static class MyAppendable implements Appendable {
    private final StringBuilder stringBuilder = new StringBuilder();
    
    @Override
    public Appendable append(char c) throws IOException {
        stringBuilder.append(c);
        return this;
    }
    
    @Override
    public Appendable append(CharSequence csq) throws IOException {
        if (csq == null) {
            append("null");
        } else {
            append(csq, 0, csq.length());
        }
        return this;
    }
    
    public Appendable append(CharSequence csq, int start, int end) throws IOException {
        if (csq == null) {
            csq == "null";
        }
        
        // According to doc, toString() must return string representation
        String s = csq.toString();
        stringBuilder.append(s, start, end);
        return this;
    }
}

public static void main(String[] args) {
    MyAppendable myAppendable = new MyAppendable();
    new Gson().toJson("test", myAppendable);
    // Prints `com.` (first 4 chars of `com.google.gson.internal.Streams.AppendableWriter.CurrentWrite`)
    System.out.println(myAppendable.stringBuilder);
}