1 Programming section 3

2 Importing classes purpose packages CLASSPATH creating and using a package

3 Importing classes Purpose Serious Java programming deals with many classes … … and so some sort of organisation is needed. Organisation in Java is achieved using the concept of a package (c.f. the use of folders or directories to organise files). A program can import classes from a package

4 Importing classes Packages Packages are collections of classes and sub-packages. (c.f. directories can hold files and subdirectories) The standard package or collection of classes used in Java is called java.lang

5 Importing classes Standard classes Standard classes in java.lang can be used directly e.g. System.out.println(“Hello world”);  out is a member variable (attribute) of the System class. It is of class PrintStream  println is a method of the PrintStream class

6 Importing classes A number of package classes If a number of classes required are in some other package then import all the public classes in the package. e.g. import java.awt.*; : Font f = new Font();

7 Importing classes Name clashes This fails if FTP is in both packages import com.naviseek.web.*; import com.prefect.http.*; : FTP out = new FTP(); This succeeds since ambiguity removed : com.prefect.http.FTP out = new com.prefect.http.FTP();

8 Protecting methods and attributes Basic Java Language Section

9 Protecting Your Class Usually you don’t want other objects directly changing your attributes because if you modify the type of name of the attributes you have to modify the other classes… So you can protect any method or attribute from outside interference.. by providing a setter to change its value You can also protect parts of your class using private and protected in their declaration

10 Problems with Accessing attributes directly class Hero { int bullets=10; } Hero h=new Hero(); h.bullets=-20; This is a silly value for the number of bullets but because the variable is accessed directly no error checking can be performed by the hero class

11 Using setters to change attributes class hero { int bullets=0; void set_Bullets(int b) { bullets=b; } Hero h=new Hero(); h.set_Bullets(-20); before we change the bullets attribute we could add error checking to detect bad values for b

12 Protecting Your Class Private – this means the method or attribute is not available outside the class it is defined in not even subclasses can access it Protected – only subclasses and classes in the same package can access it Public - anyone can access it Any class can always access the attributes and methods it defines

13 Protection class Hero { protected int bullets; public set_Bullets(int b) { bullets = b } private boolean spy; } now bullets cannot be accessed from outside the class from another package directly so the setter must be used to modify bullets and can provide error checking

14 Example of protection package examples; class Example1 { void doit() { Hero john = new Hero(); john.set_bullets(10); // legal because set_Bullets is public john.bullets=20; // illegal because height is protected john.spy=true; // illegal because spy is private }

15 Protection with inheritance package examples; class Hero2 extends Hero { void doit() { set_bullets(10); // legal because grow is public bullets=20; // legal because height is protected and we are a subclass of Person spy=true; // illegal because spy is private }

16 Arrays Mass Storage

17 Arrays Arrays allow large numbers of simple data types such as int or instances of classes to be easily accessed. int array[]=new int[10]; array[0]=4; array[9]=1234; System.out.println(array[0]); indicates that the array will hold 10 items accessing an element of an array also uses [] arrays start at 0 this is the last element of the array indicates variable is an array

18 Arrays holding Objects Arrays can also hold instances of a class or any of its subclass Monster array[]=new Monster[10]; array[0]=new Monster(); array[9]=new Dragon(); array[0].take_damage(); indicates that the array will hold 10 instances of the Monster class or subclass Dragon is a subclass of Monster so this is fine array[0] is an instance of Monster so we can call take_damage on it

19 Dangers of Arrays of Objects with an array of simple types such as ints every element in the array exists even if we don’t assign a value to them int array[]=new int[2]; System.out.println(array[0]); with an array of Objects elements are null until we assign an instance to them Monster array[]=new Monster[2]; array[0].take_damage(); null pointer exception! no problem!!

20 Initialising Object Arrays unlike arrays of simple types arrays of any type of object require each element to be instantiated and inserted into the array... Monster array[]=new Monster[2]; array[0]=new Monster(); array[1]=new Monster(); array[0].take_damage();

21 Finding the number of elements in an array Fortunately we can ask the array how long it is using.length int array[]=new int[10]; int index; for (index=0;index<array.length;index=index+1) { array[index]=0; } Now we can make the array bigger or smaller and the code will not crash and will initialise the entire array

22 Problems with Arrays I The single biggest problem with an array is that it is not variable length. You have to declare the length of the array as a constant and you can not tag on extra elements int array[]=new array[20]; array[30]=1234; array out of bounds exception!

23 Problems with arrays II You also can not remove elements in an array even if they are no longer needed. For instance suppose I have an array of 4 Monsters which the Hero is fighting. If the Hero kills 2 it would be nice to remove two from the array... the only thing you can do is to replace the dead instance with null.

24 Strings Mass Storage

25 Strings The String class allows you to store and manipulate sequences of characters Like chars, Strings are case sensitive so “hello” is not the same as “Hello” Strings are in fact Objects The length method tells you the length of a string in chars e.g. “hello”.length()==5 You can get a copy of the character at any position in a string using charAt() “hello”.charAt(0)==‘h’ “hello”.charAt(4)==‘o’

26 Relationship between String and Arrays It is possible to get a copy of the contents of a String as a char array String s="hello world"; char con[]=new char[s.length()]; s.getChars(0,s.length(),con,0); make sure the array is large enough to store the entire String Starting char in the Stringending char +1 in the String array to copy chars into starting position in the array

27 Relationship between String and Arrays It is also possible to create a string from a char array String s="hello world"; char con[]=new char[s.length()]; s.getChars(0,s.length(),con,0); con[0]='H'; s=new String(con); System.out.println(s); However, manipulating char arrays is not easy so in general it is better to manipulate the String by keeping it in its String format

28 Manipulating Strings There are many manipulation functions defined for the string object one of the most useful is subString You can get a copy of part of a string using subString “hello”.subString(0,2)==“hel” “hello”.substring(2,3)==“ll” starting index ending index

29 Adding to your String Unlike arrays, Strings can be appended and pre-pended to easily String s=“world”; s=“hello “+s; s=s+”!”; This gives the String “hello world!”

30 Seeing if Two Strings are Equal using == For simple types you can use == and != to see if something is equal or not equal. == and != do not work correctly for objects String s=“hello there”; String s1=“hello there”; s==s is true s1==s1 is true s==s1 is false!!!!!!!!!!!!!! This is because for objects == and != check where the instance is in memory and not the contents of the instances

31 Seeing if Two Strings are Equal using equals To see if two Strings are identical you need to use equals() or equalsIgnoreCase() String s=“hello there”; String s1=“hello there”; s.equals(s) is true s.equals(s1) is true This is because equals and equalsIgnoreCase check the contents of the instances and not their locations in memory

32 Converting other types into Strings Java will convert almost any simple type into a String for you String s=“”; int i=166; s=“”+i; This does not work for Objects because Java can not work out how to convert an object into a String “” converts a copy of i into a String “” is an empty String

33 Converting Strings into other types. Java provides static methods in classes called Integer, Long, Float and Double to help you: int i=Integer.parseInt("100"); long l=Long.parseLong("100"); float f=Float.parseFloat("100"); double d=Double.parseDouble("100"); If and only if the entire String looks like another type can the conversion take place. int i=Integer.parseInt(“hello 100"); Exception

34 More String Functions There are many more String manipulation functions than we have time for to look at here Take a look at the Java documentation for the String class and have some fun!