Java中的 IdentityHashMap putAll() 方法

Java.util.IdentityHashMap.putAll() 是用于复制操作的 IdentityHashMap 类的内置方法。该方法将所有元素（即映射）从一个映射复制到另一个映射。

句法：

new_Identityhash_map.putAll(exist_Identityhash_map)

参数：该方法采用一个参数exist_Identityhash_map ，该参数引用我们要从中复制的现有地图。

返回值：该方法不返回任何值。

异常：如果我们要从中复制的地图为 NULL，该方法将抛出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 Identity_hash = new 
                      IdentityHashMap();
  
        // 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 new_Identityhash_map = new 
                        IdentityHashMap();
        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 Identity_hash = new 
                      IdentityHashMap();
  
        // 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 new_Identityhash_map = new 
                      IdentityHashMap();
        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：可以对任何类型的映射执行相同的操作，这些映射具有不同数据类型的变化和组合。