CODEWITHSUNDEEP
javafor-looparraylistwhile-loophashmap
TheBigDoubleA
TheBigDoubleA

Reputation: 452

How to add an ArrayList to a Hashmap in Java

I have the following HashMap (called "trafficInput") with cities. Each city consists of a list of daily timestamps in milliseconds with two more data points each. One city ("paris") is empty:

{"paris":[],"london":[[1485907200000,182184411,41274],[1485993600000,151646118,36697],[1486080000000,48486138,18998],[1486166400000,5405780,5246],[1486252800000,1194440,1370]],"zurich":[[1485907200000,30200160,155827],[1485993600000,26681354,160269]]}

I want to iterate through this HashMap and add any daily timestamps (with 0 as the two other data points) since February 1, 2017 up until today if they are not yet included in the list. So the optimal output would be:

{"paris":[[1485907200000,0,0],[1485993600000,0,0],[1486080000000,0,0],[1486166400000,0,0],[1486252800000,0,0]],"london":[[1485907200000,182184411,41274],[1485993600000,151646118,36697],[1486080000000,48486138,18998],[1486166400000,5405780,5246],[1486252800000,1194440,1370]],"zurich":[[1485907200000,30200160,155827],[1485993600000,26681354,160269],[1486080000000,0,0],[1486166400000,0,0],[1486252800000,0,0]]}

I've coded up the following:

long startTime = 1485907200; // 01 Feb 2017 00:00:00 GMT
long currentTime = (System.currentTimeMillis() / 1000L);
while (startTime < currentTime) {
    for (String city : trafficInput.keySet()) {
        for (long[] cityAllValues : trafficInput.get(city)) {
            long[] newCityValues = {startTime*1000, 0, 0};
            ArrayList newCityValuesList = new ArrayList<>();
            newCityValuesList.add(newCityValues);
            trafficInput.put(city, newCityValuesList);
        }
    }
startTime = startTime+86400;
}

Unfortunately, the code simply overwrites all existing values, without appending them. What am I missing here?

Upvotes: 0

Views: 118

Answers (1)

BackSlash
BackSlash

Reputation: 22233

The put method does not append, it overwrites. You need to append the values yourself, so instead of doing

ArrayList newCityValuesList = new ArrayList<>(); // create new empty arraylist
newCityValuesList.add(newCityValues);
trafficInput.put(city, newCityValuesList);

You should grab the existing values list and add to them:

ArrayList newCityValuesList = trafficInput.get(city); //get existing values
newCityValuesList.add(newCityValues);
trafficInput.put(city, newCityValuesList);

This way it will still overwrite, but since you grabbed the existing data the result will be the original data + the appended values

Upvotes: 1

Related Questions