Java 中的 Hashmap 方法，示例| Set 2 (keySet()，values()，containsKey()..)
原文:https://www . geesforgeks . org/hashmap-methods-Java-examples-set-2-key set-values-contains key/
Java 中的 HashMap 类方法，示例| Set 1 (put()、get()、isEmpty()和 size())
这篇文章讨论了更多的方法。
-
KeySet():Java . util . HashMap . KeySet()它返回此地图中包含的键的集合视图。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。
```java Syntax: public Set keySet() Return: a set view of the keys contained in this map
```
-
values():Java . util . hashmap . values()它返回此地图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() It returns true if this map maps one or more keys to the specified value.
```java 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
```
实施:
```java // 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:
```java 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。
```java Syntax: public Set entrySet() Return: complete set of keys and values
```
- 。getOrDefault:Java . util . HashMap . getOrDefault()如果使用我们在 HashMap 中作为参数传递的键没有值 find，则方法返回默认值。如果键的值已经存在于 HashMap 中，它不会对它做任何事情。 为尚未映射的键赋值是非常好的方法，不会干扰已经存在的键和值集。
```java 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) 方法是一个 Java . util . hashmap 类方法。 第一个方法接受一组键和值，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()方法将不做任何事情。 同时，如果在哈希表中找到键和旧值，第二种方法将只替换已经存在的键-旧值集。
```java 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) method is being used to insert a new key-value set to the HashMap if the respective set is present. Null value is returned if such key-value set is already present in the HashMap.
```java 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 // 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:
```java 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): Removes the mapping for this key from this map if present.
```java // 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:
```java 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，它会在内部被翻译成迭代器，但是如果不显式使用迭代器，我们就不能在迭代过程中移除任何条目。这样做时，迭代器可能会抛出 ConcurrentModificationException。因此，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/util/hashmap . html
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