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CS 121 – Intro to Programming:Java - Lecture 2 Announcements Course home page: First Owl assigment.

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Presentation on theme: "CS 121 – Intro to Programming:Java - Lecture 2 Announcements Course home page: First Owl assigment."— Presentation transcript:

1 CS 121 – Intro to Programming:Java - Lecture 2 Announcements Course home page: First Owl assigment is up, due Friday. Second assignment up soon. First programming assignment now up due in class, on paper, next Tuesday Recommended IDE - jGRASP It comes with the disk in your textbook (in windows format however). I’ll make public some additional notes shortly on how get jGRASP, where to put it, etc.

2 Design Goals of the Programming Enterprise Correctness Efficiency Implementation Goals Robustness - what happens when you type with your elbows Adaptability - what happens when people start actually using your beautiful program - and they want to make changes to it Reusability - How well can the pieces of what you’ve written be repackaged for future use

3 Java’s Object Model Classes - a class is a blueprint or template for an object an Object is an example or an instantiation of a class definition an instance variable or attribute is a characteristic of an Object a method is an activity that serves an object Example: A Student class is a template for a student Attributes - name, age, credits, year, etc Methods - getAge, creditsLeft, calcGPA, and so forth Writing a class definition prepares this template Different students mean different objects -- with different attribute values!

4 Hospital class attributes? methods? Bank Account class attributes? methods? Person class Baseball player class Mother class

5 Old MadDonald does Java Old Macdonald had a farm // first two lines make chorus œœßœei, ei, o; œœßœand on that farm he had a pig œœßœei ei O œœßœWith an oink oink here œœßœAnd a oink oink there œœßœHere a oink there a oink œœßœEverywhere a oink oink œœßœOld Macdonald had a farm œœßœei, ei, o; œœßœOld Macdonald had a farm œœßœei, ei, o; and on that farm he had a dog œœßœei ei O (etc…)

6 Overall structure Chorus Pig verse Chorus Dog verse Chorus Who are the players? Chorus object Verse object - it’s parameterized (pig, dog, etc) A coordinating “song” class

7 public class MacSong{ public static void main(String[] args){ MacChorus m = new MacChorus(); MacVerse p = new MacVerse("pig", "oink"); MacVerse d = new MacVerse("dog", "woof"); m.chorus(); p.verse(); m.chorus(); d.verse(); m.chorus(); } } Three objects made - a chorus object, and two verse objects. The verse objects differ in that their attributes hold different values. The verse method exploits this to give different verses.

8 public class MacChorus{ public void chorus(){ // a method that serves MacChorus System.out.println("Old Macdonald had a farm"); System.out.println("ei, ei, o;"); } }

9 public class MacVerse{ String name; String noise; // attributes public MacVerse(String animalName, String animalNoise){ name = animalName; noise = animalNoise; } public String getName(){return name;} public String getNoise(){return noise;}; public void verse(){ System.out.println("and on that farm he had a " + name); System.out.println("ei ei O"); System.out.println("With a " + noise + " " + noise + " here"); System.out.println("And a " + noise + " " + noise + "there"); System.out.println("Here a " + noise + " there a " + noise); System.out.println("Everywhere a " + noise + " " + noise); } }

10 Primitive Data Types objects are Java’s main currency Too tedious for them to be the only currency - making numbers a kind of object is a pain, however. Primitive data types integers (4), floats(2), char, boolean. That these aren’t actually objects will turn out to be a pain, too. Statement like these are fairly common: int count = 0; boolean okSoFar = false; char averageGrade = ‘C’; // note the single quotes Details in section 2.4

11 Operator Precedence Pretty important, but not very interesting num = / 3; not the same as num = (2 + 8) / 3; *, / evaluated first, unless overridden by parentheses. Moral: use parentheses pretty much always… Make sure you read up on 2.4, 2.5, 2.6 for Thursday!

12 Strings A very important class. String greeting; greeting = new String(“ola”); greeting2 = new String(“howdy”); greeting = greeting2; System.out.println(greeting); Some caveats: 1)Strings are not primitives (unlike float, double, int, etc) 2)Strings are so important that there’s a shorthand for String creation: greeting = “ola”; // works fine 3) Cell model for variables is different: now a cell hold reference to location of (actual) object -- rather than the object itself

13 As we’ve seen, classes/objects are served by methods Certainly true for Strings String pupName = “spot”; int len = pupName.length(); char what = pupName.charAt(1); char what = pupName.charAt(0); String huh = pupName.concat(“less”); String bigHuh = pubName.toUpperCase();

14 A word on +: String noise = “moo”; int age = 20; age = age + age; noise = noise + noise; noise = 20+noise; noise = noise; noise = noise+20+30;

15 Let’s mess with jGRASP, and some changes to OldMacdonald -change “woof” to “bark” --add a dotted line divider before each verse


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