Java IdentityHashMap putAll()方法

java.util.IdentityHashMap.putAll()是IdentityHashMap类的一个内建方法，用于复制操作。该方法将所有的元素，即映射，从一个地图复制到另一个地图。

语法

new_Identityhash_map.putAll(exist_Identityhash_map)

参数： 该方法需要一个参数existence_Identityhash_map，指的是我们要复制的现有地图。

返回值： 该方法不返回任何值。

异常： 如果我们要复制的地图是空的，该方法会抛出NullPointerException。

下面的程序说明了java.util.IdentityHashMap.putAll()方法的工作：

程序1： 将字符串值映射到整数键。

// Java code to illustrate the putAll() method
import java.util.*;
  
public class Identity_Hash_Map_Demo {
    public static void main(String[] args)
    {
  
        // Creating an empty IdentityHashMap
        Map<Integer, String> Identity_hash = new 
                      IdentityHashMap<Integer, String>();
  
        // Mapping string values to int keys
        Identity_hash.put(10, "Geeks");
        Identity_hash.put(15, "4");
        Identity_hash.put(20, "Geeks");
        Identity_hash.put(25, "Welcomes");
        Identity_hash.put(30, "You");
  
        // Displaying the IdentityHashMap
        System.out.println("Initial Mappings are: " + 
                                           Identity_hash);
  
        // Creating a new Identityhash map and copying
        Map<Integer, String> new_Identityhash_map = new 
                        IdentityHashMap<Integer, String>();
        new_Identityhash_map.putAll(Identity_hash);
  
        // Displaying the final IdentityHashMap
        System.out.println("The new map: " + 
                                    new_Identityhash_map);
    }
}

输出:

Initial Mappings are: {30=You, 15=4, 10=Geeks, 25=Welcomes, 20=Geeks}
The new map: {15=4, 30=You, 10=Geeks, 25=Welcomes, 20=Geeks}

程序2： 将整数值映射到字符串键。

// Java code to illustrate the putAll() method
import java.util.*;
  
public class IdentityHash_Map_Demo {
    public static void main(String[] args)
    {
  
        // Creating an empty IdentityHashMap
        Map<String, Integer> Identity_hash = new 
                      IdentityHashMap<String, Integer>();
  
        // Mapping int values to string keys
        Identity_hash.put("Geeks", 10);
        Identity_hash.put("4", 15);
        Identity_hash.put("Geeks", 20);
        Identity_hash.put("Welcomes", 25);
        Identity_hash.put("You", 30);
  
        // Displaying the IdentityHashMap
        System.out.println("Initial Mappings are: " + 
                                          Identity_hash);
  
        // Creating a new Identityhash map and copying
        Map<String, Integer> new_Identityhash_map = new 
                      IdentityHashMap<String, Integer>();
        new_Identityhash_map.putAll(Identity_hash);
  
        // Displaying the final IdentityHashMap
        System.out.println("The new map: " + 
                                   new_Identityhash_map);
    }
}

输出:

Initial Mappings are: {Welcomes=25, 4=15, You=30, Geeks=20}
The new map: {Welcomes=25, 4=15, You=30, Geeks=20}

注意： 同样的操作可以通过不同数据类型的变化和组合对任何类型的Mapping进行操作。