java - Java8 Streams - Remove Duplicates With Stream Distinct

Question

I have a stream such as:

Arrays.stream(new String[]{"matt", "jason", "michael"});

I would like to remove names that begin with the same letter so that only one name (doesn't matter which) beginning with that letter is left.

I'm trying to understand how the distinct() method works. I read in the documentation that it's based on the "equals" method of an object. However, when I try wrapping the String, I notice that the equals method is never called and nothing is removed. Is there something I'm missing here?

Wrapper Class:

static class Wrp {
    String test;
    Wrp(String s){
        this.test = s;
    }
    @Override
    public boolean equals(Object other){
        return this.test.charAt(0) == ((Wrp) other).test.charAt(0);
    }
}

And some simple code:

public static void main(String[] args) {
    Arrays.stream(new String[]{"matt", "jason", "michael"})
    .map(Wrp::new)
    .distinct()
    .map(wrp -> wrp.test)
    .forEach(System.out::println);
}

Answer 1

Whenever you override equals, you also need to override the hashCode() method, which will be used in the implementation of distinct().

In this case, you could just use

@Override public int hashCode() {
   return test.charAt(0);
}

...which would work just fine.