Java ArrayList: Get Distinct Values from ArrayList which contain HashMap

Question

Please see problem defination as describe below. I have ArrayList which contain HashMap. In HashMap one or many key-value pairs exist. ArrayList contain following:

HashMap1 contains key: column1 value:0001 key: column2 value:1 key: column3 value:1

HashMap2 contains key: column1 value:0001 key: column2 value:1 key: column3 value:2

Both HashMap1 and HashMap2 are kept inside ArrayList.

Now I enter a String like (column1, column2) Then I need output like Create new ArrayList containing following data:

HashMap1 contains key: column1 value:0001 key: column2 value:1

When I enter String like (column1, column3) Create new ArrayList containing following data:

HashMap1 contains key: column1 value:0001 key: column3 value:1

HashMap2 contains key: column1 value:0001 key: column3 value:2

i.e. I want to get distinct values from ArrayList according to Input String. Please Help me Logically as How can I achieve the same.

I've re-read this several times and I still don't follow...

Richard H
– Richard H

2010-11-23 13:46:40 +00:00
Commented Nov 23, 2010 at 13:46 — Richard H
– Richard H, Commented Nov 23, 2010 at 13:46

Peter Lawrey · Accepted Answer · 2010-11-23 13:45:28Z

1

WHat you can do is;

for each map in the list.
  copy the map.
  map.keySet().retainAll(keys-you-want-to-keep)
  set-of-results.add(map)

This way you will have the set of sub maps.

answered Nov 23, 2010 at 13:45

Peter Lawrey

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Miguel Silva · Accepted Answer · 2010-11-23 14:07:02Z

Nothing is more strict than code. So here is the to do what you want. Take a special look at the method selectColumns. The list of Maps is stored on the class field list.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;


public class Matrix {

    private List<Map<String, String>> list = new ArrayList<Map<String,String>>();

    public Matrix(int numRows) {
        for (int i = 0; i < numRows; i++) {
            list.add(new HashMap<String, String>());
        }
    }

    private void set(int row, String string, String string2) {
        list.get(row).put(string, string2);
    }

    public Matrix selectColumns(String... columns) {
        Matrix select = new Matrix(list.size());
        for (String column : columns) {
            for (int row = 0; row < list.size(); row++) {
                select.set(row, column, list.get(row).get(column));
            }
        }
        return select;
    }

    public static Matrix createDummy(int dim) {
        Matrix matrix = new Matrix(dim);
        for (int i = 0; i < dim; i++) {
            for (int j = 0; j < dim; j++) {
                matrix.set(i, "col" + j, "val:" + i + "|" + j);
            }
        }
        return matrix;
    }

    public static void main(String[] args) {
        Matrix mainMatrix = Matrix.createDummy(3);
        System.out.println(mainMatrix.list);
        System.out.println(mainMatrix.selectColumns("col1", "col2").list);
    }

}

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