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  • 2018-08-29  08:29:43        
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    Java.util.HashMap.putAll()是HashMap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_hash_map.putAll(exist_hash_map)

    参数:该方法采用一个参数exist_hash_map,该参数引用我们要复制的现有HashMap

    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.HashMap.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 Hash_Map_Demo {
    public static void main(String[] args) {
         
        // Creating an empty HashMap
        HashMap<Integer, String> hash_map = new HashMap<Integer, String>();
    
        // Mapping string values to int keys
        hash_map.put(10, "Geeks");
        hash_map.put(15, "4");
        hash_map.put(20, "Geeks");
        hash_map.put(25, "Welcomes");
        hash_map.put(30, "You");
    
        // Displaying the HashMap
        System.out.println("Initial Mappings are: " + hash_map);
    
        // Creating a new hash map and copying
        HashMap<Integer, String> new_hash_map = new HashMap<Integer, String>();
        new_hash_map.putAll(hash_map);
    
        // Displaying the final HashMap
        System.out.println("The new map looks like this: " + new_hash_map);
    }
    }
    

    输出:

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

    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 Hash_Map_Demo {
        public static void main(String[] args)
        {
    
            // Creating an empty HashMap
            HashMap<String, Integer> hash_map = new HashMap<String, Integer>();
    
            // Mapping int values to string keys
            hash_map.put("Geeks", 10);
            hash_map.put("4", 15);
            hash_map.put("Geeks", 20);
            hash_map.put("Welcomes", 25);
            hash_map.put("You", 30);
    
            // Displaying the HashMap
            System.out.println("Initial Mappings are: " + hash_map);
    
            // Creating a new hash map and copying
            HashMap<String, Integer> new_hash_map = new HashMap<String, Integer>();
            new_hash_map.putAll(hash_map);
    
            // Displaying the final HashMap
            System.out.println("The new map looks like this: " + new_hash_map);
        }
    }
    

    输出:

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

    注意:对于具有不同数据类型的变体和组合的任何类型的映射,都可以执行相同的操作。

     
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