(Java)指定将二进制数转换为字符串时的位数(长度)？

public static String toBinaryString(int num, int len) {
    String binaryString = Integer.toBinaryString(num);
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < len - binaryString.length(); i++) {
        sb.append("0");
    }
    sb.append(binaryString);
    return sb.toString();
}