How does a leading zero change a numeric literal in Java?

Question

My friends and I have puzzled over this statement in Java after seeing it and the answer. How does this work?

System.out.printf("%d", 077);

equals 63?

Answer 1

077 is an octal number which equals 7 x 8¹ + 7 x 8⁰ which is 63 decimal. To display the octal value you could do

System.out.printf("%o", 077);