CS106A, Stanford Handout #25. Methods 2

Transcription

1 CS106A, Stanford Handout #25 Fall, Nick Parlante Methods 2 System.out.println() Console System.out is a standard system object. It implements a println() method that takes a String argument and prints that text to the "console" area. (Note: "l" is a lowercase "L", from the word "line".) Here are a few ways to use println()... Print two strings: System.out.println("hello"); System.out.println("there"); hello there Print an int or double value directly: int i = 6; System.out.println(i); 6 Mix Strings with int or double using '+' between each string and number. This converts each int or double expression to String form and then prints the whole thing. The '+' operator in this context does not do addition. It joins Strings and numbers together to make a big String. int i = 2; System.out.println("i:" + i + " i more:" + (i+1) + " a double:" + 7.0); i:2 i more:3 a double:7.0 System.out.println() prints the string, and then ends the line, like typing a return for that line. There is also a method System.out.print(), that prints the string, and does not end the line, so that subsequent printing will appear on the same line, until finally a println() ends the line. System.out.print("hello"); System.out.print("there"); System.out.println(); hellothere The console is a standard feature in many programming languages, providing an area where the program can append text. In Eclipse, the console is a little area of text that appears at the bottom of the screen in run-mode.

2 2 Essentially, a program can produce output in two ways either by drawing something on screen we can see (moving Binky around or drawing graphics), or by writing out text we can see, such as to the console. We can use println() as an easy debugging technique. We can sprinkle println() calls in the code to see when a method is called, what the ivars are at that time, what the parameters are at that time, and so on. You can leave these println() calls in your code, just comment them out. In the real world, debugging is done with a combination of println() calls in the code and using the debugger. Method Parameters We add a parameter to a method to provide a way for the caller of the method to supply some extra data to use in the computation. As an example, suppose we want to write a printsong() method that prints out the following little song... Look at that Look at that, oh yeah Look at that, don't you know it Over here Over here, oh yeah Over here, don't you know it Time for snacks Time for snacks, oh yeah Time for snacks, don't you know it Nap time Nap time, oh yeah Nap time, don't you know it The song is 12 lines long, so we could write printsong() with 12 System.out.println() calls, one for each line. However, notice that each little 3-line section has the same structure, just with a different phrase at the start of the line... <phrase> <phrase>, oh yeah <phrase>, don't you know it The best solution is to decompose out a printsection() method that takes a String phrase parameter. We can call printsection() once for each section, passing in the String phrase to use for that section. Parameter Syntax The parameters for a method are listed within the parenthesis after the method name. Each parameter has a type and a name. In this case we define the printsection() method to take a single String type parameter named "phrase" (that line is in bold below)...

3 3 // Prints a 3 line section using the given phrase // (Takes a single String parameter named "phrase") public void printsection(string phrase) { System.out.println(phrase); System.out.println(phrase + ", oh yeah"); System.out.println(phrase + ", don't you know it"); // Prints the 12 line song by calling printsection() public void printsong() { printsection("look at that"); printsection("over here"); printsection("time for snacks"); printsection("nap time"); In terms of top-down design, printsong() is the high-level task, and we decompose out the lower-level printsection() method. Each method is short and concentrates on one level of the problem. Mains In our first examples we had, for example, the Hider class and a separate HiderClient that contained a main(). In reality, every class can contain a main(). When the program starts, Eclipse asks us which main we want to run that main() runs and the others are ignored. The main() can create objects of any class and call methods on them. Often, for any class, we will put a simple "test" main() in that class with a little client test code that exercises the objects of that class. Squirrel We will look at a Squirrel example of creating an object that encapsulates some state and uses decomposition to avoid code duplication. The main() in the Squirrel class creates a couple of squirrels and calls dooneday() on them. A typical squirrel day is made of a few activities... A day of playing around is made of running in front of bikes twice, and then running up a tree twice. Each squirrel encapsulates a String myname and an int mynervous. The nervousness is kept in the range The various methods in the Squirrel class adjust the nervousness up and down. The squirrel can run in front of a bike for a number of minutes, which raises its nervousness by the number of minutes. At the start, the squirrel prints "<name> Run in front of bike...", and at the end it prints "<name> Eeek, they almost got me", followed by the current nervousness status. The squirrel will run up a tree for a number of minutes, which lowers its nervousness by the number of minutes. At the start, the squirrel prints

