In java 8 lambda expression，i find a strange problem

Question

public class TransTitle {
public static String transTitle(String s) {
    String[] split = s.split(" |\\.");
    String res = Arrays.stream(split).filter(t -> !t.isEmpty()).collect(Collectors.joining("_")) + ".java";
    String res2 = Arrays.stream(split).filter(t ->( t!="")).collect(Collectors.joining("_")) + ".java";
    System.out.println(res);
    System.out.println(res2);
    return res;
}

public static void main(String[] args) {
    String ans = TransTitle.transTitle("215. Kth");
    // System.out.println(ans);
}

}

The res and res2 is not same.They are 215_Kth.java and 215__Kth.java. I cant understand why. Can anyone explain it to me?