Impure Functions

public void doSomething(int a, int b) { ... } // void methods must have side-effects

public boolean flipACoin() { // A no-argument pure function can be a constant?
    double result = Math.random(); // Produces output based on randomness
}

public int countUnique(List<String> names) {
    Set<String> nameSet = new HashSet<>(names);
    this.numUniqueNames = nameSet.size(); // Uh oh. Side effects detected
    return this.numUniqueNames;
}

public void filterANames(List<String> names) {
    for(int i = names.size() - 1; i >= 0; i --) {
        if(names.get(i).startsWith("A")) {
            result.remove(i); // This looks dangerous
        }
    }
    return result;
}