Java - always keep two decimal places even in zeroes

I am trying to keep two decimal places, even if then numbers are zeroes, using DecimalFormatter:

DecimalFormat df = new DecimalFormat("#.00");

m_interest   = Double.valueOf(df.format(m_principal * m_interestRate));
m_newBalance = Double.valueOf(df.format(m_principal + m_interest - m_payment));
m_principal  = Double.valueOf(df.format(m_newBalance));

However for some values this gives two decimal places, and for others it doesnt. How can i fix this?