How to delete duplicates from an arraylist

Question

I am trying to delete duplicates from arraylist :

public static List<List<String>> bigList = new ArrayList<>();
    for (int i = 0; i < bigList.size(); i++) {
        bigList.get(i).stream()
            .map(str -> new LinkedHashSet<>(Arrays.asList(str.split(","))))
            .distinct()
            .map(set -> set.stream().collect(Collectors.joining(",")))
            .collect(Collectors.toList());
    }
}

When executing the code, I still get duplicates in my list. I want to delete the duplicates from bigList and bigList.get(i).

You are not assigning the result of .collect(Collectors.toList()) to anything. So you are constructing a new list, which is just immediately thrown away. — Michael
– Michael, Commented May 14, 2020 at 23:38
@LSoft Wrap your line with bigList.set( i , … ) to replace the old list with the new list. — Basil Bourque
– Basil Bourque, Commented May 14, 2020 at 23:50
Nothing in the code you posted has anything to do with doubles. — Louis Wasserman
– Louis Wasserman, Commented May 14, 2020 at 23:57
Your for loop will not run at all; you do bigList = new ArrayList<>() and you loop on i < bigList.size() — well, bigList.size() is zero so there will be no iterations. — Stephen P
– Stephen P, Commented May 15, 2020 at 0:00
@BasilBourque instead of looping and calling get, followed by calling set, it’s much simpler to use bigList.replaceAll(list -> list.stream()….collect(Collectors.toList())); Alternatively, the OP could truly remove the duplicates from the lists, instead of creating new lists. — Holger
– Holger, Commented May 15, 2020 at 10:29

Basil Bourque · Accepted Answer · 2020-05-15 16:05:05Z

Capture the new list

Your code creates a new list each time through the for loop. But the new list is immediately lost, headed for garbage collection. You neglected to capture a reference to that newly-created list.

So, the solution is: Replace the old list stored as an element in the outer list with the new list. Wrap your stream line with bigList.set( i , … ).

public static List< List< String > > bigList = new ArrayList<>(); // A list of lists of strings.
…
    for (int i = 0; i < bigList.size(); i++) {
        bigList.set( 
            i ,       // Index of outer list, where the new list should go.
            bigList   
            .get(i)
            .stream()
            .map( str -> new LinkedHashSet<>( Arrays.asList( str.split( "," ) ) ) )
            .distinct()
            .map( set -> set.stream().collect( Collectors.joining( "," ) ) )
            .collect( Collectors.toList() )  // Produce a new list. Stored in place of the old list.
        ) ;
    }

For clarity, break the code into separate lines.

public static List< List< String > > bigList = new ArrayList<>(); // A list of lists of strings.
…
    for (int i = 0; i < bigList.size(); i++) {
        List< String > oldList = bigList.get( i ) ;
        List< String > newList = 
            oldList
            .stream()
            .map( str -> new LinkedHashSet<>( Arrays.asList( str.split( "," ) ) ) )
            .distinct()
            .map( set -> set.stream().collect( Collectors.joining( "," ) ) )
            .collect( Collectors.toList() )  // Produce a new list. Stored in place of the old list.
        ) ;
        bigList.set( i , newList ) ;  // Replace the old list with the newly-created list.
    }

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How to delete duplicates from an arraylist

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Capture the new list

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