Download java - WordPress.com

Topics
1.Brief History of Java
2.What is Java?
3.Differences to C++
4.Java Development Tools
5.The First Simple Program
6.Structure of a Class
7.Variables
7.1.Declaration
7.2.Scope
7.3Constants
8.Data Types
8.1.Primitive Data Types
8.2.Reference Data Types
9.Operators
9.1.Arithmetic
9.2.Comparison and Conditional
9.3.Assignment
9.4.Others
10.Type Conversions and Casting
11.Mathematical Functions
Chapter 2
Comp321: Object Oriented Programming
2
1.Brief History of Java
0 Java was originally developed by Sun Microsystems, a US company,
in 1991. Its first name was Oak and it was designed for the
development of software for consumer electronic devices such as
televisions and video recorders. As such, one of the main goals for
the language was that it is simple, portable and highly reliable.
0 The team based the new language on the existing C and C++
languages, but they removed features of C and C++ that were seen
as being the sources of problems.
0 In 1993, the WWW (World Wide Web) was starting to be used,
transforming the previously text-based internet into a more
graphic-rich environment. The team developing the Oak language
came up with the idea of creating small programs, called applets,
that would be able to run on any computer that was connected to
the internet.
Chapter 2
Comp321: Object Oriented Programming
3
…cont
0 In 1995, Oak was renamed Java due to some legal issues – the
name does not have any particular meaning. Java was supported by
many IT companies, including the big internet browser developers,
Netscape and Microsoft.
0 Since then, Java has become one of the main languages used for
internet programming and also as a general-purpose objectoriented language for stand-alone applications.
Chapter 2
Comp321: Object Oriented Programming
4
2.What is Java?
0 Java technology consists of a programming language and a
platform on which programmes can be run.
0 Java is different from other programming languages because a Java
program is both compiled and interpreted but in many other
languages, the program is either compiled or interpreted.
0 The Java language is object-oriented and programs written in Java
are portable – that is, they can be run on many different platforms
(e.g. Windows, Mac, Linux, Solaris).
0 Mac, short for Macintosh, is an operating system owned by the
Apple Corporation. The Mac OS has a GUI that looks like Windows
but has a different underlying architecture. It is a competitor to
Windows. The Mac OS has to be run on Apple Mac computers,
which, again, have a different underlying architecture to PCs like
IBM, Acer etc.
Chapter 2
Comp321: Object Oriented Programming
5
...cont
0 Solaris is a platform from Sun Microsystems. Like Windows, Solaris
has server and workstation operating systems.
0 The Java compiler translates the program into an intermediate
language called Java bytecodes.
0 Java bytecodes are platform-independent – this means they can
be run on different operating systems (e.g. Windows and Mac).
0 There are different Java interpreters for different platforms. The
interpreter parses and runs each of the Java bytecode instructions.
Chapter 2
Comp321: Object Oriented Programming
6
Compiling and Interpreting of a Java program
Chapter 2
Comp321: Object Oriented Programming
7
Compiling and Interpreting of a Java program
0 Compilation only needs to happen one time, whereas
interpretation happens every time the program is executed.
0 The source code for a Java program is saved to one or more .java
files.
0 The compiler generates a .class file for each .java file.
0 A .java file can be opened and viewed in any text editor, but a .class
file is bytecode so it cannot be viewed in the same way.
0 Every Java interpreter is actually an implementation of something
called the Java VM (Virtual Machine).
0 It is called a Virtual Machine because it is like a computer that does
not physically exist
0 The VM exists only within the memory of the computer. The Java
bytecodes are like a set of machine code instructions for the Java
VM. A web browser that is Java-enabled is an example of an
interpreter.
Chapter 2
Comp321: Object Oriented Programming
8
…cont
0 The big benefit of the Java way of doing things is that the program
code can be written once, compiled and then run on many different
platforms.
0 See the figure below for an illustration of this concept. There is one
Java VM, but many different implementations of it – i.e. many
different interpreters for the Java bytecodes.
Chapter 2
Comp321: Object Oriented Programming
9
…cont
0 Figure - a Java program can be compiled once and then run on
different platforms
Chapter 2
Comp321: Object Oriented Programming
10
…cont
0 As mentioned above, Java includes a programming language and a
platform.
0 A platform refers to the hardware (PCs, servers, printers etc) and
software (operating systems, application software etc) in which a
program is run. So, for example, the platform we use here in your
computer laboratory is Windows XP/7 running on IBM-compatible
PCs.
0 The Java platform is a bit different to other platforms, in that it
consists of software only that means it is hardware-independent.
This is what makes it possible to run compiled Java programs on
different platforms.
0 The Java platform consists of:
0 The Java VM
0 The Java API (Application Programming Interface).
Chapter 2
Comp321: Object Oriented Programming
11
…cont
0 The API provides libraries of pre-written, ready-to-use
components that provide standard functionality.
