Operations With 3 Sets In Java

Question

I have 3 sets and i'm using sets

 HashSet<String> set_1 =new HashSet<String>();  
 set_1.add("1");  
 set_1.add("2");  
 set_1.add("3");  

HashSet<String> set_2 =new HashSet<String>();  
 set_2.add("4");  
 set_2.add("5");  
 set_2.add("6"); 

HashSet<String> set_3 =new HashSet<String>();  
 set_3.add("7");  
 set_3.add("8");  
 set_3.add("9");

i need to calculate the union, intersection, difference and power of these sets

i tried this with union

Set<String> uni_temp = new HashSet<String>();
uni_temp.addAll(set_1);
uni_temp.addAll(set_2);
uni_temp.addAll(set_3);

but it only makes the union of set_1 and set_2 like so

[1, 2, 3, 4, 5, 6]

so after you do uni_temp.addAll(set_3) and you do System.out.println(uni_temp.size()) it doesn't print out 9? — Ben Arnao, Apr 11 '17 at 19:00
Your example code is still full of statements like `set.add("7");`, when your variables are named `set_1`, `set_2`, etc. Are you sure you don't have a copy-paste error? Most likely you're adding `set_1` or `set_2` twice and `set_3` not at all. This is why it's important to create a [mcve] and then copy that actual code into your question, instead of trying to show "something like" what you're really doing. — azurefrog, Apr 11 '17 at 19:14

Sash Sinha · Answer 1 · 2017-04-11T19:23:13.773

If you initialized set_1, set_2, and set_3 like below, your union attempt would work:

Set<String> set_1 = new HashSet<String>(Arrays.asList("1", "2", "3"));
Set<String> set_2 = new HashSet<String>(Arrays.asList("4", "5", "6"));
Set<String> set_3 = new HashSet<String>(Arrays.asList("7", "8", "9"));

Set<String> uni_temp = new HashSet<String>();
uni_temp.addAll(set_1);
uni_temp.addAll(set_2);
uni_temp.addAll(set_3);

System.out.println(Arrays.toString(uni_temp.toArray())); //[1, 2, 3, 4, 5, 7, 8, 9]

Edit: Your updated code would work if you added the strings to appropriate variable names instead of the unknown variable set:

HashSet<String> set_1 = new HashSet<String>();  
set_1.add("1");  
set_1.add("2");  
set_1.add("3");  

HashSet<String> set_2 = new HashSet<String>();  
set_2.add("4");  
set_2.add("5");  
set_2.add("6"); 

HashSet<String> set_3 = new HashSet<String>();  
set_3.add("7");  
set_3.add("8");  
set_3.add("9");

In general lets say you have two sets with variable names a and b:

To get the intersection: a.retainAll(b);

To get the difference: a.removeAll(b);

i tried your code to make the union operation but it doesn't work the intersection and difference operations do work with 2 `sets` but i need them to work with 3 `sets` — BlazePage001, Apr 11 '17 at 19:33

score 1 · Answer 2 · answered Apr 12 '17 at 11:38

You can try like:

HashSet<String> union = new HashSet<String>(set_1);
union.addAll(set_2);
union.addAll(set_3);
System.out.println(union);

HashSet<String> intersection = new HashSet<String>(set_1);
intersection.retainAll(set_2);
intersection.retainAll(set_3);
System.out.println(intersection);

score 0 · Answer 3 · answered Apr 11 '17 at 19:05

Using your example you should get the union from your code as shown in the following example

    Set<Integer> set1 = new HashSet<>();
    Set<Integer> set2 = new HashSet<>();
    Set<Integer> set3 = new HashSet<>();

    for(int i = 1; i < 4; i++)
    {
        set1.add(i);
        set2.add(i + 3);
        set3.add(i + 6);
    }

    Set<Integer> uni_temp = new HashSet<Integer>();
    uni_temp.addAll(set1);
    uni_temp.addAll(set2);
    uni_temp.addAll(set3);

    java.util.Iterator<Integer> iterator = uni_temp.iterator();
    while(iterator.hasNext())
    {
        System.out.println(iterator.next());
    }

Output

Note I am using Integers instead of Strings

score 0 · Answer 4 · answered May 14 '19 at 12:47

0

You can also use Sets.union() from Google Collections

Sets.union(Sets.union(a, b), c);

answered May 14 '19 at 12:47

Danon

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1

Note that this creates an unmodifiable view of the union of a,b,c. If either of the backing sets changes after you create the view, then so does the view. This behavior may not be desirable. – Борат Сагдиев Sep 15 '22 at 14:14

Operations With 3 Sets In Java

4 Answers4