How to get key and value of map in JAVA

Editor to share with you how to get map key and value in JAVA, I believe most people do not know much about it, so share this article for your reference, I hope you can learn a lot after reading this article, let's go to know it!

There are two ways to get the key and value of map:

Map.keySet (): first get the key of map, then get the corresponding value; map..entrySet () according to key: query the key and value of map at the same time, you only need to query it once.

When traversing key and value at the same time, the performance difference between keySet and entrySet depends on the specific conditions of key, such as complexity (complex objects), discreteness, conflict rate and so on. In other words, it depends on the cost of finding value by HashMap. EntrySet's operation of fetching all key and value at once has a performance overhead, and when this loss is less than the cost of finding value by HashMap, the performance advantage of entrySet will be reflected.

The keySet method is more appropriate when only traversing the key, because entrySet takes out the useless value, wasting performance and space.

When only traversing the value, the vlaues method is the best choice, and entrySet is slightly better than the keySet method.

Through the map.keySet () method

Method 1: get the value of key and then get value

For (String key: map.keySet ()) {String value = map.get (key); System.out.println (key+ "" + value);}

Method 2: use an iterator to get key

Iterator iter = map.keySet (). Iterator (); while (iter.hasNext ()) {String key=iter.next (); String value = map.get (key); System.out.println (key+ "" + value);}

Through the map.entrySet () method

Method 1: loop each key-value pair in map, and then get key and value

For (Entry vo: map.entrySet ()) {vo.getKey (); vo.getValue (); System.out.println (vo.getKey () + "+ vo.getValue ());}

Method 2: use an iterator to get key

Iterator iter = map.entrySet (). Iterator (); while (iter.hasNext ()) {Entry entry = iter.next (); String key = entry.getKey (); String value = entry.getValue (); System.out.println (key+ "" + value);}

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