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。
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。
??Java中的HashMapJava中HashMap的putAll()方法: HashMap.putAll() - Break易站
原文：https://www.cnblogs.com/breakyizhan/p/13296111.html