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📜  Java中的Hashmap方法与示例|设置 2 (keySet(), values(), containsKey()..)

📅  最后修改于: 2022-05-13 01:55:21.435000             🧑  作者: Mango

Java中的Hashmap方法与示例|设置 2 (keySet(), values(), containsKey()..)

Java中的HashMap 类方法与示例|设置 1（put()、get()、isEmpty() 和 size()）

在这篇文章中，将讨论更多方法。

  • keySet(): Java.util.HashMap.keySet()它返回此映射中包含的键的 Set 视图。集合由地图支持，因此对地图的更改会反映在集合中，反之亦然。 
    Syntax:
public Set keySet()
Return: a set view of the keys contained in this map
  • values(): Java.util.HashMap.values()它返回此映射中包含的值的集合视图。集合由地图支持，因此对地图的更改会反映在集合中，反之亦然。 
    Syntax:
public Collection values()
Return: a collection view of the values contained in 
this map
  • containsKey(): Java.util.HashMap.containsKey()如果此映射将一个或多个键映射到指定值，则返回true。 
    Syntax:
public boolean containsValue(Object value)
Parameters:
value - value whose presence in this map is to be tested
Return: true if this map maps one or more keys to 
the specified value

    执行： 

    // Java program illustrating usage of HashMap class methods
// keySet(), values(), containsKey()
import java.util.*;
public class NewClass
{
    public static void main(String args[])
    {
        // 1   Creation of HashMap
        HashMap Geeks = new HashMap<>();
  
        // 2   Adding values to HashMap as ("keys", "values")
        Geeks.put("Language", "Java");
        Geeks.put("Platform", "Geeks For geeks");
        Geeks.put("Code", "HashMap");
        Geeks.put("Learn", "More");
  
        // 3  containsKey() method is to check the presence
        //    of a particluar key
        // Since 'Code' key present here, the condition is true
        if (Geeks.containsKey("Code"))
            System.out.println("Testing .containsKey : " +
                                           Geeks.get("Code"));
  
        // 4 keySet() method returns all the keys in HashMap
        Set Geekskeys = Geeks.keySet();
        System.out.println("Initial keys  : " + Geekskeys);
  
  
        // 5  values() method return all the values in HashMap
        Collection Geeksvalues = Geeks.values();
        System.out.println("Initial values : " + Geeksvalues);
  
        // Adding new set of key-value
        Geeks.put("Search", "JavaArticle");
  
        // Again using .keySet() and .values() methods
        System.out.println("New Keys : " + Geekskeys);
        System.out.println("New Values: " + Geeksvalues);
    }
}

    输出： 

    Testing .containsKey : HashMap
Initial keys  : [Language, Platform, Learn, Code]
Initial values : [Java, Geeks For geeks, More, HashMap]
New Keys : [Language, Platform, Search, Learn, Code]
New Values: [Java, Geeks For geeks, JavaArticle, More, HashMap]
  • .entrySet() : Java.util.HashMap.entrySet()方法返回 HashMap 中存在的一组完整的键和值。 
    Syntax:
public Set entrySet()
Return:
complete set of keys and values
  • .getOrDefault : Java.util.HashMap.getOrDefault()方法返回一个默认值，如果使用我们在 HashMap 中作为参数传递的键没有找到值。如果键的值已经存在于 HashMap 中，则它不会对其执行任何操作。
    这是将值分配给尚未映射的键的非常好的方法，而不会干扰已经存在的键和值集。 
    Syntax:
default V getOrDefault(Object key,V defaultValue)
Parameters:
key - the key whose mapped value we need to return
defaultValue - the default for the keys present in HashMap
Return:
mapping the unmapped keys with the default value.
  • .replace() : Java.util.HashMap.replace(key, value)或Java.util.HashMap.replace(key, oldvalue, newvalue)方法是Java.util.HashMap 类方法。
    第一种方法接受一组键和值，它将用参数中传递的新值替换键的现有值。如果不存在这样的集合，replace() 方法将什么也不做。
    同时，如果在 HashMap 中找到 key 和 old_Value，则第二种方法只会替换已经存在的 key-old_value 集。 
    Syntax:
replace(k key, v value)
          or
replace(k key, v oldvalue, newvalue)
Parameters:
key      - key in set with the old value.
value    - new value we want to be with the specified key
oldvalue - old value in set with the specified key
newvalue - new value we want to be with the specified key
Return:
True - if the value is replaced
Null - if there is no such set present
  • .putIfAbsent Java.util.HashMap.putIfAbsent(key, value)方法用于将新的键值集插入到 HashMap（如果存在相应的集）。如果 HashMap 中已经存在这样的键值集，则返回空值。 
    Syntax:
public V putIfAbsent(key, value)
Parameters:
key      - key with which the specified value is associates.
value    - value to associates with the specified key.

