COE318 Lecture Notes Week 9 (Oct 31, 2011)

Transcription

1 COE318 Software Systems Lecture Notes: Week 9 1 of 12 COE318 Lecture Notes Week 9 (Oct 31, 2011) Topics Casting reference variables equals() and hashcode() overloading Collections and ArrayList utilities (toarray, sort) junit continued Casting reference variables Besides casting primitive data, it is sometimes necessary to cast reference variables. For example, suppose we have: Animal tweety = new Bird( Tweety ); Now suppose Bird's have a method called fly() tat most other Animals do not have. Writing tweety.fly() was cause a compilation error since the Animal interface does not specify a fly() method. However, since we know that tweet is a Bird, we can cast it thus: ((Bird) tweety).fly(); The compiler allows this but, in general, it cannot verify that it is correct. If the cast is incorrect, a runtime exception (ClassCastException) will be thrown. It is often preferable (for readability) to write: Bird b = (Bird) tweety; b.fly(); You can also ensure that no exception can occur at runtime with the safer: if(tweety instanceof Bird) { Bird b = (Bird) tweety;//cannot fail at runtim b. Fly(); Note: the compiler will disallow any cast to a type that is not related (ancestor or descendent) of the type being cast. Thus, the compiler will not allow, for example: Resistor r = (Resistor) (new ComplexNumber(1, 2));//ILLEGAL

2 COE318 Software Systems Lecture Notes: Week 9 2 of 12 equals(object o)and hashcode() It is very common to override equals() and hashcode() (inherited from Object). If you override equals(), you must also override hashcode(). Note: hashcode() is used to qucily dtermine if two objects are potentially equal. If two objects are equal, they must have the same hashcode(). If two objects are not equal, it is preferable but not essential that they have different hashcodes. It is essential that you maintain the signature of equals. The parameter is of type Object! Before checking for actual equality, you have to check that the object is not null and that it is fot proper type. If not, return false. Otherwise, cast it to the right type and check for actual equality. Example for Resistor (where they are equal if they have the same resistance): public boolean equals(object ob) { if(ob == null) { return false; if(! (ob instanceof Resistor)) { return false; Resistor r = (Resistor) ob; return r.r == this.r; public int hashcode() { return (int) r*10000; equals using overloading In Java (and most object oriented languages) you can have more than one method in the same class with the same name! (You cannot do this C, for example.) However, the number and/or types of the parameters must differ. This is called overloading. (Don't confuse this concept from the similarly named but quite

3 COE318 Software Systems Lecture Notes: Week 9 3 of 12 different concept of overriding.) The compiler chooses the version that matches. Here's an example of using 2 equals methods for Resistor: public boolean equals(object ob) { if(ob == null) { return false; if(! (ob instanceof Resistor)) { return false; return equals((resistor) ob); public boolean equals(resistor res) { if (res == null) return false; return r == res.r; Utility methods in Collections and ArrayList The Collections class has several static utility methods (such as sort) that are broadly applicable to anything that implements the List interface (such as ArrayList). There are 2 overloaded sort methods: one that sorts Comparable objects and another where the user supplies a class that implements the Comparable interface. The signatures are: public static <T> void sort(list<t> list) public static <T> void sort(list<t> list, Comparator<? super T> cmprtr) The ArrayList (or anything else that implements the List interface) can also be converted to an ordinary array. (You will need this for one of the later labs.) public <T> T[] toarray(t[] a)

4 COE318 Software Systems Lecture Notes: Week 9 4 of 12 Example: Simple Resistor class Not the Resistor for Lab 6; the one for lab 1. The junit tests illustrate how to use Collections.sort(). How to implemnt the Comparable interface. How to write equals(). Using AssertTrue and AssertFalse in junit. Using AssertEquals for floating point numbers. Note also that the Resistor constructor throws an IllegalArgumentExcpetion if the resistance is not positive. The last test (shown below) shows how to write a junit test to verify that an expected exception did in fact occur. //How to verify that an exception has occurred //Fail if exception does NOT public void testillegalr() { new Resistor(-1); import java.util.comparator; import java.util.arraylist; import java.util.collections; import org.junit.after; import org.junit.afterclass; import org.junit.before; import org.junit.beforeclass; import org.junit.test; import static org.junit.assert.*; public class ResistorTest { public ResistorTest() public static void setupclass() throws Exception public static void teardownclass() throws Exception {

5 COE318 Software Systems Lecture Notes: Week 9 5 of 12 Resistor r1, r2, public void setup() { r1 = new Resistor(10); r2 = new Resistor(20); r3 = new public void teardown() { /** * Test of getcurrent method, of class Resistor. public void testgetcurrent() { r1.setcurrent(2); assertequals(2.0, r1.getcurrent(), 0.0); /** * Test of equals method, of class Resistor. public void testequals() { assertfalse(r1.equals(r2)); assertfalse(r1.equals("abc")); assertfalse(r1.equals(null)); asserttrue(r1.equals(r1)); public void testsort() { ArrayList<Resistor> rs = new ArrayList<Resistor>(); rs.add(r1); rs.add(r3); rs.add(new Resistor(2)); assertequals(rs.get(0).getresistance(), 10.0, 0); System.out.println("Unsorted\n" + rs); Collections.sort(rs);