0 A library groups a number of related classes and interfaces
together into what is called a Java package. You can choose what
packages you want to use in your program.
0 In order to write Java programs, you need to have the Java SDK
(Software Development Kit) installed on your PC. This includes the
API and the VM, as well as compilers and debuggers for Java.
0 Java is a programming language which is based on the OOP (objectoriented paradigm).
0 'Paradigm' means a set of ideas, a concept or hypothesis.
0 You should already be familiar with some OO concepts from your
study of C++. However, there are some differences between Java
and C++.
Chapter 2
Comp321: Object Oriented Programming
12
3.Differences to C++
0 Java is a true OO language, while C++ is C with an object-oriented
extension (C is not an OO language).
0 The following is a list of some major C++ features that were
intentionally omitted from Java or significantly modified. You may
not have covered all of these C++ features in your C++
programming course, but you should be familiar with some of
them.
0 There are, of course, other differences between Java and C++,
which you will discover as we go through the course.
Chapter 2
Comp321: Object Oriented Programming
13
0 The differences are:
...cont
0 Java does not support operator overloading .
0 Java does not have template classes.
0 Java does not directly support multiple inheritance of classes, but it
does allow this to be achieved using a feature called an interface.
0 Java does not support global variables – every variable and method
is declared within a call and forms part of that class (this relates to
the scope of variables.
0 Java does not use pointers (but similar functionality is achieved
using implicit references to objects).
0 Java has replaced the destructor function with a finalize() function.
0 There are no header files in Java (but there is a way to import all
the classes from another package or library).
Chapter 2
Comp321: Object Oriented Programming
14
4.Java Development Tools
0 To get started with writing Java programs, you need to have the
Java SDK installed on your PC.
0 The current version of the Java itself is Java 2; the current version
of the SDK is 6.9. The latest version of the SDK can be downloaded
for free from the Sun web site (http://java.sun.com).
0 The SDK provides various tools for working with Java code, as well
as the Java VM and the API.
0 Some of the tools are:
0 Javac (Java compiler)
0 Java (Java interpreter i.e. to run a Java program)
0 Javadoc (for creating HTML-format documentation from Java source
code)
0 Jdb (Java Debugger)
Chapter 2
Comp321: Object Oriented Programming
15
...cont
0 The Java API consists of many built-in classes and methods that
you can use in your code. This reflects the fact that the Java
technology is built using Java itself.
0 Some of the most commonly used packages are the following:
0 Language support package (java.lang) – classes required for
0
0
0
0
implementing basic features of Java.
Utilities package (java.util) – classes that provide various utility
functions such as date and time functions.
Input/Output package (java.io) – classes required for manipulation of
input/output to/from programs
Networking Package (java.net) – classes for communicating with other
programs/PCs over networks and internet
Applet Package (java.applet) – classes that are used to create Java
applets.
Chapter 2
Comp321: Object Oriented Programming
16
5.The first simple Program
1. /* This is a simple Java program. */
2. class Example {
3.
// Your program begins with a call to main().
4. public static void main(String args[]) {
5.
System.out.println(“Hello comps!");
6.
}
7. }
0 When the program is run, the following output is displayed:
Hello comps!
Chapter 2
Comp321: Object Oriented Programming
17
…cont
0 Line 1: is multiple line comment which is like in C++.
0
0
0
0
0
0
/*multiple line comment*/
Line 2: class Example {
This line uses the keyword class to declare that a new class is being
defined.
Example is an identifier that is the name of the class. The entire class
definition, including all of its members, will be between the opening
curly brace ({) and the closing curly brace (}).
The use of the curly braces in Java is identical to the way they are used
in C, C++, and C#.
For the moment, don’t worry too much about the details of a class
except to note that in Java, all program activity occurs within one. This
is one reason why all Java programs are (at least a little bit) objectoriented.
Line 3: is the single-line comment which is like in C++.
// single line comment
Chapter 2
Comp321: Object Oriented Programming
18
…cont
0 Line 4: public static void main(String args[]) {
0 This line begins the main( ) method. As the comment preceding it
suggests, this is the line at which the program will begin executing.
All Java applications begin execution by calling main( ). (This is
just like C/C++.)
0 The public keyword is an access specifier, which allows the
programmer to control the visibility of class members. When a
class member is preceded by public, then that member may be
accessed by code outside the class in which it is declared. (The
opposite of public is private, which prevents a member from
being used by code defined outside of its class.)
0 In this case, main( ) must be declared as public, since it must be
called by code outside of its class when the program is started.
Chapter 2
Comp321: Object Oriented Programming
19
…cont
0 The keyword static allows main( ) to be called without having to
instantiate a particular instance of the class. This is necessary since
main( ) is called by the Java interpreter before any objects are made.
