Review Session. CS2110 Prelim #1 3/8/15. Primitive types vs classes. Default values. Wrapper Classes (Boxing) Strings are immutable.

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1 Review Session Primitive types vs classes Varialedeclarations: o int i = 5; o Animal a = new Animal( Bo ); How does == ehave? CS2 Prelim # a i Animal@x36 Animal@x36 5 name Bo Default values Wrapper Classes (Boxing) What value does a field contain when it is declared ut not instantiated? o Animal a; o Oject o; o int i; o oolean ; o char c; o doule d; //null //null // //false // \ (null yte) //. class Character contains useful methods Examples of useful static Character methods: o Character.isDigit(c) o IntCharacter.isLetter(c) Autooxing o Integer x = ; o int y = x; String literals String instantiation: Constructor: String s = new String( dog ); Literal: String s2 = dog ; Roughly equivalent, ut literal is preferred s String@x62 s2 String@x28 String@x28 String@x62 dog Strings are immutale Once a String is created, it cannot e changed Methods such as tolowercase and sustring return new Strings, leaving the original one untouched In order to modify Strings, you instead construct a new String and then reassign it to the original variale: o String name = Gries ; o name = name +, ; o name = name + David ; dog

2 String concatenation Other String info Operator + operator is called catenation, or concatenation If one operand is a String and the other isn t, the other is converted to a String Important case: Use + exp to convert exp to a String. Evaluates left to right. Common mistake: o System.out.println( sum: ); Prints sum: 56 o System.out.println( sum: + (5 + 6)); Prints sum: Always use equals to compare Strings: o str.equals(str2) Very useful methods: o length, sustring (overloaded), indexof, charat Useful methods: o lastindexof, contains, compareto D Array Review Java arrays Animal[] pets = new Animal[3]; Java arrays do not change size! pets.length is 3 pets[] = new Animal(); pets[].walk(); Why is the following illegal? pets[] = new Oject(); pets null Array@x null null 2 null Array@x A@xa A@x2 Big A@x2 A@xa Cornell Ithaca String[] = { Cornell, Ithaca ; String[] Big = Arrays.copyOf(, 4); = Big; A@x2 2 3 Cornell Ithaca 2D arrays: An array of D arrays. 2D arrays: An array of D arrays. Java only has D arrays, whose elements can also e arrays. int[][] = new int[2][3]; This array has 2 int[] arrays of length 3 each. 2 How many rows in? How many columns in row? How many columns in row?.length [].length [].length

3 2D arrays: An array of D arrays. 2D arrays: An array of D arrays. int[][] = new int[2][]; The elements of are of type int[]. int[][] = new int[2][]; [] = new int[] {,4,,3,9,3; [] = new int[] {,2,3; null null is called a ragged array Exceptions The superclass of exceptions: Throwale A Throwale instance: ArithmeticException Exceptions class Throwale: Superclass of Error and Exception Does the crashing Contains the constructors and methods Throwale() Throwale(String) class Error: A very serious prolem and should not e handled Example: StackOverflowError class Exception: Reasonale application might want to crash or handle the Exception in some way ArithmeticException@x2 Throwale detailmessage Exception RuntimeException ArithmeticException / y zero There are so many exceptions we need to organize them. Exception Throwale RuntimeException ArithmeticException Error Buling up exceptions Exceptions will ule up the call stack and crash the methods that called it. Method call: first(); Console: Exception in thread main java.lang.arithmeticexception: at Ex.third(Ex.java:) at Ex.second(Ex.java:7) at Ex.first(Ex.java:3) AE AE AE Exceptions class Ex { void first() { second(); AE = ArithmeticException void second() { third(); void third() { int c = 5/; Try-catch locks An exception will ule up the call stack and crash the methods that called it unless it is caught. catch will handle any exceptions of type Exception (and its suclasses) that happened in the try lock Console: in error class Ex { void first() { second(); void second() { try { System.out.println( in ); third(); Exceptions System.out.println( out ); catch (Exception e){ System.out.print( error ); Exception Type ArithmeticException! void third() { int c = 5/; 3

4 How to write an exception class Exceptions A Little More Geometry! Astract Classes /** An instance is an exception */ pulic class OurException extends Exception { /** Constructor: an instance with message m*/ pulic OurException(String m) { super(m); Shape x y /** Constructor: an instance with default message */ pulic OurException() { this( Default message! ); Square area() size Triangle area() ase height Circle area() radius A Partial Solution: Astract Classes Prolems not solved Astract Classes Add method area to class Shape: pulic doule area() { return ; pulic doule area() { throw new RuntimeException( area not overridden );. What is a Shape that isn t a Circle, Square, Triangle, etc? What is only a shape, nothing more specific? a. Shape s = new Shape(...); Should e disallowed 2. What if a suclass doesn t override area()? a. Can t force the suclass to override it!. Incorrect value returned or exception thrown. Solution: Astract classes Astract Classes Solution: Astract methods Astract Classes Astract class Can t e instantiated. (new Shape() illegal) pulic astract class Shape { pulic astract class Shape { Can have implemented methods, too pulic doule area() { return ; pulic astract doule area(); Astract method Suclass must override. Place astract method only in astract class. Semicolon instead of ody. 4

