Java Programming. Computer Science 112 Class 2

Transcription

1 Java Programming Computer Science 112 Class 2

2 The first assignment It is extra credit (5/0) Mostly a test of your determination. I gave points for trying. println(" * * *"); // etc print(" * "); print(number); println(" * *"); // Or use "+" from last week's slides.

3 The second assignment That's the flowchart paper It's graded for real, but this one is "tried/didn't" only And they are due at the end of the class after the one they is assigned. If I happen to mention the answer to one or two of them I always do For this first one, I'm just using it for attendance. Wave it around when I say your name.

4 Let's do rock paper scissors...for no reason at all. Unless anyone has a question about a specific previous one. Yay flowcharts!

5 Recap: Variables Get used to this format (it's an idiom): <type> <name> = <value>; Types: int, float, boolean int tx = 0; int tcount = 5; float tvalue = 7.0f; boolean tawesome = true; You've seen an Object; those are in a few weeks so don't worry yet <type> <name> = new <type>(); Scanner tkeyboard = new Scanner();

6 Tricky bits Integer Division 9 / 4 equals 2, 9 % 4 equals 1 / is divide or Div, % is remainder or Mod 9 / 4.0 equals 2.25 Telling the computer one is a float makes the answer a float Can also "cast" with a type in () 9 / (float) 4 Order of operations Please excuse my dear aunt Sally Eating all oreos String equality String is a class, int is a primitive == is for primitives, tastring.equals("dog") for String

7 Commands on variables The variable you are changing is always on the left int tnumber = 5; int tothernumber = 4; tnumber = tnumber + tothernumber; tnumber has changed, but it is not retroactive Programs go one line at a time downwards The right side is computed before the assigning happens

8 Swapping badly Say I want to switch what is in two variables. "I want to switch two variables." int tx = 1; int ty = 2; tx = ty; ty = tx; Does that work? (Rhetorical questions are pretty much always no.)

9 Swapping badly Say I want to switch what is in two variables. int tx = 1; int ty = 2; tx = ty; // tx is now 2 ty = tx; // ty is set to 2... shoot. Line at a time, top to bottom, nothing reaches back up.

10 Swapping goodly Each command finishes before the next one starts We can't lose the value of one of the variables by overwriting it, so we make a place to hold it for a moment Variables are free. Make as many as you want. They are just Post-It Notes (r) 1 2 tx ty ttemp = ty; 1 2 tx ty ty = tx; 1 1 tx ty tx = ttemp; ttemp 2 ttemp 2 ttemp 2 tx ty ttemp

11 Column attack! Super quick questions - 1 per row. 1) x = / 2 - (5 * 2) 2) x = (7 * 2 + 1) / 5-3 3) x = 14; y = 5 % 2; x = (x + y) / 3-5; 4) x = 5; y = 6; y = x + x / x - x % x - y 5) x = 3; y = 4; z = x & y; 6) x = "ro"; y = "ze"; z = y + x;

12 Branching: The "if" statement boolean teasycheck = true; if( teasycheck == true ) { System.out.println("I love branching."); } else { System.out.println("I like pie."); }

13 Connection to flow charts? The decision in the diamond is converted to the statement in the if-clause. There are only two ways out. The statement is true and we run what is inside the if It is false and we run the else (optional) instead Can afford rent? No Yes Pay rent Run! if( tbalance > 3000 ) { tbalance = tbalance ; } else { // Knees broken }

14 (OT) What's with all the t's? Every programmer says their coding style method is the best. But mine is. Indentation styles: Allman K&R BSD Variable names: Hungarian Camel The only thing that matters is what your boss says to do

15 And I'm your boss Allman indentation Curly bracket is on next line Space inside parens, not outside Scope capitalization (Half Hungarian) Lowercase identifying prefix, capital camel after "t" is for "temp" You'll lose a point if I can trace a bug back to bad coding style. (Get a 10 and I'll not care.) if( tpants == 0 ) { } PutOnPants();

16 Back to what can be in an "if" Anything that results in true or false <, >, <=, >= are familiar!= means "not equal" = vs == is the third most common bug you will make this semester A single-equals means you are setting a variable A double-equals is testing a variable if( tpotapopotatos == 3 ) { } System.out.println("Boil 'em mash 'em etc");

