Java HashMap 移位值

Question

I have hash map like this below.And i want to shift values up when upper values are null.

HashMap 
Key1 Val1
Key2 
Key3 
Key4 Val4
Key5 
Key6 Val6

Become like this

HashMap 
Key1 Val1
Key2 Val4
Key3 Val6
Key4 
Key5 
Key6 

And then delete null value key like this.

HashMap 
Key1 Val1
Key2 Val4
Key3 Val6

Answer 1

You can't do this meaningfully with a HashMap because it doesn't have a defined ordering, so there is no "second key" - Key2 - to move Val4 up to.

But you can do it if you have an ordered Map , such as a TreeMap (this code works generally on any mutable Map ; but it only gives predictable results on a map with defined iteration order):

Iterator<? extends Map.Entry<?, V>> entryIterator = map.entrySet().iterator();
// Find non-null values, set the next entry's value to that.
for (String val : map.values()) {
  if (val != null) {
    entryIterator.next().setValue(val);
  }
}

// Remove the rest of the entries, because we have no non-null
// values to assign to them.
while (entryIterator.hasNext()) {
  entryIterator.next();
  entryIterator.remove();
}

Ideone Demo