Discussion 6

Packages and Bits

Announcements

1. Project 1 Checkpoint due Friday, 2/25
2. Lab 6 (part of Enigma) due Friday, 2/25
3. HW 4 due Tuesday, 3/1

Access Modifiers (in classes)

"default" is also called package-private.

Bits (Lecture preview)

How do computers represent numbers? In binary (base 2, using 0's and 1's).

The prefix 0b indicates that the number is binary:
0b101 = 4 + 0 + 1

Why should I care, when I can just use base 10?

• Java numbers use a finite amount of bits
• Cool math tricks
• This is what's "under the hood" - 61C!

Additional (surplus) reading: Sections 4.1-4.6 of Dive into Systems

Worksheet

1. Reduce

See worksheet for skeleton

2. Inheritance Infiltration

            
              public class PasswordChecker {
                /** Returns true if the provided login and password are correct. */
                public boolean authenticate(String login, String password) {
                  // Does some secret authentication stuff...
                }
              }
              public class User {
                private String username;
                private String password;
                public void login(PasswordChecker p) {
                  p.authenticate(username, password);
                }
              }
            
          

            
              public class PasswordExtractor extends _______________PasswordChecker {
                String extractedPassword;
                public String extractPassword(User u) {



                }





              }
            
          

3A. A Bit on Bits

            
              /** Returns whether the ith bit of num is a 1 or not. */
              public static boolean isBitIOn(int num, int i) {
                int mask = 1 __________________________________________;
                return ________________________________________________;
              }
            
          

Relevant bit operations:

• x & y
• x << n

3B. A Bit on Bits

            
              /** Returns the input number but with its ith bit changed to a 1. */
              public static int turnBitIOn(int num, int i) {
                int mask = 1 __________________________________________;
                return ________________________________________________;
              }
            
          

Relevant bit operations:

• x | y
• x << n (again!)