Java中的WeakHashMap putall() 方法

Java.util.WeakHashMap.putAll() 是 WeakHashMap 类的一个内置方法，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_weakhash_map.putAll(exist_weakhash_map)

参数：该方法采用一个参数exist_weakhash_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.WeakHashMap.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 Weak_Hash_Map_Demo {
    public static void main(String[] args)
    {
  
        // Creating an empty WeakHashMap
        Map weak_hash = new 
                      WeakHashMap();
  
        // Mapping string values to int keys
        weak_hash.put(10, "Geeks");
        weak_hash.put(15, "4");
        weak_hash.put(20, "Geeks");
        weak_hash.put(25, "Welcomes");
        weak_hash.put(30, "You");
  
        // Displaying the WeakHashMap
        System.out.println("Initial Mappings are: " + 
                                           weak_hash);
  
        // Creating a new weakhash map and copying
        Map new_weakhash_map = new 
                        WeakHashMap();
        new_weakhash_map.putAll(weak_hash);
  
        // Displaying the final WeakHashMap
        System.out.println("The new map: " + 
                                    new_weakhash_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 WeakHash_Map_Demo {
    public static void main(String[] args)
    {
  
        // Creating an empty WeakHashMap
        Map weak_hash = new 
                      WeakHashMap();
  
        // Mapping int values to string keys
        weak_hash.put("Geeks", 10);
        weak_hash.put("4", 15);
        weak_hash.put("Geeks", 20);
        weak_hash.put("Welcomes", 25);
        weak_hash.put("You", 30);
  
        // Displaying the WeakHashMap
        System.out.println("Initial Mappings are: " + 
                                          weak_hash);
  
        // Creating a new weakhash map and copying
        Map new_weakhash_map = new 
                      WeakHashMap();
        new_weakhash_map.putAll(weak_hash);
  
        // Displaying the final WeakHashMap
        System.out.println("The new map: " + 
                                   new_weakhash_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。