Question

Is it possible to configure the Java StringBuilder class to append a new line between each call to append?

Answer 1

I don’t think StringBuilder has that functionality built-in. Since you can’t just extend it, you could try using the decorator pattern…

class NewlineStringBuilder {

    private StringBuilder delegate;

    public NewlineStringBuilder() {
        delegate = new StringBuilder();
    }

    public NewlineStringBuilder appendLine(String str) {
        delegate.append(str).append("\n");
        return this;
    }

    public String toString() {
        return delegate.toString();
    }
}

Alternately Java can support this shorthand, which is useful in that it is not necessary for the decorator to reimplement all methods that need to be available from the decorated class, however it is more dangerous as the raw resource is completely unexposed so that trade-off must be weighed:

StringBuilder message = new StringBuilder();
class AppendingStringBuilder {
    private final StringBuilder builder;
    AppendingStringBuilder(StringBuilder builder) {
        this.builder = builder;
    }
    public AppendingStringBuilder append(String str) {
        builder.append(str + "\n");
        return this;
    }
}
AppendingStringBuilder messageHelper;
// ...