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    Java.util.IdentityHashMap.putAll() 是IdentityHashMap类的内置方法,用于复制操作。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,该参数引用我们要复制的现有map集合

    返回值:该方法不返回任何值。

    异常:如果我们要复制的映射为NULL,则该方法抛出NullPointerException

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

    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}

     
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