4 4 "<name> Run up tree...", and at the end it prints "<name> That was fun", followed by the current nervousness status. In above cases, the squirrel prints its current nervousness status, made of its nervousness number. If the nervousness is especially high, the squirrel adds " I can't take it any more". If it is especially low, it adds " and that's ok". The squirrel class has lots of behavior, but it can be carved up into methods, where each method just does one thing. We decompose out runuptree() and runinfrontofbike() methods to handle those two cases. In common, they print a status line once the squirrel has a new nervousness number. We decompose out a printstatus(), and both methods can just call it. Finally, the dooneday() method just calls the others. /* * Squirrel encapsulates a String name and an int nervousness in the range * Implements runuptree(), runinfrontofbike(), and dooneday(). */ class Squirrel { private String myname; // name, used in status printout private int mynervous; // current nervousness // hard limits of nervousness public static final int MAX = 10; public static final int MIN = 0; // when stress is below HAPPY, that's good // when stress is above SAD, that's bad public static final int HAPPY = 3; public static final int SAD = 8; // Creates a new squirrel with the given name and nervousness public Squirrel(String name, int nervous) { myname = name; mynervous = nervous; // Plays around: given a number of minutes, runs in front of bike // twice, each for that many minutes. Then runs up tree twice, // each for that many minutes. public void dooneday(int minutes) { runinfrontofbike(minutes); runinfrontofbike(minutes); runuptree(minutes); runuptree(minutes);

5 5 // Runs up a tree for the given number of minutes, // decreasing stress. Prints status when done. public void runuptree(int minutes) { System.out.println(myName + " Run up tree..."); mynervous = mynervous - minutes; if (mynervous < MIN) { mynervous = MIN; printstatus("that was fun"); // Runs in front of bike for the given number of minutes, // increasing stress. Prints status when done. public void runinfrontofbike(int minutes) { System.out.println(myName + " Run in front of bike..."); mynervous = mynervous + minutes; if (mynervous > MAX) { mynervous = MAX; printstatus("eeek, they almost got me"); // Prints the name and the given message, adding // status info at its end. Appends an extra message // if the nervousness is very low or very high. // This is a "utility" method used by other methods. public void printstatus(string message) { System.out.print(myName + " " + message + " (" + mynervous + ")"); if (mynervous < HAPPY) { // things are good System.out.print(" and that's ok!"); if (mynervous > SAD) { // things are bad System.out.print(" I can't take it any more!"); System.out.println(); // ends the above line of status text System.out.println(); // prints a blank line after the status text // main() -- creates a couple squirrels and calls things on them. // (i.e. this is client code of some Squirrel objects). // This main() could be in any class, but here we put it in the Squirrel // class itself. It is common to put a simple "test" main() in a class // like this that exercises the objects of that class. public static void main(string[] args) { // Create two squirrels with initial stress 5 Squirrel alice = new Squirrel("Alice", 5); Squirrel bob = new Squirrel("Bob", 5); alice.dooneday(2); bob.dooneday(4);

6 6 /* Output Alice Run in front of bike... Alice Eeek, they almost got me (7) Alice Run in front of bike... Alice Eeek, they almost got me (9) I can't take it any more! Alice Run up tree... Alice That was fun (7) Alice Run up tree... Alice That was fun (5) Bob Run in front of bike... Bob Eeek, they almost got me (9) I can't take it any more! Bob Run in front of bike... Bob Eeek, they almost got me (10) I can't take it any more! Bob Run up tree... Bob That was fun (6) Bob Run up tree... Bob That was fun (2) and that's ok! */