Java Review Session

1. Java Review Session CS 402 Melissa Carroll Berk Kapicioglu Most slides by: Matt Hibbs

2. First Off • Slides available on course website • This presentation doesn’t get into very fine detail, so for more info check out Java in a Nutshell, which is available online to Princeton students for free. You can search/find it at: catalog.princeton.edu

3. Why Java? • Cross Platform • Bytecode and the JVM run on most platforms • Security • JVM can restrict access to local machine • Safety • Garbage Collection (fewer memory leaks) • Ease of Coding • Many available packages

4. Java Basics • http://java.sun.com (SDK) • Development Environments • Eclipse • Emacs • Notepad and Command Line

5. In C int main (int argc, char** argv) { printf(“Hello World!\n”); return 0; }/* end main */ In Java public class HelloWorld { public static void main (String[] args) { System.out.println(“Hello World!”); }/* end main */ }//end HelloWorld Hello World • Things to notice: • Similar syntax • Classes • System.out.println() • main is void: public static void main

6. How Java Differs from C • Exclusively Object-Oriented Language • EVERYTHING must live in a class (mostly) • No Global Variables • No Pointers • Also, no ‘*’ ‘->’ or ‘&’ operators • Blessing and a Curse • Garbage Collection • Loss of Power

7. How Java Differs from C • No Preprocessor (no #include, #define, etc.) • No goto statement • Declare/Define Variables & Methods anywhere (within a class) • No struct, enum, or typedef • Can’t overload Operators • Use new rather than malloc()

8. Java – Data Types • Primitives • boolean, char, byte, short, int, long, float, double • char is Unicode (16 bits) • boolean is true/false (not 1/0) int i = 1; if (i) { } //BAD if (i == 1) { } //GOOD

9. Java – Data Types • Primitives • Type conversions • Can’t convert boolean boolean b = false; int i = b; //BAD int j = b ? 1 : 0; //GOOD • Converting ‘up’ is automatic • Converting ‘down’ requires a cast double f = 37.5; // it used to be float but // gave compiler error int i = f; //BAD int j = (int) f; //GOOD (truncates)

10. Java – Data Types • References (by reference vs. by value) • Everything that’s not primitive (classes and arrays) • Think of these as hidden pointers Foo a = new Foo(); Foo b = a; • a and b now reference the same object

11. Java – Arrays • Mostly what you would expect String[] textLines; String[] textLines2 = new String[15]; textLines2[3] = “Hi, how are you today?”; int[] fib = {1, 1, 2, 3, 5, 8, 13, 21}; System.out.print(fib[5]); // 8 • textLines is just a Reference, no array was actually made • textLines2 is an un-initialized array • fib is an initialized array

12. Java Development

13. Java Platform • Lots of built-in objects and functions for various purposes • Graphics (java.awt) • Math Functions (java.math) • Networking (java.net) • Databases (java.sql) • We’ll mostly use the platform for data structures (java.util) and I/O (java.io)

14. Java Packages • Java organizes classes into larger groups called packages • Both the Java Platform and any classes that you write are organized into packages • For code that you write, don’t worry so much about what package you’re in (the default is fine)

15. Using the Java Platform • http://java.sun.com/j2se/1.4.2/docs/api/index.html • You can always use the full package path name to access classes java.io.FileReader fr = new java.io.FileReader (“test.txt”); • But that’s pretty annoying, so you can use import statements at the beginning of your code to avoid this import java.io.FileReader; //... FileReader fr = new FileReader(“test.txt”); //...

16. Object Orientation

17. Java - Classes • Similar to C++ classes in many ways • Here’s a simple Java class that we’ll break down: public class Circle { private float radius; public static final double PI = 3.14159; public Circle(float r) { radius = r; } public float area() { return PI * radius * radius; } }

18. Java - Classes • Naming public class Circle { • The basic form is: • <modifiers> class <name> { • Typically need an access modifier • public, protected, private • Can have additional modifiers • static, final, abstract • If a Class is public, it MUST be the only public class in its file, and this file MUST be called <name>.java

19. Java – Access Modifiers • Access modifiers control who has access to a variable or class public – anything can access it protected – only classes in same package can access (not important for 402) private – only accessible within the class that defines it

20. Java - Classes • Instance Field private float radius; • Any variable declared in a class is a field • Typically have an access modifier • In this case the field is uninitialized, so it will get the default value (0).

