sort list of object on java 8 with custom criteria

Question

I have a list of MyObject

List<MyObject> myObjects;

where MyObject is a model class something like below

public class MyObject{

private String fName;
private String lname;
private String code;

//getter setter

}

there are four possible value of code let's say ABC,DEF,XYZ and PQR.

Now i want to sort the list based on following criteria. All object with code value XYZ should come first, followed by PQR,ABC and DEF.

XYZ
PQR
ABC
DEF

I want to achieve this using java 8 if possible. How can i sort my ArrayList.

Use a custom `Comparator`. [How to sort List of objects by some property](https://stackoverflow.com/questions/5805602/how-to-sort-list-of-objects-by-some-property) would be a good place to start — MadProgrammer, Oct 09 '19 at 21:22
`myObjects.sort((o1, o2) -> { if(o1.getCode().equals("XYZ")) return -1; else if(o2.getCode().equals("XYZ")) return 1; // else return o1.getCode().compareTo(o2.getCode()); });` — pvpkiran, Oct 09 '19 at 21:39
Remember, you're comparing the keys of two objects and how the relate to each other — MadProgrammer, Oct 09 '19 at 21:41
assign a numeric value to each code: XYZ -> 1, PQR -> 2, ABC -> 3, DEF -> 4. Get the numeric value for o1, and the one for o2. Compare the numeric values. — JB Nizet, Oct 09 '19 at 21:44

Eritrean · Accepted Answer · 2019-10-10T16:36:44.513

If there are only four possible values that your code variable can take, you could save them in a map and compare the values when sorting your list:

public static void main(String[] args) {             
    List<MyObject> myObjects = new ArrayList<>();
    myObjects.add(new MyObject("fName1", "lname1", "ABC"));
    myObjects.add(new MyObject("fName2", "lname2", "PQR"));
    myObjects.add(new MyObject("fName3", "lname3", "XYZ"));
    myObjects.add(new MyObject("fName4", "lname4", "DEF"));

    Map<String,Integer> map = new HashMap<>();
    map.put("XYZ", 1);
    map.put("PQR", 2);
    map.put("ABC", 3);
    map.put("DEF", 4);

    Comparator<MyObject> sortByCode = (obj1,obj2)->Integer.compare(map.get(obj1.code), map.get(obj2.code));
    System.out.println("Before sorting");
    System.out.println(myObjects);

    System.out.println("After sorting");
    myObjects.sort(sortByCode);
    System.out.println(myObjects);
}

@user9735824 I have updated my code to sort your original list without creating a new one. — Eritrean, Oct 10 '19 at 16:38

score 1 · Answer 2 · answered Oct 09 '19 at 21:30

You'd need to create your own Comparator for comparing instances of MyObject according to your logic:

Comparator<MyObject> cmp = (o1, o2) ->{
    //Implement comparison logic here
    //Compare o1 and o2 and return -1,0, or 1 depending on your logic
};

Then given a list such as this:

List<MyObject> listToSort = ...

You can either sort it in-place using the old Collections.sort() function:

Collections.sort(listToSort, cmp);

Or, if you want, using Java 8 streams:

listToSort.stream().sorted(cmp).collect(Collectors.toList()); //Using streams

Thanks for your answer but the main problem i am having is actually Implementing comparison logic — user9735824, Oct 09 '19 at 21:36

sort list of object on java 8 with custom criteria

2 Answers2