out.println(userMap);System.out.println(clonedMap);}
}

运行结果：

{1=User{name='jay', age=26}, 2=User{name='fany', age=25}}Changes DO NOT reflect in other 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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