SetUnionDemo.java

package collection;

import java.util.HashSet;
import java.util.Set;
import java.util.stream.Collectors;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

public class SetUnionDemo
{
    @Test
    public void overlappingSets()
    {
        Set<String> aSet = Set.of("hello", "world", "over");
        Set<String> bSet = Set.of("over", "and", "out");

        Set<String> zz = union(aSet, bSet);
        Assertions.assertEquals(5, zz.size());
        String result = zz.stream().sorted().collect(Collectors.joining(","));
        Assertions.assertEquals("and,hello,out,over,world", result);
    }

    @Test
    public void nullEntryASet_nullBSet()
    {
        Set<String> aSet = new HashSet<>();
        aSet.add("hello");
        aSet.add(null);
        aSet.add("world");
        aSet.add(null);
        aSet.add("over");

        Set<String> bSet = null;

        Set<String> zz = union(aSet, bSet);
        Assertions.assertEquals(3, zz.size());
        String result = zz.stream().sorted().collect(Collectors.joining(","));
        Assertions.assertEquals("hello,over,world", result);
    }

    @Test
    public void nullASet_BSet()
    {
        Set<String> aSet = null;
        Set<String> bSet = Set.of("bb", "zz", "aaa");

        Set<String> zz = union(aSet, bSet);
        Assertions.assertEquals(3, zz.size());
        String result = zz.stream().sorted().collect(Collectors.joining(","));
        Assertions.assertEquals("aaa,bb,zz", result);
    }

    @Test
    public void nullASet_nullBSet()
    {
        Set<String> aSet = null;
        Set<String> bSet = null;

        Set<String> zz = union(aSet, bSet);
        Assertions.assertNull(zz);
    }

    @Test
    public void emptyASet_emptyBSet()
    {
        Set<String> aSet = Set.of();
        Set<String> bSet = Set.of();

        Set<String> zz = union(aSet, bSet);
        Assertions.assertNull(zz);
    }

    @Test
    public void nullEntryASet_emptyBSet()
    {
        Set<String> aSet = new HashSet<>();
        aSet.add(null);
        Set<String> bSet = Set.of();

        Set<String> zz = union(aSet, bSet);
        Assertions.assertNull(zz);
    }

    @Test
    public void emptyASet_nullEntryBSet()
    {
        Set<String> aSet = Set.of();
        Set<String> bSet = new HashSet<>();
        bSet.add(null);

        Set<String> zz = union(aSet, bSet);
        Assertions.assertNull(zz);
    }

    public static Set<String> union(Set<String> setA, Set<String> setB)
    {
        if ((setA == null) && (setB == null))
            return null;

        /*
        if (setA == null || setA.isEmpty())
            return setB; // doesn't remove null entries from setB

        if (setB == null || setB.isEmpty())
            return setA; // doesn't remove null entries from setA
         */

        Set<String> result = new HashSet<>();
        if (setA != null) result.addAll(setA);
        if (setB != null) result.addAll(setB);
        result.remove(null); // remove null entry (if any)

        if (result.isEmpty())
            return null; // return null if the result is empty?

        return result;
    }
}