List filter in Java8 using isPresent method

You should use noneMatch()

if (mylist.stream().noneMatch(str -> str.equalsIgnoreCase(testString))) {
    System.out.println("Value is not Present");
}

You should be using Stream#noneMatch for this. It will make your code more readable and more concise. Also, try to avoid putting to much logic inside of your if statement, extract a max in readable variables

List<String> mylist = new ArrayList<>();
mylist.add("test");
mylist.add("test1");

Predicate<String> equalsIgnoreCasePredicate = str -> str.equalsIgnoreCase("test");
boolean noneMatchString = mylist.stream().noneMatch(equalsIgnoreCasePredicate);

if (noneMatchString) {
    System.out.println("Value is not Present");
}