Java 简明教程

Java - Map Interface

Map Interface

Map 接口将唯一键映射到值。键是您稍后用来检索值的 object。

The Map interface maps unique keys to values. A key is an object that you use to retrieve a value at a later date.

  1. Given a key and a value, you can store the value in a Map object. After the value is stored, you can retrieve it by using its key.

  2. Several methods throw a NoSuchElementException when no items exist in the invoking map.

  3. A ClassCastException is thrown when an object is incompatible with the elements in a map.

  4. A NullPointerException is thrown if an attempt is made to use a null object and null is not allowed in the map.

  5. An UnsupportedOperationException is thrown when an attempt is made to change an unmodifiable map.

Map Interface Methods

Sr.No.

Method & Description

1

void clear( ) Removes all key/value pairs from the invoking map.

2

boolean containsKey(Object k) Returns true if the invoking map contains k as a key. Otherwise, returns false.

3

boolean containsValue(Object v) Returns true if the map contains v as a value. Otherwise, returns false.

4

Set entrySet( ) Returns a Set that contains the entries in the map. The set contains objects of type Map.Entry. This method provides a set-view of the invoking map.

5

boolean equals(Object obj) Returns true if obj is a Map and contains the same entries. Otherwise, returns false.

6

Object get(Object k) Returns the value associated with the key k.

7

int hashCode( ) Returns the hash code for the invoking map.

8

boolean isEmpty( ) Returns true if the invoking map is empty. Otherwise, returns false.

9

Set keySet( ) Returns a Set that contains the keys in the invoking map. This method provides a set-view of the keys in the invoking map.

10

Object put(Object k, Object v) Puts an entry in the invoking map, overwriting any previous value associated with the key. The key and value are k and v, respectively. Returns null if the key did not already exist. Otherwise, the previous value linked to the key is returned.

11

void putAll(Map m) Puts all the entries from m into this map.

12

Object remove(Object k) Removes the entry whose key equals k.

13

int size( ) Returns the number of key/value pairs in the map.

14

Collection values( ) Returns a collection containing the values in the map. This method provides a collection-view of the values in the map.

Classes that Implement Map

以下是可以使用 Map 的功能来实现 Map 的类 -

The following are the classes that implement a Map to use the functionalities of a Map -

  1. HashMap

  2. EnumMap

  3. LinkedHashMap

  4. WeakHashMap

  5. TreeMap

Interfaces that Extend Map

以下是一些扩展了 Map 接口的接口 -

The following are the interfaces that extend the Map interface -

  1. SortedMap

  2. NavigableMap

  3. ConcurrentMap

Examples of Map Interface

Example 1

Map 在 HashMap 等类的实现中，以下是解释映射功能的示例 -

Map has its implementation in various classes like HashMap. Following is an example to explain map functionality −

import java.util.HashMap;
import java.util.Map;
public class CollectionsDemo {

   public static void main(String[] args) {
      Map<String, String> m1 = new HashMap<>();
      m1.put("Zara", "8");
      m1.put("Mahnaz", "31");
      m1.put("Ayan", "12");
      m1.put("Daisy", "14");

      System.out.println();
      System.out.println(" Map Elements");
      System.out.print("\t" + m1);
   }
}
Map Elements
	{Daisy = 14, Ayan = 12, Zara = 8, Mahnaz = 31}

Example 2

Map 在 TreeMap 等类的实现中，它根据键对条目进行排序。以下是使用 TreeMap 说明映射功能的示例 -

Map has its implementation in various classes like TreeMap which sorts the entries based on keys. Following is an example to explain map functionality using TreeMap −

import java.util.Map;
import java.util.TreeMap;

public class CollectionsDemo {

   public static void main(String[] args) {
      Map<String, String> m1 = new TreeMap<>();
      m1.put("Zara", "8");
      m1.put("Mahnaz", "31");
      m1.put("Ayan", "12");
      m1.put("Daisy", "14");

      System.out.println();
      System.out.println(" Map Elements");
      System.out.print("\t" + m1);
   }
}
 Map Elements
	{Ayan=12, Daisy=14, Mahnaz=31, Zara=8}

Example 3

映射在各种类中有着其实现，例如 HashMap。下面举例说明了使用 HashMap 来向映射添加和移除元素的映射功能：

Map has its implementation in various classes like HashMap. Following is an example to explain map functions using HashMap to add and remove elements to the map−

import java.util.HashMap;
import java.util.Map;
public class CollectionsDemo {

   public static void main(String[] args) {
      Map<String, String> m1 = new HashMap<>();
      m1.put("Zara", "8");
      m1.put("Mahnaz", "31");
      m1.put("Ayan", "12");
      m1.put("Daisy", "14");

      System.out.println();
      System.out.println(" Map Elements");
      System.out.print("\t" + m1);

      m1.remove("Daisy");
	  System.out.println(" Map Elements");
      System.out.print("\t" + m1);
   }
}
 Map Elements
	{Daisy=14, Ayan=12, Zara=8, Mahnaz=31} Map Elements
	{Ayan=12, Zara=8, Mahnaz=31}