5 Astract Classes, Astract Methods Astract Classes. Cannot instantiate an oject of an astract class. (Cannot use new-expression). A suclass must override astract methods. (ut no multiple inheritance in Java, so ) pulic interface Whistler { methods are automatically void whistle(); pulic and astract int MEANING_OF_LIFE= 42; fields are automatically pulic, static, and final (i.e. constants) class Human extends Mammal implements Whistler { Must implement all methods in the implemented interfaces Multiple interfaces Solution: pulic interface Singer { Classes can implement several void singto(human h); interfaces! They must implement all the methods in those interfaces they implement. class Human extends Mammal implements Whistler, Singer { Interface Whistler offers promised functionality to classes Human and Parrot! Whistler Mammal Animal Bird Must implement singto(human h) and whistle() Human Dog Parrot Casting Casting Human h = new Human(); Oject o = (Oject) h; Animal a = (Animal) h; Mammal m = (Mammal) h; Singer s = (Singer) h; Whistler w = (Whistler) h; Whistler Oject Animal Mammal All point to the same memory address! Human Singer Human h = new Human(); Oject o = h; Animal a = h; Mammal m = h; Singer s = h; Whistler w = h; Automatic up-cast Forced down-cast Whistler Oject Animal Mammal Human Singer 5

6 Casting up to an interface automatically Shape implements Comparale<T> class Human implements Whistler { void listento(whistler w) {... Human h = new Human(...); Human h = new Human(...); h.listento(h); Parrot p = new Parrot(...); h.listento(p); Whistler Arg h of the call has type Human. Its value is eing stored in w, which is of type Whistler. Java does an upward cast automatically. Same thing for p of type Parrot. Oject Animal Mammal Human pulic class Shape implements Comparale<Shape> {... /** */ pulic int compareto(shape s) { doule diff= area() - s.area(); return (diff ==? : (diff <? - : +)); Beauty of interfaces String sorting Arrays.sort sorts an array of any class C, as long as C implements interface Comparale<T> without needing to know any implementation details of the class. Classes that implement Comparale: Boolean Byte Doule Integer String BigDecimal BigInteger Calendar Time Timestamp and others Arrays.sort(Oject[] ) sorts an array of any class C, as long as C implements interface Comparale<T>. String implements Comparale, so you can write String[] strings=...;... Arrays.sort(strings); During the sorting, when comparing elements, a String s compareto function is used Astract Classes vs. Astract class represents something Sharing common code etween suclasses Similarities: Can t instantiate Must implement astract methods Interface is what something can do A contract to fulfill Software Engineering purpose Four loopy questions //Precondition Initialization; // invariant: P while ( B ) { S 2. Does it stop right? Does P and!b imply the desired result? Loop Invariants. Does it start right? Does initialization make invariant P true? 3. Does repetend S make progress toward termination? 4. Does repetend S keep invariant P true? 6

7 Add elements ackwards Loop Invariants Add elements ackwards Loop Invariants Precondition Invariant Postcondition h?????? h s = sum s = sum int s = ; int h =.length-; while (h >= ) { s = s + [h]; h--; INV:??? h s = sum. Does it start right? 2. Does it stop right? 3. Does it keep the invariant true? 4. Does it make progress toward termination? Linear search time Binary search time ([..n-] is sorted) Linear search for v in an array of length n n??? worst-case time. v is not in [..n-], so linear search has to look at every element. Takes time proportional to n. expected (average) case time. If you look at all possiilities where v could e and average the numer of elements linear search has to look at, you would get close to n/2. Still time proportional to n. h= -; t= n; // invariant: P (elow) while (h < t-) { int e= (h+t)/2; if ([e] <= v) h= e; else t= e; // [..h] <= v < [t..n-] inv P: [h+..t-] starts out with n elements in it. Each iteration cuts size of [h+..t-] in half. worst-case and expected case time: log n h t n <= v? > v Insertion sort of [..n-] Selection sort of [..n-] h= ; // invariant: P (elow) while (h < n) { Push [h] down into its sorted position in [..h]; h= h+; inv P: Worst-case time for Push: h swaps Average case time for Push: h/2 swaps n- = n (n-) / 2 Worst-case and average case time: proportional to n^2 h n sorted? h= ; // invariant: P (elow) while (h < n) { Swap [h] with min value in [h..n-]; h= h+; inv P: h n sorted? To find the min value of [h..n-] takes time proportional to n - h. n + (n-) = n (n-) / 2 Worst-case and average case time: proportional to n^2 7

