Sorting ArrayList of HashMap based on key value

Question

I have a list of HashMaps. Each HashMap consists of several kay-value pairs and everything comes as a string. I am storing all the hashmaps inside an arraylist. Now I need to sort the arraylist based on the key inside the hashmap.

Here is my sample data:

{
 "productID":"5643",
 "productName":"Apple - iPod touch",
 "outsidePrice":"189.99", 
 "merchantID":"134439",
 "ourPrice":"184.99", 
 "storeName":"Ebay",
}


{
 "productID":"3243",
 "productName":"Apple - iPad",
 "outsidePrice":"389.99", 
 "merchantID":"54439",
 "ourPrice":"384.99", 
 "storeName":"Apple",
}

I am storing this data inside this structure.

ArrayList<HashMap<String, String>> data_list = new ArrayList<HashMap<String, String>>();

I have a huge list of items like this. Now I need to sort the arraylist based on the productName, Price, storeName, productID fields inside the hashmap.

"I have a huge list of items like this". You should be storing this in SQLite. — Simon
– Simon, Commented Nov 25, 2012 at 7:20
I don't need that. Say the list has 25 item details like this. The data will be keep on changing and it is temporary. It won't occupy much memory. — intrepidkarthi
– intrepidkarthi, Commented Nov 25, 2012 at 7:23

Simon · Accepted Answer · 2012-11-25 08:57:15Z

7

I recommend that you use a custom product class to do this for you. It will ultimately make your code easier to maintain and more robust, IMHO.

How about this?

A class to represent your data:

class Product{

     public string productId;
     public string productName;
     public BigDecimal outsidePrice;
     public int merchantId;
     public BigDecimal ourPrice;
     public string storeName;

// whatever constuctors you need

}

A List of your products:

  List<Product> products;

Now define a Comparator to sort, one for each field that you need to sort on. Here is an example for productId.

public class ProductProductIdComparator implements Comparator<Product>{

     @Override
     public int compare(Product product1, Product product2) {

        if (product1.productId > product2.productId){
            return +1;
        }else if (product1.productId < product2.productId){
            return -1;
        }else{
            return 0;
        }
    }
}

And finally, a Collections sort which accepts a comparator as an argument:

Collections.sort(products, new ProductProductIdComparator());

edited Nov 25, 2012 at 8:57

answered Nov 25, 2012 at 7:48

Simon

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1 Comment

Erik-RW Over a year ago

This is a good approach. Since Java should be OOP, Entity classes are to favor against String Lists

Perception · Accepted Answer · 2012-11-25 07:37:55Z

6

The Collections class provides a utility method for sorting a list in place, using a Comparator.

final List<Map<String, String>> dataList = new ArrayList<HashMap<String, String>>(4);
Collections.sort(dataList, new Comparator<Map<String, String>>() {
    @Override
    public int compare(final Map<String, String> map1, final Map<String, String> map2) {
        // Get fields from maps, compare
    }
}

answered Nov 25, 2012 at 7:37

Perception

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1 Comment

intrepidkarthi Over a year ago

I will try it and come back

Sajid · Accepted Answer · 2012-11-25 07:28:58Z

-1

You can use arrays.sort() to achieve this in short, at some minor memory cost.

HashMap[] result = Arrays.sort(list.toArray(), new Comparator() {
  public void compare(Object o1, Object o2) {
    HashMap<String, String> a = (HashMap<String, String>)o1;
    HashMap<String, String> b = (HashMap<String, String>)o2;
    // return value as per contract of Comparator.compare() doing whatever comparisons you need.
  }

  public boolean equals(Object obj) { return this == obj; }
});

answered Nov 25, 2012 at 7:28

Sajid

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4 Comments

intrepidkarthi Over a year ago

I don't understand the answer. I don't have just two sets of data. I have 25 sets of hashmap inside my arraylist.

Sajid Over a year ago

What this does is first convert the ArrayList to an array with toArray(). It then uses the built in sort in the Arrays class. Arrays.sort() runs a merge sort, comparing each pair of elements by calling the comparator you are passing in. So basically, your comparator just needs to return -1 if o1 is before o2, 0 if they are they same, and 1 if o1 is after o2. Arrays.sort() will take care of the rest.

Perception Over a year ago

This code does not work. 1) The Arrays method does a sort in place, you can't assign a value from calling it. 2) The original collection is unaffected by your call, since you are not sorting it, but an array created from it.

Mordechai Over a year ago

Why convert to array, Collection.sort` does then same.

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