Replacing an existing element by another element in Set in Java

Question

Is it possible replace an element of java.util.Set by another element? Given below a simple example.

Set<String> set=new LinkedHashSet<String>() {{
    add("A");
    add("B");
    add("C");
    add("D");
}};

if(set.contains("C")) {
    set.remove("C");
    set.add("E");
}

for(String str:set) {
    System.out.println(str);
}

This example first removes an element and adds another element afterwards. Doing so would add the element at the bottom of the Set.

It outputs after completion of the foreach loop as follows.

A
B
D
E

Is it somehow possible to just replace an existing element by another element retaining its original position so that it outputs like the following?

A
B
E
D

You need to use a list instead. Order of elements in a set is undefined. — Bhesh Gurung
– Bhesh Gurung, Commented Jul 30, 2014 at 13:23
@sᴜʀᴇsʜᴀᴛᴛᴀ I know you know...sometimes we all get Layer8 problems except Jon Skeet... — gtgaxiola
– gtgaxiola, Commented Jul 30, 2014 at 13:30

user1907906 · Accepted Answer · 2014-07-30 13:25:11Z

1

No. LinkedHashSet.add is inherited from HashSet.add. Using it on a LinkedHashSet will add the element at the end of the underlying linked list.

answered Jul 30, 2014 at 13:25

user1907906

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Comments

Syam S · Accepted Answer · 2014-07-30 13:42:34Z

As all the answers above you cannot do that with Set. You can achieve this by adding the set to a list, then do operation and then back to set. But that is very expensive. Alternatively If you are particular about this kind of implementation you can use Apache Commons ListOrderedSet. Please note that as far as I know this doesn't support generics. So either you'll have to put extra care on type safety or else you could do explicit casting. Example

import java.util.Set;

import org.apache.commons.collections.set.ListOrderedSet;

public class Sample {

    public static void main(String[] args) {
        @SuppressWarnings("unchecked")
        Set<String> set = new ListOrderedSet();
        set.add("A");
        set.add("B");
        set.add("C");
        set.add("D");

        System.out.println(set); // Prints [A, B, C, D]
        int indexToRemove = ((ListOrderedSet) set).indexOf("C");
        if (indexToRemove != -1) {
            set.remove("C");
            ((ListOrderedSet) set).add(indexToRemove, "E");
        }
        System.out.println(set); // Prints [A, B, E, D]
    }
}

Pravin · Accepted Answer · 2014-11-14 14:12:27Z

0

public static void main(String[] args) {
    Set<String> set = new LinkedHashSet<String>();
    set.add("A");
    set.add("B");
    set.add("C");
    set.add("D");
    System.out.println(set);

List<String> list = new ArrayList<String>();
list.addAll(0, set);
System.out.println(list);

if(list.contains("C")){
       int i = list.indexOf("C");
       list.add(i, "E");
       list.remove("C");
}

set.removeAll(set);
//System.out.println(list);
//System.out.println(set);
set.addAll(list);
System.out.println(set);
}

with this solution you have to use unwanted List :(

output :[A,B,E,D]

edited Nov 14, 2014 at 14:12

answered Jul 30, 2014 at 13:38

Pravin

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