8 Quicksort of [..n-] Quicksort of [..n-] partition(, h, k) takes time proportional to size of [h..k] Best-case time: partition makes oth sides equal length depth: proportional to log n therefore: time n log n time n to partition time n to partition time n to partition /** Sort [h..k] */ void QS(int[], int h, int k) { if ([h..k] size < 2) return; j= partition(, h, k); // [h..j-] <= [j] <= [j+..k] QS(h, j-); QS(j+, k) Someone proved that the average or expected time for quicksort is n log n Quicksort of [..n-] What method calls are legal partition(, h, k) takes time proportional to size of [h..k] Animal an; an.m(args); Worst-case time: partition makes one side empty legal ONLY if Java can guarantee that method m exists. How to guarantee? time n to partition time n- to partition time n-2 to partition m must e declared in Animal or inherited. depth: proportional to n therefore: time n^2 Java Summary On the Resources ta of the course wesite We have selected some useful snippets We recommend going over all the slides (int) 3.2 any numer type Casting among types casts doule value 3.2 to an int any numer expression may e automatic cast narrow wider yte short int long float doule must e explicit cast, may truncate char is a numer type: (int) 'V' (char) 86 Unicode representation: 86 Page A-9, inside ack cover 'V' 48 8

9 Declaration of class Circle Multi-line comment starts with /* ends with */ /** An instance (oject) represents a circle */ pulic class Circle { Put declarations of fields, methods in class ody: { pulic: Code everywhere can refer to Circle. Called access modifier Page B-5 Precede every class with a comment Put class declaration in file Circle.java 49 Overloading Possile to have two or more methods with same name /** instance represents a rectangle */ pulic class Rectangle { private doule sideh, sidev; // Horiz, vert side lengths /** Constr: instance with horiz, vert side lengths sh, sv */ pulic Rectangle(doule sh, doule sv) { sideh= sh; sidev= sv; /** Constructor: square with side length s */ pulic Rectangle(doule s) { sideh= s; sidev= s; Lists of parameter types must differ in some way 5 Use of this this evaluates to the name of the oject in which is appears /** Constr: instance with radius radius*/ pulic Circle(doule radius) { this.radius= radius; Page B-28 Memorize this! 5 /** An instance represents a shape at a point in the plane */ pulic class Shape { private doule x, y; // top-left point of ounding ox /** Constructor: a Shape at point (x, y) */ pulic Shape (doule x, doule y) { x= x; y= y; /** return x-coordinate of ounding ox*/ pulic doule getx() { return x; /** return y-coordinate of ounding ox*/ pulic doule gety() { return y; Class Shape 52 Oject: superest class of them all Class doesn t explicitly extend another one? It automatically extends class Oject. Among other components, Oject contains: Constructor: pulic Oject() { /** return name of oject */ pulic String tostring() /** return value of this oject and o are same, i.e. of this == o */ pulic oolean equals(oject o) c.tostring() is Circle@x Page C-8 53 pulic class Circle { private doule radius; private static int t; pulic Circle(doule r) { doule r= r; radius= r; Java has 4 kinds of variale Field: declared non-static. Is in every oject of class. Default initial val depends on type, e.g. for int Class (static) var: declared static. Only one copy of it. Default initial val depends on type, e.g. for int Parameter: declared in () of method header. Created during call efore exec. of method ody, discarded when call completed. Initial value is value of corresp. arg of call. Scope: ody. Local variale: declared in method ody. Created during call efore exec. of ody, discarded when call completed. No initial value. Scope: from declaration to end of 54 lock. 9

10 Basic class Box pulic class Box { private Oject oject; pulic void set(oject o) { oject = o; pulic Oject get() { return oject; New code Box<Integer> = new Box<Integer>();.set(new Integer(35)); Integer x=.get(); parameter T (you choose name) Written using generic type pulic class Box<T> { private T oject; pulic void set(t o) { oject = o; pulic T get() { return oject; Replace type Oject everywhere y T 55 Linked Lists (These slides are from the class lectures and availale on the wesite as well) 57 Linked Lists Idea: maintain a list (2, 5, 7) like this: a a6 a8 h a v 2 v 5 v 7 next a6 next a8 next null This is a singly linked list To save space we write names like a6 instead of N@35acd Easy to insert a node in the eginning! h a a a6 a8 v 2 v 5 v 7 (2, 5, 7) next a6 next a8 next null a a6 a8 h a3 v 2 v 5 v 7 a3 next a6 next a8 next null v 8 next a (8, 2, 5, 7) 58 Easy to remove a node if you have its predecessor! 59 a h a v 2 next a6 (2, 5, 8, 7) k a6 a6 v 5 next a2 a2 v 8 next a8 a8 v 7 next null Recursion h a (2, 5, 7) a a6 v 2 v 5 next a6 next a8 k a6 a2 v 8 next a8 a8 v 7 next null 59

11 6 Sum the digits in a non-negative integer /** return sum of digits in n. * Precondition: n >= */ pulic static int sum(int n) { if (n < ) return n; // { n has at least two digits // return first digit + sum of rest return sum(n/) + n% ; E.g. sum(7) = 7 E.g. sum(873) = sum(87) + 3; sum calls itself! 62 Stack Frame A frame contains information aout a method call: At runtime, Java maintains a stack that contains frames for all method calls that are eing executed ut have not completed. local variales parameters return info Method call: push a frame for call on stack, assign argument values to parameters, execute method ody. Use the frame for the call to reference local variales, parameters. End of method call: pop its frame from the stack; if it is a function, leave the return value on top of stack.