Question

I just stumbled over a weird piece of code while refactoring. It looks like a candidate for factoring out the common part of both readString()-methods, only it seems to be impossible (it’s a freaking brain teaser for me):

private final StringBuilder readStringBuilder = new StringBuilder(128);

@Override
public String readString() throws IOException {
    final int l = readInt();
    if (l <= 0) {
        switch (l) {
            case -1: return null;
            case 0: return "";
            default: throw new IOException("invalid string length encoding: " + l);
        }
    }
    readStringBuilder.setLength(0);
    for (int i=0; i<l; ++i) {
        readStringBuilder.append(readChar());
    }
    return readStringBuilder.toString();
}

@Override
public String readString(final StringCache cache) throws IOException {
    final int l = readInt();
    if (l <= 0) {
        switch (l) {
            case -1: return null;
            case 0: return "";
            default: throw new IOException("invalid string length encoding: " + l);
        }
    }
    readStringBuilder.setLength(0);
    for (int i=0; i<l; ++i) {
        readStringBuilder.append(readChar());
    }
    return cache.get(readStringBuilder, readStringBuilder);
}

You see that both methods do almost the same, the method body is completely identical, except for the return statement. But since there are early termination exits, I can’t find a method signature that can take the main body – natuarally the returns type would be StringBuilder, only in the early termination case it would be String…

Any ideas how to factor the body into a separate method? (and take note that toString() on an empty StringBuilder does create a new String instead of returning a constant string literal)

EDIT: Definition of the StringCache is:

public interface StringCache {
    public String get(final CharSequence charSeq, final CharSequence notFoundResult);
}

Answer 1

Looks like having readStringBuilder as an instance field is some form of (premature?) optimization. Assuming this is the case, how about something like this:

private StringBuilder readStringHelper() throws IOException {
  StringBuilder readStringBuilder = new StringBuilder(128);
    final int l = readInt();
    if (l <= 0) {
        switch (l) {
            case -1: return null;
            case 0: break;
            default: throw new IOException("invalid string length encoding: " + l);
        }
    }
    for (int i=0; i<l; ++i) {
        readStringBuilder.append(readChar());
    }
    return readStringBuilder;
}

@Override
public String readString() throws IOException {
    StringBuilder readStringBuilder = readStringHelper();
    return readStringBuilder==null ? null : readStringBuilder.toString();
}

@Override
public String readString(final StringCache cache) throws IOException {
    StringBuilder readStringBuilder = readStringHelper();
    return readStringBuilder== null ? null : cache.get(readStringBuilder, readStringBuilder);
}