21. Java - Classes • Class fields and methods • Use static modifier • Are instance-independent • Refer to using class name e.g. Circle.PI • No need to include class name if using within class • If only used static methods, would not be OO

22. Java - Classes • Constant class field public static final double PI = 3.14159; • Again, typically have an access modifier • If you want to have a constant variable, also use the static and final modifiers • static means that there is only one PI no matter how many instances of Circle we make • final means that PI cannot be changed

23. Java - Classes • Constructors public Circle(float r) {radius = r;} • This is what gets called when an instance of your class is created • Typically used to just initialize fields • If you don’t write one, you get a default constructor for free, which does nothing • Refer to other constructors with this() Name of the class

24. Java - Classes • Methods public float area() { return PI * radius * radius; } • Again, need an access modifier • Need a return type (void is a valid choice) • Can optionally pass in arguments

25. Java - Classes • Here’s the whole thing again: public class Circle { private float radius; public static final double PI = 3.14159; public Circle(float r) { radius = r; } public float area() { return PI * radius * radius; } }

26. Java - Objects • So, we’ve made a class, now what? • We can start making circles… public Circle c1; public Circle c2 = new Circle(); public Circle c3 = new Circle(2.5); • c1 didn’t actually make a circle, it just made a circle reference • c2 used the default constructor, and has radius 0 • c3 used our constructor, and has radius 2.5

27. Java - Objects • Now that we have circles, we can call methods on them public float f = c3.area(); System.out.print(f); //19.6349

28. Java - Interfaces • Interfaces are one of the ways that Java allows for Inheritance (the other is with subclasses) • For example, we could make an interface called Shape that our Circle class can implement public interface Shape { public float area (); }// end Shape public class Circle implements Shape { //... }//end Circle • Now, since we know the Circle implements Shape, we know that Circle must contain an area() method • Classes can implement one interface, no interfaces, or multiple interfaces (many do)

29. Java – Reference Oddities • Since we only deal with references to objects, some behavior may seem odd Circle hulaHoop = new Circle (3); Circle CD = hulaHoop; CD.radius = 1; //we can’t actually do this (let’s pretend) System.out.print(hulaHoop.radius); // 1

30. Packages to Look At • Have a look at java.util, especially java.util.HashMap and java.util.LinkedList • Both are what they sound like and have fairly straightforward functions • These may or may not be useful in upcoming assignments

31. What to Focus on for 402 • Do focus on • Basic syntax like control flow, declarations • Syntax for declaring classes and methods, instantiating objects • Writing output to stdout, stderr (for debugging) • Basic OO like interfaces • Java.util • Don’t worry so much about • Reading input from stdin and files • Misc packages like java.security

32. Example Demo

33. Suggestions • Play around with Java • Find an IDE your comfortable with, and just write some programs to get a feel for it. • Use the Java API specification at http://java.sun.com/j2se/1.4.2/docs/api/index.html • If you have any problems, e-mail Berk (bkapicio@cs) or Melissa (mkc@cs) or feel free to ask your friends and/or classmates for help

34. Suggested Programs • A rcenet rsesercah sduty fnuod taht it deosn’t mttaer in waht oredr the ltteers of a wrod are, so lnog as the frist and lsat letetrs are in the rgiht pclae. The rset can be a toatl mses and you can siltl raed it whtuoit a porbelm.

35. Suggested Programs • A recent research study found that it doesn’t matter in what order the letters of a word are, so long as the first and last letters are in the right place. The rest can be a total mess and you can still read it without a problem. • As a toy, write a program that garbles text like this – it would get you familiar with I/O and text manipulation.

36. Suggested Programs • Using the built in HashMap, write a tester as though you had written this class yourself (possibly in CS 226) • Maybe you could help out the TAs by writing a program that uses student names as a key and maps them to student ID numbers (You don’t really need to do this, we can just look it up)

37. Questions?