How to iterate through a Map in java?

问题

I need to iterate through a BucketMap and get all keys but how do I get to something like buckets[i].next.next.next.key for instance without doing it manually as I tried here:

public String[] getAllKeys() {

    int j = 0;      //index of string array "allkeys"

    String allkeys[] = new String[8];

    for(int i = 0; i < buckets.length; i++) { //iterates through the bucketmap

        if(buckets[i] != null) {             //checks wether bucket has a key and value
            allkeys[j] = buckets[i].key;     //adds key to allkeys
            j++;                             // counts up the allkeys index after adding key

            if(buckets[i].next != null) {         //checks wether next has a key and value
                allkeys[j] = buckets[i].next.key; //adds key to allkeys
                j++;
            }
        }
    }

    return allkeys;
}

Also how can I initialize the String[] allkeys using the version of j we get after the iteration is done as the index?

回答1:

For basic utilisation, the HashMap is the best, I've put how to iterate over it, easier than using an iterator :

public static void main (String[] args) {
    //a map with key type : String, value type : String
    Map<String,String> mp = new HashMap<String,String>();
    mp.put("John","Math");    mp.put("Jack","Math");    map.put("Jeff","History");

    //3 differents ways to iterate over the map
    for (String key : mp.keySet()){
        //iterate over keys
        System.out.println(key+" "+mp.get(key));
    }

    for (String value : mp.values()){
        //iterate over values
        System.out.println(value);
    }

    for (Entry<String,String> pair : mp.entrySet()){
        //iterate over the pairs
        System.out.println(pair.getKey()+" "+pair.getValue());
    }
}

A quick explanation :

for (String name : mp.keySet()){
        //Do Something
}

means : "For all string from the keys of the map, we'll do something, and at each iteration we will call the key 'name' (it can be whatever you want, it's a variable)

Here we go :

public String[] getAllKeys(){ 
    int i = 0;
    String allkeys[] = new String[buckets.length];
    KeyValue val = buckets[i];

    //Look at the first one          
    if(val != null) {             
        allkeys[i] = val.key; 
        i++;
    }

    //Iterate until there is no next
    while(val.next != null){
        allkeys[i] = val.next.key;
        val = val.next;
        i++;
    }

    return allkeys;
}

回答2:

See if this helps,

  HashMap< String, String> map = new HashMap<>();
  Set<String> keySet = map.keySet();
  Iterator<String> iterator = keySet.iterator();
  while(iterator.hasNext())
  {
    //iterate over keys
  }

//or iterate over entryset 
Iterator<Entry<String, String>> iterator2 = map.entrySet().iterator();

while(iterator2.hasNext())
{
    Entry<String, String> next = iterator2.next();
    //get key
    next.getKey();
    //get value
    next.getValue();
}

回答3:

With Java 8, I would suggest you to use Stream API.

It will allow you to iterate through the Map in a much more convenient approach:

public void iterateUsingStreamAPI(Map<String, Integer> map) {
    map.entrySet().stream()
      // ...
      .forEach(e -> System.out.println(e.getKey() + ":" + e.getValue()));
}

Check out for more examples about iteration through maps in Java.

来源：https://stackoverflow.com/questions/43015098/how-to-iterate-through-a-map-in-java

标签

java

loops

hashmap

iterator

initialization