Java Review Session CS 402 Melissa Carroll Berk

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

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

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

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

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

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

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()

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

Java – Data Types • Primitives – Type conversions • Can’t convert 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)

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

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

Java Development

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)

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)

Using the Java Platform • http: //java. sun. com/j 2 se/1. 4. 2/docs/api/index. html • You can always use the full package path name to access classes java. io. File. Reader fr = new java. io. File. Reader (“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. File. Reader; //. . . File. Reader fr = new File. Reader(“test. txt”); //. . .

Object Orientation

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; } }

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

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

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).

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

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

Java - Classes • Constructors public Circle(float r) {radius = r; } Name of the class – 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 free, which does nothing – Refer to other constructors with this()

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

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; } }

Java - Objects • So, we’ve made a class, now what? • We can start making circles… public Circle c 1; public Circle c 2 = new Circle(); public Circle c 3 = new Circle(2. 5); – c 1 didn’t actually make a circle, it just made a circle reference – c 2 used the default constructor, and has radius 0 – c 3 used our constructor, and has radius 2. 5

Java - Objects • Now that we have circles, we can call methods on them public float f = c 3. area(); System. out. print(f); //19. 6349

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)

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

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

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

Example Demo

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/j 2 se/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

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.

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.

Suggested Programs • Using the built in Hash. Map, 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)

Questions?