17 And after that, it gets even better Complex tests can use AND, OR, and NOT AND is a double ampersand "&&" OR is a double bar " " (on backslash key usually) NOT is an exclamation point "!" Parentheses help control complex tests if( (thavemonkeys == false && tallowedmonkeys == true) (tparentshome < 1) ) { BuyMonkey(); }

18 Is today a weekday? "It is not Saturday and it is not Sunday" if(!today.equals("saturday") &&!today.equals("sunday") ) return true; else return false; "It is not Saturday or Sunday" makes sense in English, but fails spectacularly in programming because of order of operations. if(!today.equals("saturday") "Sunday" )

19 "else if" An if only has two choices, but you may have three (or more) outcomes "else if" will check sequentially if( tbananas < 5 ) System.out.println("I need more."); else if( tbananas == 5 ) System.out.println("I have enough."); else if( tbananas > 5 ) System.out.println("You can't have too many."); A series of diamonds, not one with three spokes

20 Strings are special Objects are the most magical best part of Java, but we aren't there yet. We have Scanner and String so far Note, the types here are capitalized int, float, and boolean aren't Objects To do anything with them, you use the DOT Scanner tkeyboard = new Scanner(System.in); String tfirstline = tkeyboard.nextline(); String tsecondline = tkeyboard.nextline(); if( tfirstline.equals(tsecondline) ) { } System.out.println("Oh hey, this line has dots too.");

21 Switch statements If you have many possible outcomes, you may want to use a "Switch". But you won't, because I would fire you. Sixteen years and I've never used it once. Provably worse than else-if Powered by "goto" Java version and platform specific It's on page 173. It's not on the test. This slide required by law. Don't use them. Really.

22 Enumerations are cool though if( tlanguage == 0 ) System.out.println("Hi"); else if( tlanguage == 1 ) System.out.println("Hola."); else if( tlanguage == 2 ) System.out.println("nuqneH"); Those are "Magic Numbers", and they are bad.

23 Turn Magic Numbers in to Words enum Language { English, Spanish, Klingon } Language tlanguage = Language.Klingon; Makes your own primitive type (like int) Expandable, readable, clear. But kinda funky to set up. So I don't require them.

24 One last thing about Scanner Scanner works great for Strings, but when you try nextint to get an int instead of nextline to get a String, it leaves an invisible character in the buffer that will mess up your next read. Always reading Strings is safer. And if you aren't doing math, you can even just leave it as the letter 2. String tinput = tscanner.nextline(); // Yay this works fine always! int tinputint = tscanner.nextint(); String tdummy = tscanner.nextline(); // Eats the garbage from nextint.

25 One more last thing about Scanner That "import" line at the top of the example code is magic Using pretty much any capital-letter class needs one so Eclipse can find it Just like a book on a shelf But Eclipse loves you - red squiggly line power!

26 So how are you doing? If I can afford to buy 20 monkeys, I will buy 5 iguanas unless iguanas cost more than ostriches which means I'll buy 3 ostriches. If I can only afford 10 monkeys, I'll buy enough ostriches to equal my iguana count. If I can't afford 10 monkeys, I'll just steal one and name it George. int tmonkeys = 14; int tostriches = 5; int tiguanas = 10; float tbudget = f; float tmonkeycost = 400.0f; float tostrichcost = 600.0f; float tiguanacost = 25.0f; We'll make a flowchart, then you have to code it with me.

27 Run it With those numbers you should get "I bought 5 ostriches". Change budget to "I bought 5 iguanas" But how can you be sure?

28 THE DEBUGGER Using the debugger is the most important skill you can have. You'd think "does it work" is the most important, but you need to know it works. I'm going to minimize and do it for real, but for later reading see next slide

29 Next slide Right click the margin of a line and pick Toggle Breakpoint When the code gets there, it will pause and move your windows around. When you are done debugging you get them put back with the buttons in the top right. Now the Step Over, Step In, and Step Out buttons let you see exactly what is going on. "Step Over" is pretty much all you need until functions later. Press Continue to resume normal operation.