1. Overview
In this tutorial, we'll discuss the differences between Set and List in Java with the help of a simple example.
2. Conceptual Difference
Both List and Set are members of Java Collections. However, there are a few important differences:
- A List can contain duplicates, but a Set can't
- A List will preserve the order of insertion, but a Set may or may not
- Since insertion order may not be maintained in a Set, it doesn't allow index-based access as in the List
Please note that there are a few implementations of the Set interface which maintain order, for example, LinkedHashSet.
3. Code Example
3.1. Allowing Duplicates
Adding a duplicate item is allowed for a List. However, it isn't for a Set:
@Test
public void givenList_whenDuplicates_thenAllowed(){
List<Integer> integerList = new ArrayList<>();
integerList.add(2);
integerList.add(3);
integerList.add(4);
integerList.add(4);
assertEquals(integerList.size(), 4);
}
@Test
public void givenSet_whenDuplicates_thenNotAllowed(){
Set<Integer> integerSet = new HashSet<>();
integerSet.add(2);
integerSet.add(3);
integerSet.add(4);
integerSet.add(4);
assertEquals(integerSet.size(), 3);
}
3.2. Maintaining Insertion Order
A Set maintains order depending on the implementation. For example, a HashSet is not guaranteed to preserve order, but a LinkedHashSet is. Let's see an example of ordering with LinkedHashSet:
@Test
public void givenSet_whenOrdering_thenMayBeAllowed(){
Set<Integer> set1 = new LinkedHashSet<>();
set1.add(2);
set1.add(3);
set1.add(4);
Set<Integer> set2 = new LinkedHashSet<>();
set2.add(2);
set2.add(3);
set2.add(4);
Assert.assertArrayEquals(set1.toArray(), set2.toArray());
}
Since a Set is not guaranteed to maintain order, it can't be indexed.
4. Conclusion
In this tutorial, we saw the difference between a List and a Set in Java.
The source code is available over on GitHub.
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