Tarkvaraprojekt seminar XI 27.aprill Ivo Mägi Java Puzzlers

Transcription

1 Tarkvaraprojekt seminar XI 27.aprill 2006 Ivo Mägi Java Puzzlers

2 Administratiivinfot Presentatsioonide kuupäevad 11. mai AJU Darth Vader Hades23 HAL invitro 18.mai Kosmos Lendoravad Macrosoft Spectrum

3 Thanks to:joshua Bloch&Neal Gafter

4 Introduction Eight Java puzzles Short program with curious behavior. What does it print? The mystery revealed. How to fix the problem? The moral. Covers language and core libraries Every correct and reasoned answer earns two bonuspoints for your team.

5 1. The name game public class Names { private Map m = new HashMap(); public void Names() { m.put("mickey", "Mouse"); m.put("mickey", "Mantle"); public int size() { return m.size(); public static void main(string args[]) { Names names = new Names(); System.out.println(names.size());

6 What does it print? a. 0 b. 1 c. 2 d. throws an Exception

7 What does it print? a. 0 b. 1 c. 2 d. throws an Exception The class does not have a constructor!

8 How do you fix it? public class Names { private Map m = new HashMap(); public void Names() { m.put("mickey", "Mouse"); m.put("mickey", "Mantle"); public int size() { return m.size(); public static void main(string args[]) { Names names = new Names(); System.out.println(names.size());

9 2. Animal Farm public class AnimalFarm { public static void main(string[] args) { final String pig = "length: 10"; final String dog = "length: " + pig.length(); System.out.println("Animals are equal: " + pig == dog);

10 What Does It Print? a. Animals are equal: true b. Animals are equal: false c. It varies d. None of the above

11 What Does It Print? a. Animals are equal: true b. Animals are equal: false c. It varies d. None of the above: false The + operator binds tighter than ==

12 How Do You Fix It? public class AnimalFarm { public static void main(string[] args) { final String pig = "length: 10"; final String dog = "length: " + pig.length(); System.out.println("Animals are equal: " + pig.equals(dog));

13 The Moral Parenthesize when using string concatenation Spacing can be deceptive; parentheses never lie Don t depend on interning of string constants Use equals, not ==, for strings

14 3. The Woofer class Dog { public static void bark() { System.out.print("woof "); class Basenji extends Dog { public static void bark() { public class Bark { public static void main(string args[]) { Dog woofer = new Dog(); Dog nipper = new Basenji(); woofer.bark(); nipper.bark();

15 What does it print? a. woof b. woof woof c. It won t compile d. Nothing

16 What does it print? a. Woof b. Woof woof c. It won t compile d. Nothing Static methods cannot be overriden!

17 How do you fix it? class Dog { public static void bark() { System.out.print("woof "); class Basenji extends Dog { public static void bark() { public class Bark { public static void main(string args[]) { Dog woofer = new Dog(); Dog nipper = new Basenji(); woofer.bark(); nipper.bark();

18 The moral Static methods can never be overridden; they can only be hidden. Both variables are statically declared as Dogs. So the compiler makes the same decision in both cases, invoking the bark() method in Dog. When you invoke a static method, you should qualify it with the name of the type on which it was defined, rather than qualifying it with an expression. If Java language specification group had the choice today, they would disallow it.

19 4. And my name is public class Name { private String first, last; public Name(String first, String last) { this.first = first; this.last = last; public boolean equals(object o) { if (!(o instanceof Name)) return false; Name n = (Name)o; return n.first.equals(first) && n.last.equals(last); public static void main(string[] args) { Set s = new HashSet(); s.add(new Name("Donald", "Duck")); System.out.println(s.contains(new Name("Donald", "Duck")));

20 What does it print? a. true b. false c. it won t compile d. throws Exception

21 What does it print? a. true b. false c. it won t compile d. throws Exception You violated the hashcode contract!

22 How do you fix it? Add a hashcode method always when overriding equals: public int hashcode() { return 31 * first.hashcode() + last.hashcode();

23 The moral Name class violates the hashcode contract! The Name class overrides the equals() method but the hashcode contract demands that equal objects must have equal hash codes. Name inherits its hashcode method from Object, so it returns an identity-based hash code.