0 The keyword void simply tells the compiler that main() does not
return a value.
0 In main( ), there is only one parameter, albeit a complicated one.
String args[ ] declares a parameter named args, which is an array of
instances of the class String. (Arrays are collections of similar objects.)
Objects of type String store character strings.
0 In this case, args receives any command-line arguments present when
the program is executed. This program does not make use of this
information, but other programs which we’ll see later will do.
Chapter 2
Comp321: Object Oriented Programming
20
0 Line:5
…cont
System.out.println(“Hello Comps!");
0 This line outputs the string “Hello Comps!” followed by a new line
on the screen.
0 Output is actually accomplished by the built-in println() method.
In this case, println( ) displays the string which is passed to it.
0 As you will see, println( ) can be used to display other types of
information, too.
0 The line begins with System.out. - System is a predefined class
that provides access to the system, and out is the output stream
that is connected to the console
Chapter 2
Comp321: Object Oriented Programming
21
6.Structure of a Class
0 All Java code is written inside classes.
0 The class definition is the first part of the code that appears. This
consists of the access modifier for the class (public or private), the
class keyword and the name of the class.
0 By convention, class names begin with an upper case letter.For
example, to define a class called Circle :
public class Circle
{
}
Chapter 2
Comp321: Object Oriented Programming
22
…cont
0 Inside the class, the code can be divided into fields, constructors
and methods.
0 The fields are the data members of the class. The data members
can be class variables or instance variables.
0 The constructors are special methods that are called when an
object is instantiated from the class. There can be more than one
constructor for a class – as long as each constructor has a different
parameter list. For example, in a Circle class, the radius for a new
object could be passed to the constructor, or if no radius is passed,
a default value could be assigned for the radius. In fact, this also
applies to methods. This feature of Java is called method
overloading.
Chapter 2
Comp321: Object Oriented Programming
23
…cont
0 The methods implement the behaviour of the objects belonging to
the class. Generally, methods return information about the state of
an object or they change the state of an object. These are
sometimes called accessor and mutator methods.
0 The order of these parts of a class is generally not important, but
placing the fields and then the constructors at the beginning does
make the class readable and easy to get around for programmers.
Of course, comment blocks should also be used to document the
code.
Chapter 2
Comp321: Object Oriented Programming
24
public class Circle
{
…cont (Eg)
//data
private double radius;
//constructors
public void Circle ()
{ radius = 1.0;}
public void Circle (double r)
{ radius = r; }
//methods
public double calcArea()
{
return 3.14* radius * radius;
}
}
public void draw()
{
……
}Chapter 2
Comp321: Object Oriented Programming
25
7.1.Variables Declaration
0 Remember that a variable is like a storage location to store a data value
0
0
0
0
and that a variable can take different values at different times during the
execution of a program.
Some Java variable declarations include:
int x;
FirstJava myObject = new FirstJava ();
int sum = 0;
A variable must be given an explicit name and data type.
The name must be a legal identifier (an identifier is simply a name given
to an item used in a Java program). A legal identifier begins with a letter
and can consist of alpha-numeric characters (letters and digits) and can
also include the underscore (_) and dollar ($) characters. Identifiers are
case-sensitive and cannot have spaces in them.
Some data types have default values that are used for initialisation if a
variable is not explicitly initialised in code. For example, the integer data
26
Comp321: Object Oriented Programming
type has a default value of 0.
7.1.Variables Declaration(cont)
0 A convention used in Java programming is that variable names begin
0
0
0
0
0
0
with a lowercase letter while class names begin with an uppercase letter.
If a variable or class name has more than one word in it, the words are
joined together and the first letter of each word after the first will begin
with a capital letter. This convention makes code easier to read and
understand.
A variable declaration is used to give a variable a name and a data type –
the format is to put the type followed by the name i.e. type name
A variable must be initialised before it is used (the compiler will not
allow an uninitialized variable to be used). Initialisation simply means
assigning some initial value to a variable e.g.
int i ;
i = 0; //to initialise an integer variable to the value 0.
However, declaration and initialisation are often combined into one
statement e.g.
Comp321: Object Oriented Programming
27
int i = 0;
7.2.Variables Scope
0 A variable has scope.
0 Scope refers to the parts of the code in which the variable name
can be used without prefixing it with the class or object name. The
scope is determined by where in the class definition a variable is
declared.
0 Generally, Java variables have either class scope or local scope.
0 Static variables or fields , which is another way of saying 'class
variable’ have class scope – because they can be referenced within
the class and its subclasses without prefixing with the class name.
0 Instance variables are variables that belong to instances (objects)
of a class. These also have class scope, because they must be
prefixed with the instance name when used outside the class or its
subclasses.
Chapter 2
Comp321: Object Oriented Programming
28
7.2.Variables Scope(cont)
0 The difference between static, or class, variables and instance
variables is that an instance variable can take different values for
each object while a class variable has the same value for all objects
of that class.
