collections - Merge two maps with same keys in java and add the value together

Question

Closed. This question needs details or clarity. It is not currently accepting answers. question from:https://stackoverflow.com/questions/65945496/merge-two-maps-with-same-keys-in-java-and-add-the-value-together

Answer 1

Either the values in col map should be of numeric type to allow for arithmetic operations, or another map Map<String, Integer> should be created to store the results of calculations.

Also, there's no need to have a nested loop to calculate the sums while reading the data because the calculation results will be incorrect.

There are several ways to accumulate the sums in the map per key.

1. Use method Map::compute

Map<String, Integer> col = new HashMap<>(); // changed value type to Integer
// ...
Integer number = 0;
while (cmd.hasNextLine()) {
    String line = cmd.nextLine();
    if (line.charAt(9) == 'A') {
        number = Integer.valueOf(line.substring(23, 28)); 
        Name = line.substring(29, 34);
        col.compute(Name, (key, prev) -> (prev == null ? 0 : prev) + number);     
    }
    // no need for nested loop
}

// print map contents
col.forEach((name, sum) -> System.out.print("%s %04d%n", name, sum));

2. Use method Map::merge along with Integer::sum which can be replaced with a lambda (sum, val)-> sum + val:

Integer number = 0;
while (cmd.hasNextLine()) {
    String line = cmd.nextLine();
    if (line.charAt(9) == 'A') {
        number = Integer.valueOf(line.substring(23, 28)); 
        Name = line.substring(29, 34);
        col.merge(Name, number, Integer::sum);     
    }
}
// print map contents
col.forEach((name, sum) -> System.out.print("%s %04d%n", name, sum));