如何遍历java中的Map？

Question

我需要遍历BucketMap并获取所有keys但是我如何在不手动操作的情况下获得诸如buckets[i].next.next.next.key类的东西，就像我在这里尝试的那样：

public String[] getAllKeys() {
    //index of string array "allkeys"
    int j = 0;
    String allkeys[] = new String[8];
    //iterates through the bucketmap
    for (int i = 0; i < buckets.length; i++) {
        //checks wether bucket has a key and value
        if (buckets[i] != null) {
            //adds key to allkeys
            allkeys[j] = buckets[i].key;
            // counts up the allkeys index after adding key
            j++;
            //checks wether next has a key and value
            if (buckets[i].next != null) {
                //adds key to allkeys
                allkeys[j] = buckets[i].next.key;
                j++;
            }
        }
    }
    return allkeys;
}

另外，如何使用迭代完成后获得的j版本作为索引来初始化String[] allkeys ？

Answer 1

对于基本使用，HashMap 是最好的，我已经介绍了如何迭代它，比使用迭代器更容易：

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());
    }
}

快速解释：

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

意思是：“对于来自地图键的所有字符串，我们会做一些事情，并且在每次迭代时我们将调用键'name'（它可以是任何你想要的，它是一个变量）

---

开始了 ：

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;
}

Answer 2

看看这是否有帮助，

  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();
}

Answer 3

对于 Java 8，我建议您使用 Stream API。

它将允许您以更方便的方法遍历 Map：

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

查看有关Java 中映射迭代的更多示例。