24 5. We will sort you! public class Sorter { public static void main(string args[]) { Integer big = new Integer( ); Integer small = new Integer( ); Integer zero = new Integer(0); Integer[] arr = new Integer[] {big, small, zero; Arrays.sort(arr, new Comparator() { public int compare(object o1, Object o2) { return ((Integer)o2).intValue() - ((Integer)o1).intValue(); ); System.out.println(Arrays.asList(arr));

25 What does it print? a. Sorted list in ascending order b. Sorted list in descending order c. It varies d. Will throw an Exception

26 What does it print? a. Sorted list in ascending order b. Sorted list in descending order c. It varies d. Will throw an Exception You will encounter an overflow!

27 How do you fix it? public class Sorter { public static void main(string args[]) { Integer big = new Integer( ); Integer small = new Integer( ); Integer zero = new Integer(0); Integer[] arr = new Integer[] {big, small, zero; Arrays.sort(arr, new Comparator() { public int compare(object o1, Object o2) { return ((Integer)o2).compareTo((Integer)o1); ); System.out.println(Arrays.asList(arr));

28 The moral While all these numbers are within the range of an int, the difference between them isn't. When you subtract minus two billion from two billion, you get...? Right, and this is not genuine integer, this is an int value, which is truncated to 32 bits. Four billion doesn't fit in 32 bits, so the high order bits are discarded. The low order 32 bits turn out to be a negative number. Check your ranges!

29 6. Printer class Base { public String name = "Base"; class Derived extends Base { private String name = "Derived"; public class Printer { public static void main(string[] args) { System.out.println(new Derived().name);

30 What does it print? a. It won t compile b. It will throw an Exception c. Base d. Derived

31 What does it print? a. It won t compile b. It will throw an Exception c. Base d. Derived You cannot override instance variables you can only hide them!

32 How do you fix it? class Base { public String getname() { return "Base"; class Derived extends Base { public String getname() { return "Derived"; public class PrintName { public static void main(string[] args) { System.out.println(new Derived().getName());

33 The moral Lesson to learn from this problem is that hiding is generally a bad idea. Hiding leads to confusion in the mind of the reader.

34 7. Big problem public class BigProblem { public static void main(string[] args) { BigInteger fivethousand = new BigInteger("5000"); BigInteger fiftythousand = new BigInteger("50000"); BigInteger fivehundredthousand = new BigInteger("500000"); BigInteger total = BigInteger.ZERO; total.add(fivethousand); total.add(fiftythousand); total.add(fivehundredthousand); System.out.println(total);

35 What does it print? a b c d. 0

36 What does it print? a b c d. 0 BigIntegers are immutable!

37 How do you fix it? public class BigProblem { public static void main(string[] args) { BigInteger fivethousand = new BigInteger("5000"); BigInteger fiftythousand = new BigInteger("50000"); BigInteger fivehundredthousand = new BigInteger("500000"); BigInteger total = BigInteger.ZERO; total = total.add(fivethousand); total = total.add(fiftythousand); total = total.add(fivehundredthousand); System.out.println(total);

38 The moral Do not be misled into thinking that immutable types are mutable. Instead of modifying existing instances, operations on these types return new instances.

39 8. Let s sum this together public class Sum { public static void main(string[] args) { System.out.println(5432l+5432l);

40 What does it print? a b. it won t compile c d

41 What does it print? a b. it won t compile c d l is not a number!

42 How do you fix it? public class Sum { public static void main(string[] args) { System.out.println(5432L+5432L);

43 Kuidagi palju punkte jäi alles või mis?