Map common traversal methods - keySet and entrySet

1. Traverse using keySet

implementation steps:

(1) Use the method keySet() in the Map collection to take out all the keys of the Map collection and store them in a Set collection

(2) Traverse the set collection to obtain each key in the Map collection

(3) Through the method get(key) in the Map collection, find the value through the key

 @Testpublic void testKeySet() {Map map = new ConcurrentHashMap<>(16);map.put("one",1);map.put("two",2);map.put("three",3);// 1. Enhance the for loop to traverse the keys in the mapfor (String key : map.keySet()) {// Find the corresponding value by keyInteger value = map.get(key);System.out.println(key + "->" +value);}System.out.println("-----------------------");// 2. Use an iterator to traverse (all keys in the map collection are taken out and stored in the Set collection)Iterator iteratorKey = map.keySet().iterator();while (iteratorKey.hasNext()){String key = iteratorKey.next();Integer value = map.get(key);System.out.println(key + "->" +value);}}

2. Use entrySet to traverse

implementation steps:

(1) Map.entrySet() maps the key value of the map to a Set collection

(2) The Map.entrySet iterator will generate an EntryIterator, and the returned instance is an object containing key/value pairs

(3)getKey(): returns the key of the key-value pair

(4)getValue(): returns the value of the key-value pair

 @Testpublic void testEntrySet() {Map map = new ConcurrentHashMap<>(16);map.put("one",1);map.put("two",2);map.put("three",3);// The Map.entrySet iterator will generate an EntryIterator, and the returned instance is an object containing key/value pairs.// The iterator in keySet returns only the key object, and it also needs to get the value twice in the map.Therefore, entrySet is about twice as fast as keySet.Set> entries = map.entrySet();Iterator> entryIterator = entries.iterator();while (entryIterator.hasNext()){// Iterates out the object of each instance, including key-value pairsMap.Entry entry = entryIterator.next();String key = entry.getKey();Integer value = entry.getValue();System.out.println(key + "->" + value);}}

3. Comparison

(1) The value obtained by keySet can only be obtained by first obtaining the key

(2) entrySet can directly obtain Key and Value from the iterated instance object after iteration

(3) The traversal results of the two methods are the same. If the query speed is required, select entrySet(), and entrySet can improve performance.