HashMap的java.util.HashMap.put()方法用于将映射插入到映射中。这意味着我们可以将特定键及其映射的值插入到特定映射中。如果传递现有密钥,则先前的值将被新值替换。如果传递了一对新对,那么该对将作为整体插入。

句法:

Hash_Map.put(key, value)

参数:该方法有两个参数,都是HashMap的Object类型。

  • key:这是指需要插入Map进行映射的关键元素。
  • value:这是指上面的键映射到的值。

返回值:如果传递现有密钥,则返回先前的值。如果传递了一对新对,则返回NULL。

下面的程序用于说明java.util.HashMap.put()的工作方法

HashMap.put()程序1:传递现有密钥时。

// Java code to illustrate the put() 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);

        // Inserting existing key along with new value
        String returned_value = (String)hash_map.put(20, "All");

        // Verifying the returned value
        System.out.println("Returned value is: " + returned_value);

        // Displayin the new map
        System.out.println("New map is: " + hash_map);
    }
}

输出:

Initial Mappings are: {20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}
Returned value is: Geeks
New map is: {20=All, 25=Welcomes, 10=Geeks, 30=You, 15=4}

HashMap.put()程序2:传递新密钥时。

// Java code to illustrate the put() 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);

        // Inserting existing key along with new value
        String returned_value = (String)hash_map.put(50, "All");

        // Verifying the returned value
        System.out.println("Returned value is: " + returned_value);

        // Displayin the new map
        System.out.println("New map is: " + hash_map);
    }
}

输出:

Initial Mappings are: {20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}
Returned value is: null
New map is: {50=All, 20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}

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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