JoinBuilder.java

public class JoinBuilder {
    private static final String EMPTY = "";

    private final String separator;
    private StringBuilder sb = new StringBuilder();

    public JoinBuilder(String separator) {
        if (separator == null || EMPTY.equals(separator)) {
            separator = EMPTY;
        }

        this.separator = separator;
    }

    public JoinBuilder line(String... parts) {
        if (parts != null && parts.length > 0) {

            boolean needSeparator = true;
            if (sb == null) {
                sb = new StringBuilder();
                needSeparator = false;
            }

            for (String s : parts) {
                sb.append(s);
            }

            if (needSeparator && (separator != EMPTY)) {
                sb.append(separator);
            }
        }

        return this;
    }

    @Override
    public String toString() {
        if (sb == null) {
            return EMPTY;
        }

        return sb.toString();
    }
}