检查 Java HashMap 中是否存在特定密钥
原文:https://www . geesforgeks . org/check-if-special-key-exists-in-Java-hashmap/
HashMap 在 Java 中是实现了 Hash Table 数据结构的排序。它由键和值对组成，符号表示为< K，V >，其中 K 代表键，V 代表值。它是映射接口的一个实现，并且在分别通过 put 和 get 方法分配和访问元素时提供了恒定的时间性能。它用于形成关联的对，并基于另一对访问或修改一个对。
有多种方法可以检查特定的键是否存在，如下所述:
- Use the built-in containskey () method of HashMap class.
- Convert the keys of hashmap into a list , and then traverse them.
- Create a map from all the entries of HashMap, and then traverse it.
方法 1 :
使用这种方法，我们使用了 HashMap 类的 containsKey() 预定义方法，该方法返回一个布尔值。
语法:
Hash_Map.containsKey(*key_element*)
参数:该方法只取一个参数 key_element ，该参数是指映射应该在地图内部检查的键。
返回值:如果检测到键的存在，则该方法返回布尔真，否则返回假
算法:
- Create a function with a return type of Boolean.
- Inside the function, create a new HashMap, specifying the data types of the key and the value respectively.
- Use the put () method of HashMap class to populate HashMap with key-value pairs.
- Declare a Boolean variable to store the result.
- Call the containsKey () method of HashMap class, taking the key to check as the parameter.
- Returns a variable.
代码:
Java
// java program to check if a particular
// key exists in HashMap
import java.util.*;
class GFG {
// declaring the method
// the parameter keyToBeChecked is the
// key to be checked
boolean checkForKey(String keyToBeChecked)
{
// initializing the hashmap
HashMap<String, Integer> hashMap =
new HashMap<>();
// filling the key - value pairs in hashmap
hashMap.put("first", 1);
hashMap.put("second", 2);
hashMap.put("third", 3);
hashMap.put("fourth", 4);
// variable to store the boolean value
// for result
boolean result
= hashMap.containsKey(keyToBeChecked);
// returning the result
return result;
}
// Driver Code
public static void main(String[] args)
{
// instantiating the class
GFG ob = new GFG();
// displaying and calling the checkForKey()
// method
System.out.println(ob.checkForKey("fourth"));
}
}
输出
true
方法 2 :
我们从 HashMap 的键创建一个 ArrayList，然后遍历它们来检查指定的键是否存在。
算法:
- Repeat steps 1 to 3 mentioned in the first method.
- Next, we use the keySet () predefined method of HashMap class to initialize an ArrayList with the same data type as HashMap key.
- Next, we declare an iterator to iterate through the elements of ArrayList created above.
- Then, if the specific key matches any element in the array list created above, we will traverse the array list check in each iteration.
- Returns the corresponding output.
代码:
Java
// java program to check if a particular
// key exists in the HashMap
import java.util.*;
class GFG {
// declaring the method
// the parameter keyToBeChecked is
// the key to be checked
boolean checkForKey(String keyToBeChecked)
{
// initializing the HashMap
HashMap<String, Integer> hashMap =
new HashMap<>();
// filling the key-value pairs
// in the HashMap
hashMap.put("first", 1);
hashMap.put("second", 2);
hashMap.put("third", 3);
hashMap.put("fourth", 4);
// creating an ArrayList from the keys
ArrayList<String> listOfKeys
= new ArrayList<>(hashMap.keySet());
// declaring the iterator
Iterator<String> itr = listOfKeys.iterator();
// loop to iterate through the
// elements of the ArrayList
while (itr.hasNext()) {
// condition to check against
// the specific key
if (itr.next() == keyToBeChecked)
return true;
}
return false;
}
// Driver Code
public static void main(String[] args)
{
// instantiating the class
GFG ob = new GFG();
// displaying and calling the
// checkForKey method
System.out.println(ob.checkForKey("second"));
}
}
输出
true
进场 3 :
我们遍历这些键，检查指定的键是否存在。
算法:
- Repeat steps 1 to 3 in the first method to create a HashMap.
- Use a predefined method of HashMap class for each loop and entrySet () to create an identical Map.
- In each iteration of the for loop, use the built-in getkey () method of the Map class to get a key from the Map constructed above.
- Compare with the specified key.
- Returns true if the key matches any element of the key set, otherwise returns false.
代码:
Java
// java program to check if a particular
// key exists in the HashMap
import java.util.*;
class GFG {
// declaring the method
// the parameter keyToBeChecked specifies
// the particular key
boolean checkForKey(String keyToBeChecked)
{
// initializing the HashMap
HashMap<String, Integer> hashMap = new HashMap<>();
// filling up the key-value
// pairs in the HashMap
hashMap.put("first", 1);
hashMap.put("second", 2);
hashMap.put("third", 3);
hashMap.put("fourth", 4);
// initializing the for each loop
// using which the entries of the
// HashMap are stored in a Set
for (Map.Entry<String, Integer> mapEntries :
hashMap.entrySet()) {
// getting the keys and checking
// against the specified key
if (mapEntries.getKey() == keyToBeChecked)
return true;
}
return false;
}
// Driver Code
public static void main(String[] args)
{
// instantiating the class
GFG ob = new GFG();
// displaying and calling the
// checkForKey method
System.out.println(ob.checkForKey("seventh"));
}
}
输出
false
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