Check whether list of custom objects have same value for a property in Java 8

Question

I am new to Java 8. I have a list of custom objects of type A, where A is like below:

 class A {
      int id;
      String name;
 }

I would like to determine if all the objects in that list have same name. I can do it by iterating over the list and capturing previous and current value of names. In that context, I found How to count number of custom objects in list which have same value for one of its attribute . But is there any better way to do the same in java 8 using stream?

Answer 1

You can map from A --> String , apply the distinct intermediate operation, utilise limit(2) to enable optimisation where possible and then check if count is less than or equal to 1 in which case all objects have the same name and if not then they do not all have the same name.

boolean result = myList.stream()
                       .map(A::getName)
                       .distinct()
                       .limit(2)
                       .count() <= 1;

With the example shown above, we leverage the limit(2) operation so that we stop as soon as we find two distinct object names.

Answer 2

One way is to get the name of the first list and call allMatch and check against that.

String firstName = yourListOfAs.get(0).name;
boolean allSameName = yourListOfAs.stream().allMatch(x -> x.name.equals(firstName));

Answer 3

another way is to calculate count of distinct names using

boolean result = myList.stream().map(A::getName).distinct().count() == 1;

of course you need to add getter for 'name' field

Answer 4

One more option by using Partitioning . Partitioning is a special kind of grouping, in which the resultant map contains at most two different groups – one for true and one for false.

by this, You can get number of matching and not matching

String firstName = yourListOfAs.get(0).name;

Map<Boolean, List<Employee>> partitioned =  employees.stream().collect(partitioningBy(e -> e.name==firstName));

Java 9 using takeWhile takewhile will take all the values until the predicate returns false. this is similar to break statement in while loop

String firstName = yourListOfAs.get(0).name;        

        List<Employee> filterList =  employees.stream()
                                               .takeWhile(e->firstName.equals(e.name)).collect(Collectors.toList());
        if(filterList.size()==list.size())
        {
            //all objects have same values 
        }

Answer 5

Or use groupingBy then check entrySet size.

boolean b  = list.stream()
             .collect(Collectors.groupingBy(A::getName, 
             Collectors.toList())).entrySet().size() == 1;