在Java中初始化后分配ArrayList

Question

I am new to Java and I have a silly question. Here's my pseudocode:

private static Map<String, ArrayList<byte[]> > myMap = new HashMap<String,ArrayList<byte[]> >();
public void setValue(String key, byte[] value) {
  ArrayList<byte[]> valueList = new ArrayList<byte[]>();
  if(this.myMap.containsKey(key))
  {      
       valueList = this.myMap.get(key);
  }
  valueList.add(value);
  this.myMap.put(key, valueList);
}

What I am trying to do is to check if the given key already has some value in the map. If yes, it will retrieve the list of values and add the new value to this list. If not, it will add a new key and list of value (with just one value) in the map.

My question is about initializing the ArrayList with 'new' and then assigning it the value returned by this.myMap.get(key). Is it a good way to do it? In C++ it will be an issue. I am not sure about Java. What do you think?

Answer 1

Your attempt is a reasonable way to achieve what you ask. The classic way of doing this, though, is to attempt to retrieve and then to populate it if no value was retrieved:

Map<String, ArrayList<byte[]>> myMap = new HashMap<>();

public void setValue(String key, byte[] value) {
    ArrayList<byte[]> valueList = this.myMap.get(key);
    if (valueList == null) {
        valueList = new ArrayList<>();
        this.myMap.put(key, valueList);
    }
    valueList.add(value);
}

This saves one key lookup and one value insertion and consolidates the initialization logic to 4 consecutive lines.

If you are using Java 8, then you can do even better by using the Map#computeIfAbsent method. This is more efficient, more concise, and has the added advantage of maintaining concurrency constraints for map implementations that support concurrent operations:

public void setValue(String key, byte[] value) {
    ArrayList<byte[]> valueList = this.myMap.computeIfAbsent(key, k -> new ArrayList<>());
    valueList.add(value);
}

Or even more concisely:

public void setValue(String key, byte[] value) {
    this.myMap.computeIfAbsent(key, k -> new ArrayList<>())
            .add(value);
}