    // Java Program illustrating HashMap class methods(). 
// entrySet(), getOrDefault(), replace(), putIfAbsent
import java.util.*;
public class NewClass
{
    public static void main(String args[])
    {
        // Creation of HashMap
        HashMap Geeks = new HashMap<>();
  
        // Adding values to HashMap as ("keys", "values")
        Geeks.put("Language", "Java");
        Geeks.put("Code", "HashMap");
        Geeks.put("Learn", "More");
  
        // .entrySet() returns all the keys with their values present in Hashmap
        Set> mappingGeeks = Geeks.entrySet();
        System.out.println("Set of Keys and Values using entrySet() : "+mappingGeeks);
        System.out.println();
  
        // Using .getOrDefault to access value
        // Here it is Showing Default value as key - "Code" was already present
        System.out.println("Using .getorDefault : " 
                                    + Geeks.getOrDefault("Code","javaArticle"));
  
        // Here it is Showing set value as key - "Search" was not present
        System.out.println("Using .getorDefault : "
                                    + Geeks.getOrDefault("Search","javaArticle"));
        System.out.println();
  
        // .replace() method replacing value of key "Learn"
        Geeks.replace("Learn", "Methods");
        System.out.println("working of .replace()     : "+mappingGeeks);
        System.out.println();
  
        /* .putIfAbsent() method is placing a new key-value
            as they were not present initially*/
        Geeks.putIfAbsent("cool", "HashMap methods");
        System.out.println("working of .putIfAbsent() : "+mappingGeeks);
  
        /* .putIfAbsent() method is not doing anything
            as key-value were already present */
        Geeks.putIfAbsent("Code", "With_JAVA");
        System.out.println("working of .putIfAbsent() : "+mappingGeeks);
  
    }
}

    输出： 

    Set of Keys and Values using entrySet() : [Language=Java, Learn=More, Code=HashMap]

Using .getorDefault : HashMap
Using .getorDefault : javaArticle

working of .replace()     : [Language=Java, Learn=Methods, Code=HashMap]

working of .putIfAbsent() : [Language=Java, cool=HashMap methods, Learn=Methods, Code=HashMap]
working of .putIfAbsent() : [Language=Java, cool=HashMap methods, Learn=Methods, Code=HashMap]
  • remove(Object key)：如果存在，则从此映射中删除此键的映射。 
// Java Program illustrating remove() method using Iterator.
  
import java.util.*;
public class NewClass
{
    public static void main(String args[])
    {
        //  Creation of HashMap
        HashMap Geeks = new HashMap<>();
  
        //  Adding values to HashMap as ("keys", "values")
        Geeks.put("Language", "Java");
        Geeks.put("Platform", "Geeks For geeks");
        Geeks.put("Code", "HashMap");
  
  
        //  .entrySet() returns all the keys with their values present in Hashmap
        Set> mappingGeeks = Geeks.entrySet();
        System.out.println("Set of Keys and Values : "+mappingGeeks);
        System.out.println();
  
        //  Creating an iterator
        System.out.println("Use of Iterator to remove the sets.");
        Iterator> geeks_iterator = Geeks.entrySet().iterator();
        while(geeks_iterator.hasNext())
        {
            Map.Entry entry = geeks_iterator.next();
            //  Removing a set one by one using iterator
            geeks_iterator.remove(); // right way to remove entries from Map,
            // avoids ConcurrentModificationException
            System.out.println("Set of Keys and Values : "+mappingGeeks);
  
        }
    }
}

输出： 

Set of Keys and Values : [Language=Java, Platform=Geeks For geeks, Code=HashMap]

Use of Iterator to remove the sets.
Set of Keys and Values : [Platform=Geeks For geeks, Code=HashMap]
Set of Keys and Values : [Code=HashMap]
Set of Keys and Values : []

优势：
如果我们使用 for 循环，它会在内部转换为 Iterator，但如果不显式使用 Iterator，我们将无法在 Iteration 期间删除任何条目。这样做时，Iterator 可能会抛出 ConcurrentModificationException。所以，我们使用显式迭代器和while循环来遍历。

参考：
https://docs.oracle.com/javase/7/docs/api/java Java