Computer Science 15-111 (Sections A & B), Spring 2007

Class Notes: 24-Jan-2007

Quiz: quiz1 on Chapters 1-3 and App A.1, B.1, and B.2, on Thu 25-Jan.

Topic Outline:

1. How to Study (A Handy Guide for Freshmen in their First Advanced College Course)

a. Attend lecture and recitation!

b. Do the reading! All of it! Carefully! Critically!

c. Do some unassigned problems (say, from the reading)! Really!

d. Study the lecture notes! Carefully! Critically! Run the examples!

e. Work with classmates, friends, course staff, others!!!

f. Start homework early, ask questions often!

g. Think, think, think!!!

2. “Addition by subtraction” and Two’s Complement
From earlier notes: “So subtraction is addition (less silicon!), and no +-0”

3. Read: Dr. Adamchik’s Notes on Arrays

4. Strings and immutability

a. Not mutable
public static void failedStringChange(String s) {
s = "no effect"; // does not change caller's string!
}

b. So String “+” is very expensive!
String s = "this " + " makes " + " garbage!";

5. StringBuffer and StringBuilder: Mutable strings!

a. StringBuilder is faster but not thread-safe

b. Example:
StringBuilder sb = new StringBuilder();
for (char c='A'; c<'E'; c++)
sb.append(c); // appends with no garbage!
String s = sb.toString();
System.out.println(s); // prints ABCD

6. Copying Arrays

a. Looping structure
int[] a = { 1, 3, 5 };
int[] b = new int[a.length];
for (int i=0; i<a.length; i++) b[i] = a[i];
System.out.println(a == b); // false
System.out.println(Arrays.equals(a,b)); // true

b. System.arraycopy()
Replace:
for (int i=0; i<a.length; i++) b[i] = a[i];
with:
System.arraycopy(a,0,b,0,a.length);

c. clone()
Replace:
int[] b = new int[a.length];
for (int i=0; i<a.length; i++) b[i] = a[i];
with:
int[] b = (int[]) a.clone();

7. Copies are shallow (they share the same objects)
Object[] a = { new StringBuilder("ab"), "cd" };
Object[] b = (Object[]) a.clone();
System.out.println(Arrays.toString(a)); // [ab, cd]
System.out.println(Arrays.toString(b)); // [ab, cd]
((StringBuilder)b[0]).append('E');
b[1] = b[1] + "F";
System.out.println(Arrays.toString(a)); // [abE, cd]
System.out.println(Arrays.toString(b)); // [abE, cdF]
System.out.println(a[0] == b[0]); // true

8. Arrays.sort
int[] a = { 2, 4, 1, 3 };
Arrays.sort(a);
System.out.println(Arrays.toString(a)); // [1, 2, 3, 4]

9. Arrays and the for-in iterator
int[] a = { 2, 4, 1, 3 };
for (int i : a) System.out.println(i);

10. Primitives, Objects, Autoboxing and Auto-unboxing
Integer i = 5; // illegal before Java 1.5
int j = i + 1; // ditto
int[] a = { i }; // ditto
Object[] b = { j }; // ditto
System.out.println(i.getClass()); // java.lang.Integer
System.out.println(j.getClass()); // Will not compile!

11. ArrayLists: Variable-length arrays
ArrayList<Object> a = new ArrayList<Object>();
a.add("ab");
a.add(12); // autoboxes!
a.add(true); // ditto
System.out.println(a.size()); // 3

System.out.println(a.toString()); // [ ab, 12, true ]
String s = (String) a.get(0); // must cast!
System.out.println(s); // ab
ArrayList<String> b = new ArrayList<String>();

b.add("cd");

String t = b.get(0); // no cast!

System.out.println(t); // cd

12. WordSearch example: In recitation tomorrow