Java: How do I sort multiple ArrayList by their size?

Question

I have 9 different ArrayList and I want to have a list of the top 5.

I'm thinking of sorting those ArrayLists by their sizes.

Is it possible to do that? If so, how can I achieve that?

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After a few try i finally got it working, just want to share it with everyone.

it will be better to get the size of the arraylist and add it to the big arraylist

                                                // creates an ArrayList that holds ArrayLists 
                                                List allTheLists = new ArrayList(); 
                                                allTheLists.add(pbaustraliaList.size());
                                                allTheLists.add(pbotherList.size()); 
                                                allTheLists.add(pbunitedStatesList.size()); 
                                                allTheLists.add(pbunitedKingdomList.size()); 
                                                allTheLists.add(pbchinaList.size()); 
                                                allTheLists.add(pbgermanyList.size()); 
                                                allTheLists.add(pbindiaList.size()); 
                                                allTheLists.add(pbjapanList.size()); 
                                                allTheLists.add(pbsingaporeList.size()); 

                                                Comparator comparator = Collections.reverseOrder();
                                                Collections.sort(allTheLists,comparator);

                                                //display elements of ArrayList    
                                                System.out.println("ArrayList elements after sorting in descending order : ");    
                                                for(int i=0; i<allTheLists.size(); i++) {     
                                                    System.out.println(allTheLists.get(i));   
                                                }   

Answer 1

What you could do is the following:

// this List of lists will need to contain 
// all of the ArrayLists you would like to sort
List<ArrayList> allTheLists; 
Collections.sort(allTheLists, new Comparator<ArrayList>(){
    public int compare(ArrayList a1, ArrayList a2) {
        return a2.size() - a1.size(); // assumes you want biggest to smallest
    }
});

This will sort the list of lists by the length of each list. The first element in the sorted list will be the longest list, and the last one will be the shortest list.

Then, you can iterate through the first 5 lists to see what the top 5 were.

Some links for reference:

• Sorting tutorial
• Collections Javadoc
• Comparator Javadoc

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Depending on how you have your ArrayLists stored, the code to create a List<ArrayList> would look something like this:

// creates an ArrayList that holds ArrayLists
List<ArrayList> allTheLists = new ArrayList<ArrayList>();
allTheLists.add(yourList1);
allTheLists.add(yourList2);
...
allTheLists.add(yourList9);