打印 Java ConcurrentHashMap 中的所有键/值对

Question

I am trying to simply print all key/value pair(s) in a ConcurrentHashMap.

I found this code online that I thought would do it, but it seems to be getting information about the buckets/hashcode. Actually to be honest the output it quite strange, its possible my program is incorrect, but I first want to make sure this part is what I want to be using.

for (Entry<StringBuilder, Integer> entry : wordCountMap.entrySet()) {
    String key = entry.getKey().toString();
    Integer value = entry.getValue();
    System.out.println("key, " + key + " value " + value);
}

This gives output for about 10 different keys, with counts that seem to be the sum of the number of total inserts into the map.

Answer 1

I tested your code and works properly. I've added a small demo with another way to print all the data in the map:

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<String, Integer>();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

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

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

Answer 2

The HashMap has forEach as part of its structure. You can use that with a lambda expression to print out the contents in a one liner such as:

map.forEach((k,v)-> System.out.println(k+", "+v));

or

map.forEach((k,v)-> System.out.println("key: "+k+", value: "+v));

Answer 3

You can do something like

Iterator iterator = map.keySet().iterator();

while (iterator.hasNext()) {
   String key = iterator.next().toString();
   Integer value = map.get(key);

   System.out.println(key + " " + value);
}

Here 'map' is your concurrent HashMap.

Answer 4

  //best and simple way to show keys and values

    //initialize map
    Map<Integer, String> map = new HashMap<Integer, String>();

   //Add some values
    map.put(1, "Hi");
    map.put(2, "Hello");

    // iterate map using entryset in for loop
    for(Entry<Integer, String> entry : map.entrySet())
    {   //print keys and values
         System.out.println(entry.getKey() + " : " +entry.getValue());
    }

   //Result : 
    1 : Hi
    2 : Hello

Answer 5

The ConcurrentHashMap is very similar to the HashMap class, except that ConcurrentHashMap offers internally maintained concurrency. It means you do not need to have synchronized blocks when accessing ConcurrentHashMap in multithreaded application.

To get all key-value pairs in ConcurrentHashMap , below code which is similar to your code works perfectly:

//Initialize ConcurrentHashMap instance
ConcurrentHashMap<String, Integer> m = new ConcurrentHashMap<String, Integer>();

//Print all values stored in ConcurrentHashMap instance
for each (Entry<String, Integer> e : m.entrySet()) {
  System.out.println(e.getKey()+"="+e.getValue());
}

Above code is reasonably valid in multi-threaded environment in your application. The reason, I am saying 'reasonably valid' is that, above code yet provides thread safety, still it can decrease the performance of application.

Hope this helps you.

Answer 6

Work 100% sure try this code for the get all hashmap key and value

static HashMap<String, String> map = new HashMap<>();

map.put("one"  " a " );
map.put("two"  " b " );
map.put("three"  " c " );
map.put("four"  " d " );

just call this method whenever you want to show the HashMap value

 private void ShowHashMapValue() {

        /**
         * get the Set Of keys from HashMap
         */
        Set setOfKeys = map.keySet();

/**
 * get the Iterator instance from Set
 */
        Iterator iterator = setOfKeys.iterator();

/**
 * Loop the iterator until we reach the last element of the HashMap
 */
        while (iterator.hasNext()) {
/**
 * next() method returns the next key from Iterator instance.
 * return type of next() method is Object so we need to do DownCasting to String
 */
            String key = (String) iterator.next();

/**
 * once we know the 'key', we can get the value from the HashMap
 * by calling get() method
 */
            String value = map.get(key);

            System.out.println("Key: " + key + ", Value: " + value);
        }
    }