Question

Can anyone please explain this code to me, I don’t have much coding experience with Collections so I am having difficulties in understanding these LOC.

String[] stringList ={"1","2","1","1","2","3","2","3","2","1"}; 
List<String> al =Arrays.asList(stringList); 
Set<String> uniqueList = new HashSet<String>(al); 
for (String strCount :uniqueList) { 
     System.out.println(strCount + ": " + Collections.frequency(al, strCount)); 
} 

Why does this loop only run 3 times while uniquelist has all the members of stringList. Shouldn’t the loop run 10 times (length of uniqueList)?

Answer 1

The for loop only executes three times, because there are only 3 distinct values in your stringList.

A Set does not allow duplicate entries. The Set checks entries with .equals(), which will be true for two duplicate entries. Consequently, there are only three elements in uniqueList, namely: "1", "2" and "3".

Note, that "2".equals("2") is true.

Here is the most important part of the javadoc:

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element.