Java中的Hashtable contains()方法

Java中的Java .util.Hashtable.contains( Object value ) 方法用于检查特定值是否被哈希表中存在的任何键映射。

句法：

Hash_table.contains(Object value)

参数：该方法接受一个对象类型的参数值，并引用待验证映射的哈希表的值。

返回值：如果传递的值由 Hashtable 中的任何键映射，则该方法返回布尔真值。

异常：如果传递的值为Null，该方法将抛出NullPointerException 。

下面的程序说明了上述方法的工作：
方案一：

// Java code to illustrate the contains() method
import java.util.*;
  
public class Hash_Table_Demo {
    public static void main(String[] args)
    {
  
        // Creating an empty Hashtable
        Hashtable hash_table = 
        new Hashtable();
  
        // Mapping string values to int keys
        hash_table.put(10, "Geeks");
        hash_table.put(15, "4");
        hash_table.put(20, "Geeks");
        hash_table.put(25, "Welcomes");
        hash_table.put(30, "You");
  
        // Displaying the HashMap
        System.out.println("Initial Table is: " + hash_table);
  
        // Checking for the Value 'Geeks'
        System.out.println("Is the value 'Geeks' present? " + 
        hash_table.contains("Geeks"));
  
        // Checking for the Value 'World'
        System.out.println("Is the value 'World' present? " + 
        hash_table.contains("World"));
    }
}

输出：

Initial Table is: {10=Geeks, 20=Geeks, 30=You, 15=4, 25=Welcomes}
Is the value 'Geeks' present? true
Is the value 'World' present? false

方案二：

// Java code to illustrate the contains() method
import java.util.*;
  
public class Hash_Table_Demo {
    public static void main(String[] args)
    {
  
        // Creating an empty Hashtable
        Hashtable hash_table = 
        new Hashtable();
  
        // Mapping int values to string keys
        hash_table.put("Geeks", 10);
        hash_table.put("4", 15);
        hash_table.put("Geeks", 20);
        hash_table.put("Welcomes", 25);
        hash_table.put("You", 30);
  
        // Displaying the Hashtable
        System.out.println("Initial Mappings are: " + hash_table);
  
        // Checking for the Value '10'
        System.out.println("Is the value '10' present? " + 
        hash_table.containsValue(10));
  
        // Checking for the Value '30'
        System.out.println("Is the value '30' present? " + 
        hash_table.containsValue(30));
  
        // Checking for the Value '40'
        System.out.println("Is the value '40' present? " + 
        hash_table.containsValue(40));
    }
}

输出：

Initial Mappings are: {You=30, Welcomes=25, 4=15, Geeks=20}
Is the value '10' present? false
Is the value '30' present? true
Is the value '40' present? false