在Java hashMap中添加键值对

Question

I have a map which maps cost with position:

Map<Vector<Double>,Double> positionCost=new HashMap<Vector<Double>,Double>();

positions are Vectors of type double.

I am putting cost for each position by:

positionCost.put(position, newcost);

Now I have a vector where I save all the cost produced cost.add(newcost); for all the positions. But there is one problem - the size of HashMap is not equal to the size of vector of costs.

System.out.println("no of particles"+" "+cost.size()+positionCost.size());

I am not able to figure out why.

Answer 1

The size of the positionCost Map won't be the same as the size of the cost Vector if you are adding the same position key more than once to the Map . In that case, the latter value associated with that key will overwrite the previous value that was associated that key, and the size of the Map will stay the same.

You should add a condition before adding to the map :

if (!positionCost.containsKey(position)) {
    positionCost.put(position, newcost);
} else {
    // the key already exists in the map. It might be a bug, or it might be 
    // a valid situation that you have to decide how to handle
}

Answer 2

这是因为哈希图的大小取决于键的数量。如果键相同，则将其计为1。因此，在您的情况下，哈希图计算的是位置数。因此值不同

Answer 3

Is it possible that you are adding a position-to-cost mapping to your map multiple times for the same position? Ie the same Vector object? Or perhaps some position objects have the same hashcode? This will mean that you will replace an entry in your map at some point. The Vector object behaves differently - it merely appends the object. So I suggest printing the hashcode of the position object you are using as a key to debug.