Example octal number = 147

Method one: From left to right.
Step 1: First digit is one. Take that times 8 plus 4. Got 12.
Step 2: Take 12 times 8 + 7. Got 103, and 103 is the answer.

Method 2, bitshift:
Octal Number: 147.

Step 1: 1 = 1(bin) = Shift << 3 = 1000(result value)
Step 2: 4 = 100(bin) + 1000(result value) = 1100(result value)
Step 3: 1100(result value) Shift << 3 = 1100000
Step 4: 7 = 111(bin) + 1100000(result value) = 1100111
Step 5: 1100111 binary is 103 decimal.

for ( int i = 0; i < length; i++ ){
     c = (str.charAt(i) ^ 48);
     if ( c > 7 ) return 0;
     out = (out << 3) + c;
 }