Convert List to Set in Java - GeeksforGeeks

11 Dec, 2018
Java Set is a part of java.util package and extends java.util.Collection interface. It does not allow the use of duplicate elements and at max can accommodate only one null element. Few important features of Java Set interface are as follows:
The set interface is an unordered collection of objects in which duplicate values cannot be stored.
The Java Set does not provide control over the position of insertion or deletion of elements.
Basically, Set is implemented by HashSet, LinkedHashSet or TreeSet (sorted representation).
Set has various methods to add, remove clear, size, etc to enhance the usage of this interface.
In this post different methods to convert a List interface to Set in Java is discussed:
Naive Approach: The naive solution is to create an empty set and add every element of the specified list to the newly created set.Below is the implementation of the naive approach:// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;import java.util.function.Function;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create an empty set        Set<T> set = new HashSet<>();          // Add each element of list into the set        for (T t : list)            set.add(t);          // return the set        return set;    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}Output:List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]
Using Constructor: HashSet constructor can take another collection object to construct a new set containing the elements of the specied list.Below is the implementation of the above approach:// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create a set from the List        return new HashSet<>(list);    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}Output:List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]
Using Java 8 Stream API: HashSet constructor can take another collection object to construct a new set containing the elements of the specied list.Below is the implementation of the above approach:// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create a set from the List        return list.stream().collect(Collectors.toSet());    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}Output:List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]
Using Guava Sets.newHashSet(): Sets.newHashSet() creates a mutable HashSet instance containing the elements of the specified list.Below is the implementation of the above approach:// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create a set from the List        return return Sets.newHashSet(list);    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}Output:List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Naive Approach: The naive solution is to create an empty set and add every element of the specified list to the newly created set.Below is the implementation of the naive approach:// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;import java.util.function.Function;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create an empty set        Set<T> set = new HashSet<>();          // Add each element of list into the set        for (T t : list)            set.add(t);          // return the set        return set;    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}Output:List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Below is the implementation of the naive approach:
// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;import java.util.function.Function;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create an empty set        Set<T> set = new HashSet<>();          // Add each element of list into the set        for (T t : list)            set.add(t);          // return the set        return set;    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}
Output:
List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Using Constructor: HashSet constructor can take another collection object to construct a new set containing the elements of the specied list.Below is the implementation of the above approach:// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create a set from the List        return new HashSet<>(list);    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}Output:List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Below is the implementation of the above approach:
// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create a set from the List        return new HashSet<>(list);    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}
Output:
List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Using Java 8 Stream API: HashSet constructor can take another collection object to construct a new set containing the elements of the specied list.Below is the implementation of the above approach:// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create a set from the List        return list.stream().collect(Collectors.toSet());    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}Output:List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Below is the implementation of the above approach:
// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create a set from the List        return list.stream().collect(Collectors.toSet());    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}
Output:
List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Using Guava Sets.newHashSet(): Sets.newHashSet() creates a mutable HashSet instance containing the elements of the specified list.Below is the implementation of the above approach:// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create a set from the List        return return Sets.newHashSet(list);    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}Output:List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Below is the implementation of the above approach:
// Java Program to convert// List to Set in Java 8  import java.util.*;import java.util.stream.*;  class GFG {      // Generic function to convert list to set    public static <T> Set<T> convertListToSet(List<T> list)    {        // create a set from the List        return return Sets.newHashSet(list);    }      public static void main(String args[])    {          // Create a stream of integers        List<String> list = Arrays.asList("GeeksForGeeks",                                          "Geeks",                                          "forGeeks",                                          "A computer portal",                                          "for",                                          "Geeks");          // Print the List        System.out.println("List: " + list);          // Convert List to stream        Set<String> set = convertListToSet(list);          // Print the Set        System.out.println("Set from List: " + set);    }}
Output:
List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Java - util package
java-list
Java-List-Programs
java-set
Java-Set-Programs
Java
Java
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