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Programming section 3 1 Importing classes purpose packages CLASSPATH creating and using a package 2 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 3 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 4 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 5 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(); 6 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(); 7 Protecting methods and attributes Basic Java Language Section 8 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 9 Problems with Accessing attributes directly class Hero { int bullets=10; } 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 Hero h=new Hero(); h.bullets=-20; 10 Using setters to change attributes class hero { int bullets=0; void set_Bullets(int b) { before we change the bullets attribute we could add error checking to detect bad values for b bullets=b; } } Hero h=new Hero(); h.set_Bullets(-20); 11 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 12 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 13 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 } } 14 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 } } 15 Arrays Mass Storage 16 Arrays Arrays allow large numbers of simple data types such as int or instances of classes to be easily accessed. indicates that the array will hold 10 items indicates variable is an array int array[]=new int[10]; array[0]=4; accessing an element of an array also uses [] arrays start at 0 this is the last element of the array array[9]=1234; System.out.println(array[0]); 17 Arrays holding Objects Arrays can also hold instances of a class or any of its subclass indicates that the array will hold 10 instances of the Monster class or subclass Monster array[]=new Monster[10]; array[0]=new Monster(); Dragon is a subclass of Monster so this is fine array[9]=new Dragon(); array[0] is an instance of Monster so we can call take_damage on it array[0].take_damage(); 18 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 no problem!! 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]; null pointer exception! array[0].take_damage(); 19 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(); 20 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 21 the entire array 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 out of bounds array[30]=1234; exception! 22 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. 23 Strings Mass Storage 24 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’ 25 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"; make sure the array is large enough to store the entire String char con[]=new char[s.length()]; Starting char in the String ending char +1 in the String s.getChars(0,s.length(),con,0); array to copy chars into starting position in the array 26 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 27 String by keeping it in its String format 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 starting index “hello”.subString(0,2)==“hel” “hello”.substring(2,3)==“ll” ending index 28 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!” 29 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 30 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 s.equals(s1) is true This is because equals and equalsIgnoreCase check the contents of the instances and not their locations in memory 31 Converting other types into Strings Java will convert almost any simple type into a String for you String s=“”; “” is an empty String int i=166; “” converts a copy of i into a String s=“”+i; This does not work for Objects because Java can not work out how to convert an object into a String 32 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. Exception int i=Integer.parseInt(“hello 100"); 33 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! 34