Java8 lambda sequential or piped String transformation

Question

I work on a JCR- based application, and my query breaks when it contains illegal characters.

So I've undertaken a really simple task: given a query string and a map containing a list of "dodgy" characters, sequentially replace those dodgy chars with allowed ones. I want to use lambdas, and am sadly a little bit stuck:

public static Map<String, String> DODGY_CHARS = getDodgyCharMapping();

static Map<String, String> getDodgyCharMapping(){
    Map<String, String> map = new HashMap<>();
    map.put("'", "''");
    return map;
}

private String sanitizeQueryString(String query){
    DODGY_CHARS.keySet().forEach(key->{
        query = replaceCharacter(query, key, DODGY_CHARS.get(key));
    });
    return query;
}

The query variable inside the lambda is what J8 is not happy with, resulting in the following error:

error: local variables referenced from a lambda expression must be final or effectively final

Answer 1

The problem is that you're accessing the local variable query within the scope of the lambda expression's body. It therefore must be final or effectively final . However, declaring query as final would not solve the problem since you're assigning a value to it.

I would suggest you let the method replaceCharacter takes a StringBuilder and have it replace the contents instead of reading and re-assigning the String variable.

Answer 2

Thanks for useful replies and comments, good people. Yes, I started digging deeper - and have tried to use StringBuilder, but since I needed to replace all occurrencies within a string, that quickly escalated to 10+ lines of code with another nested loop.

Thus, instead of reducing complexity and improving code readability, that was doing right the opposite.

Compare to the lambda-free one:

private String sanitizeQueryString(String query){
    for (String key: DODGY_CHARS.keySet()){
        query = replaceCharacter(query, key, DODGY_CHARS.get(key));
    }
    return query;
}

Nice and simple, huh?

Lesson learnt: don't try to shoe-horn everything as lambda, no matter how fashionable these are!