在while循环用完之前对其中的地图列表进行排序（Java）

Question

A database call is made and result is a bunch of rows of two string columns of type A and B . eg (x_a, y_b), (x_a, y1_b), (x2_a,y_b)

The idea is to come up with a list of maps like {(x_a,{y_b,y1_b}), (x2_a,{y_b})} where the objects of type A are not repeated and to do this while pulling the results from a database.

Here's what I tried:

int i =0;
            List<String> type2 = new ArrayList<String>();
            Map<String,List<String>> type1_type2 = new HashMap<String,List<String>>();
            List<Map> list_type1_type2 = new ArrayList<Map>();

            String [] type1Array = new String[100];
            String [] type2Array = new String[100];
            int trackStart = 0;
while (res.next()){


                String type1 = res.getString(1);
                String type2 = res.getString(2);
                type1Array[i]=type1;
                type2Array[i] = type2;


                if(i>0 && !type1Array[i].equals(type2Array[i-1])){
                    int trackStop = i;
                    for(int j = trackStart; j<trackStop;j++){
                        type2.add(type2Array[j]);
                    }
                    type1_type2.put(type1Array[i-1], type2);
                    list_type1_type2.add(type1_type2);

                //debugging stuff   
                 String x = list_type1_type2.toString();
         System.out.println(x);
System.out.println(" printing because "+ type1Array[i]+" is not equal to " + type1Array[i-1]);
        type2 = new ArrayList<String>();
     type1_type2 = new HashMap<String,List<String>>();
                     trackStart=i;
                     }

                     i++;


                }

This method does not work when the last type1 values of the result object are the same.

Is there a way to do this in the same spirit (within the while(res.next)) without first storing the results of the database call in separate arrays or adding an extra for loop outside the while loop to "patch it up"?

Answer 1

The simple way to do this is to use a Guava / Google Collections SetMultiMap . This is essentially a mapping from a key (your 'A' objects) to a set of values (your 'B' objects).

[I'm not going to try to code it for you. Your current code is too horrible to read ... unless you were paying me :-) ]

However, a better idea would be to get the database to do the collation. If you can do that, you will reduce the amount of (redundant) data that gets send across the database connection ... assuming that you are using JDBC.

Answer 2

If you don't want duplicates like {x_a:[y_b, y_b]} then use a set as the value of your map:

Map<String,Set<String>> type1_type2;

I don't know what the other various list and arrays are for. You can probably just get by with the type1_type2 map. Process each (x, y) in pseudo-code:

Set s = type1_type2.get(x)
if s == null:
    s = new Set()
    type1_type2.put(x, s)
s.add(y)