How can I sum the values associated with a reoccurring key in a hashmap

问题

I want to add the values of the same keys in a hashmap. For example:

   ABC --> 123  
   DEF --> 456  
   ABC --> 123  
   XXX --> 111  
   XXX --> 222

should become:

   ABC --> 246  
   DEF --> 456  
   XXX --> 333  

Here's the code I have so far:

public class Reading {


@SuppressWarnings("unchecked")
public static void main(String[] args) throws IOException {
    //Create hashmap to store the the string and int in the excel sheet
    HashMap<String, Integer> hm = new HashMap<String, Integer>();

    String key = null;
    int value = Integer.MIN_VALUE;
    // String chkeq; // this

    @SuppressWarnings("rawtypes")
    ArrayList list = new ArrayList();

    // File path or EXCEL file
    FileInputStream fos = new FileInputStream(
            "/Users/SG/Desktop/tester.xls");

    // Sheet is the individual sheet that the data is coming from, in this
    // case Sheet1

    try {

        // Put the XLS file into
        HSSFWorkbook workbook = new HSSFWorkbook(fos);

        // Get first sheet from the project
        HSSFSheet sheet = workbook.getSheetAt(0);

        // Go through each row
        Iterator<Row> rowIterator = sheet.iterator();
        while (rowIterator.hasNext()) {


            Row row = rowIterator.next();

            // Go through each column in the rows
            Iterator<Cell> cellIterator = row.cellIterator();
            while (cellIterator.hasNext()) {

                Cell cell = cellIterator.next();

                switch (cell.getCellType()) {
                    case Cell.CELL_TYPE_BOOLEAN:
                        //System.out.print(cell.getBooleanCellValue() + "\t\t");

                        list.add(cell.getBooleanCellValue());
                        break;
                    case Cell.CELL_TYPE_NUMERIC:
                        //System.out.print(cell.getNumericCellValue() + "\t\t");

                        value = (int) cell.getNumericCellValue();
                        list.add(cell.getNumericCellValue());
                        break;
                    case Cell.CELL_TYPE_STRING:
                        //System.out.print(cell.getStringCellValue() + "\t\t");
                        key = cell.getStringCellValue();
                        //chkeq = cell.getStringCellValue();

                       /* for (int i = 0; cellIterator.hasNext(); i++) {
                            if (chkeq == cellIterator.next().getStringCellValue()) {
                                System.out.println("same" + cell.getStringCellValue());

                            }

                        }*/

                        list.add(cell.getStringCellValue());
                        break;
                }

                if (key != null && value != Integer.MIN_VALUE) {
                    hm.put(key, value);
                    key = null;
                    value = Integer.MIN_VALUE;
                }

            }
            //System.out.println("");

        }
        for (String keys : hm.keySet())
            System.out.println(keys + ":" + hm.get(keys));
        fos.close();
        // System.out.println(list);

    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}
}

Note: I have used Apache POI to extract the data from an Excel sheet.

The code outputs this:

DEF --> 456
ABC --> 123
XXX --> 222

All this is doing is overwriting the last cell that was put into the hashmap with equal key.

Is there any way to sum the values instead of writing over them?

回答1:

Put overwrites whatever is in the map.

You need to get the current value in the map for the key (if it exists) and add it to the value you want to store.

if(hm.containsKey(key))
    hm.put(key, value + hm.get(key));
else
    hm.put(key, value);

回答2:

As you observed, hm.put overwrites the previous value, if any, associated with the key. In Java 8 there is a new method that merges the incoming value with the existing one. You can use a lambda to specify how the merging is done. In this case the merge operation is addition, so we would do:

hm.merge(key, value, (oldVal, newVal) -> oldVal + newVal);

Prior to Java 8 you have to use a conditional, as in Cheruvian's answer. (+1)

回答3:

Simply if value exists for the key, retrieve value and sum it with new value and put it back to map for the same key

回答4:

If the data is in Excel then a PivotTable could do the addition.

来源：https://stackoverflow.com/questions/24638521/how-can-i-sum-the-values-associated-with-a-reoccurring-key-in-a-hashmap