[FIXED] issue with java 8 collectors Type mismatch: cannot convert from List<Object> to List<String>

Issue

i was having working code with earlier version of java 8 which i was using to get unique values from list but since i upgraded to JDK 66 its giving me an error

Type mismatch: cannot convert from List<Object> to List<String>

List<String> instList = new ArrayList<String>();

while (res.next()) {
    instList.add(res.getString("INST").toString());
}           

List<String> instListF = instList.stream().distinct().collect(Collectors.toList());

Where res is resultset i am getting from database, not sure what is wrong any idea?

Solution

Well I have also faced similar kind of error Type mismatch: cannot convert from Set<Object> to Set<String> recently. Below is the code snippet-:

public static void main(String[] args) {
    String[] arr = new String[]{"i", "came", "i", "saw", "i", "left"};
    
    Set<String> set = Arrays.asList(arr).stream().collect(Collectors.toSet());
    
    System.out.println(set.size() + "  distinct words: " + set);
}

Here is the screen shot for reference-:

Now let me explain why was I getting this error? In my case code was displaying compile time error because there was mismatch in compiler version in project properties. I had selected 1.7 but it should be 1.8 since this feature has been added in 1.8.

So please make a note of below points-:

1. Appropriate JDK has been selected in Java Build Path. e.g. JDK 1.8 in this case.
2. Appropriate compiler version must be selected under Java Compiler (as displayed in above screenshot) in project properties. e.g. 1.8

I hope this would help you.

Answered By - Ashish Kumar
Answer Checked By - David Marino (JavaFixing Volunteer)