Java中输出16位长度的二进制字符串，不足以0补齐

int num= Integer.parseInt(numberString);

StringBuilder binaries = new StringBuilder(Integer.toBinaryString(num)); 

 

public String fixLengthBins(StringBuilder bins)

{

    int len = bins.length();

    for(int i=0;i<16-len;i++)

    {

        bins.insert(0, '0');

    }

    return bins.toString();

}