6 COE318 Software Systems Lecture Notes: Week 9 6 of 12 assertequals(rs.get(0).getresistance(), 2.0, 0); System.out.println("Sorted\n" + rs); Collections.sort(rs, new Comparator<Resistor>() { public int compare(resistor t, Resistor t1) { return (int) (t1.getresistance() - t.getresistance()); ); assertequals(rs.get(0).getresistance(), 20.0, 0); System.out.println("Reverse Sorted\n" + rs); /** * Test of setcurrent method, of class Resistor. public void testsetcurrent() { //How to verify that an exception has occured //Fail if exception does NOT public void testillegalr() { new Resistor(-1); The Resistor class itself is given below: public class Resistor implements Comparable<Resistor>{ final private double r; private double v; public Resistor(double r) { if(r <= 0) { throw new IllegalArgumentException( "Resistance must be > 0"); this.r = r;

7 COE318 Software Systems Lecture Notes: Week 9 7 of 12 public double getresistance() { return r; public double getcurrent() { return v/r; public int hashcode() { int hash = 5; hash = 83 * hash + (int) (Double.doubleToLongBits(this.r) ^ (Double.doubleToLongBits(this.r) >>> 32)); return hash; public boolean equals(object ob) { if(ob == null) { return false; if(! (ob instanceof Resistor)) { return false; Resistor r = (Resistor) ob; return r.r == this.r; public void setcurrent(double i) { v = i*r; public int compareto(resistor t) { return (int) (r - t.r); public String tostring() { return "" + r;

8 COE318 Software Systems Lecture Notes: Week 9 8 of 12 The last version of Zoo shows how ot use toarray(...) for an ArrayList. public Animal[] getanimals() { Animal[] ts = new AbstractAnimal[0]; return animals.toarray(ts); Remarks on Lab 6 No main() supplied (and no need for one!) Use junit tests (you decide how many). Submit your lab when you are confident it works. (It will be tested using junit tests designed by us...) Beware: testing getnodes() is tricky. Another example (Comparable, Comparator interfaces, equals): Person import java.util.arraylist; import java.util.collections; import java.util.comparator; public class Person implements Comparable<Person> { private final String name; private final boolean ismale; public Person(String n, boolean x) { name = n; ismale = x; public String getname() { return name; public boolean ismale() { return ismale;

9 COE318 Software Systems Lecture Notes: Week 9 9 of 12 public String tostring() { return name + "(" + (ismale? "M" : "F") + ")"; public static void main(string[] args) { ArrayList<Person> people = new ArrayList<Person>(); people.add(new Person("Mary", false)); people.add(new Person("Alice", false)); people.add(new Person("Joe", true)); people.add(new Person("Bob", true)); people.add(new Person("Barb", false)); System.out.println("Unsorted: " + people); Collections.sort(people); System.out.println("Sorted (default): " + people); Custom custom = new Custom(); Collections.sort(people, custom); System.out.println("Sorted (custom): " + people); //Alternate... Collections.sort(people, new Comparator<Person>() { public int compare(person t, Person t1) { if(t.ismale()!= t1.ismale()) { return t.ismale()? -1 : 1; //Same gender so order by name return t.getname().compareto(t1.getname()); ); System.out.println("Sorted (anonymous class): " + people); public int compareto(person t) { return name.compareto(t.name); static class Custom implements Comparator<Person> { public int compare(person t, Person t1) { if(t.ismale()!= t1.ismale()) { return t.ismale()? 1 : -1; //Same gender so order by name return t.getname().compareto(t1.getname());

10 COE318 Software Systems Lecture Notes: Week 9 10 of 12 Questions 1. A Person class is shown below. Complete the methods equalsand compareto. (Person objects are considered equal if they have the same first and last names. For the compareto() method, if the last names are different then the one that comes alphabetically before the other is considered smaller ; if the last names are the same, then the first names are used to dtermine the smaller ; if the first and last names are the same, compareto should return 0.) public class Person implements Comparable<Person> { private final String first; private final String last; public Person(String first, String last) { this.first = first; this.last = last; /** * equals true if first and last name same */ public boolean equals(object obj) { public int hashcode() { public int compareto(person p) {

11 COE318 Software Systems Lecture Notes: Week 9 11 of Write junit tests for equals() and compareto() in Person. Answers 1. public class Person implements Comparable<Person> { private final String first; private final String last; public Person(String first, String last) { this.first = first; this.last = last; /** * equals true if first and last name same */ public boolean equals(object obj) { if(obj == null) return false; if(!(obj instanceof Person)) return false; Person p = (Person) obj; return last.equals(p.last) && first.equals(p.first); public int hashcode() {

12 COE318 Software Systems Lecture Notes: Week 9 12 of 12 return first.hashcode() + last.hashcode(); public int compareto(person p) { int a = last.compareto(p.last); if(a == 0) { return first.compareto(first); return a;