Traversal method of HasMap [7 types]

7 traversal methods of HashMap

The study notes can help others better~
I suggest you read the original HashMap's 7 traversal methods and performance analysis! "Correction"
注：1）尽量使用 entrySet 来实现 Map 集合的遍历，性能更高
When traversing EntrySet, directly put the obtained ley and value into the entry pair, which can be obtained directly;
while KeySet uses map.get(key) when traversing, which is equivalent to traversing the Map collection again to query the key The corresponding value is therefore less efficient.
2）Iterator迭代器 和 forEach遍历的效率相差不大

1. The concise code of each method is given below

Note: The following entry is in the form of Integer:String

1. Iterator iterator traversal - EntrySet

   Iterator<Map.Entry<Integer, String>> iterator = map.EntrySet().iterator();
   while(iterator.hasNext()){
         
         
   	Map.Entry<Integer, String> entry = ietrator.next();
   	System.out.println("key--" + enrty.getKey());	
   	System.out.println("value--" + enrty.getValue());	
   }
   

2. Iterator iterator traversal - KeySet

   Iterator<Integer> iterator = map.KeySet().iterator();
   while(iterator.hasNext()){
         
         
   	Integer key = iterator.next();
   	System.out.println("key--" + key);
   	System.out.println("value--" + map.get(key));
   }
   

3. For Each traversal - EntrySet

   for(Map.Entry<Integer, String> entry : map.EntrySet()){
         
         
   	 System.out.println("key--" + entry.getKey());
   	 System.out.println("value--" + entry.getValue());
   }
   

4. For Each traversal - KeySet

   for(Integer key : map.KeySet()){
         
         
   	System.out.println("key--" + key);
   	System.out.println("value--" + map.get(key));
   }
   

5. Lambda expression traversal (JDK1.8+)

   map.forEach((key, value)->{
         
         
   	System.out.println("key--" + key);
       System.out.println("value--" + value);
   });
   

6. Streams API traversal (JDK1.8+) - single-threaded stream()

   map.entrySet().stream().forEach((entry)->{
         
         
   	System.out.println("key--" + entry.getKey());
   	System.out.println("value--" + entry.getValue());
   });
   

7. Streams API traversal (JDK1.8+) - multi-threaded parallelStream()

   map.entrySet().parallelStream().forEach((entry) -> {
         
         
       System.out.println("key--" + entry.getKey());
   	System.out.println("value--" + entry.getValue());
   });