0 Class and instance variables are declared at the class level – not
within methods. But they can be used within methods in the class,
without prefixing with the class or instance name.
0 Variables that are declared within a method or within a block of
code have local or lexical scope, that is, they can be used only within
the enclosing block of statements or method. For example, in the
following block of code, the variable i is out of scope in the line that
begins 'System.out.println' because it was declared within the
block of statements following the if statement, so its scope is that
block. Thus the last line will not compile:
Chapter 2
Comp321: Object Oriented Programming
29
7.2.Variables Scope(cont)
0 if (...) { int i = 17; ...}
System.out.println("The value of i = " + i); // error
0 The scope of parameters to a method or cannot be used outside the
method or constructor.
0 Local variables do not have the public or private modifiers in front
of them. The lifetime of a local variable is only the time of the
method execution – they are created when a method is called and
destroyed when the method finishes.
To summarise:
0 Class variables are used to store data that is common to all objects
of the class.
0 Instance variables are used to store data that persists through the
lifetime of an object.
0 Local variables are often used as temporary storage locations
while a method or constructor completes its task.
Chapter 2
Comp321: Object Oriented Programming
30
7.3.Variables Constants
0 In Java, a variable declaration can begin with the final keyword.
This means that once an initial value is specified for the variable,
that value is never allowed to change.
0 This is the equivalent of a constant in C++ or other languages.
0 For example, to declare a constant for the value of the maximum
number of students allowed on a course:
final int MAX_STUDENTS = 100;
0 The variable can be initialised after the declaration – but if the
declaration includes the final modifier, the initial value is the last
value that can be assigned.
final int MAX_STUDENTS;
MAX_STUDENTS = 100;
0 Note that the convention in Java programming is to use all
uppercase letters for constant names, so that they stand out in the
code. Underscores are used to separate the words if there is more
than one word in the constant's name.
31
Chapter 2
Comp321: Object Oriented Programming
8.Data Types
0 Every variable must have a data type – the data type
determines the values that a variable can contain and also
what operations can be performed on it.
0 Java has two types of data type –
0 primitive and
0 reference.
Chapter 2
Comp321: Object Oriented Programming
32
8.1.Primitive Data Type
0 A variable of primitive type contains a single value of the
appropriate size and format for its type: a number, a character, or
a boolean value. For example, an integer value is 32 bits of data in
a format known as two's complement, the value of a char is 16 bits
of data formatted as a Unicode character, and so on.
0 A variable of reference type has as its value an address, or a
pointer to the values or set of values that the variable represents.
Classes, interfaces and arrays are reference types.
0 The primitive types can be divided into numeric and nonnumeric types.
0 Numeric types can further be divided into integer and real
number (or floating-point) types. Integer types hold whole
numbers, positive and negative. Real number types can hold
decimal values e.g. 1.234, -6.754.
Chapter 2
Comp321: Object Oriented Programming
33
Numeric data types in Java
Integer types
Real number/floatingpoint types
Chapter 2
Type
Description
Storage Size
byte
Byte-length integer; -128 to 127
8 bits (1 byte)
short
Short integer; -32768 to 32767
16 bits (2 bytes)
int
Integer; -231 to (231 –1)
32 bits (4 bytes)
long
Long integer; -263 to +(263-1)
64 bits (8 bytes)
float
IEEE 754 floating point;
+/- 1.4E-45 to +/- 3.4028235E+38
32 bits
double
IEEE 754 floating point;
64 bits
+/- 4.9E-324 to +/- 1.7976931348623157E+308
Comp321: Object Oriented Programming
34
…cont
0 The bigger storage size types take up more memory (i.e. short takes more
0
0
0
0
0
0
memory than byte). So, you should select a data type appropriate to the
values that will be held in a variable e.g. if the values will be in the range –
30000 to +30000, use short rather than int. All Java number values are
signed – which means they can be positive or negative.
Non-numeric data types are boolean and character.
The boolean type has only two possible values, representing the two
Boolean states – true and false. Java reserves the words true and false for
these values.
Comparison operators (e.g. >, <, = =) all return boolean type values.
Unlike C/C++, boolean values in Java cannot be converted to or from other
data types e.g. 0 and 1 cannot be converted to true and false.
The character type, called char in Java, represents any Unicode character.
The char type takes up 16 bits (2 bytes) in memory and holds only a single
character.
Java provides some built-in classes, such as String and StringBuffer, for
35
working with strings of char values.
Comp321: Object Oriented Programming
…cont
0 Literal primitive values can be used directly in code e.g.
char myChar = 'A'; //to assign the character A to a char variable
int myInt = 5; // to assign the value 5 to an integer variable
0 The declarations above also show how a value can be assigned to a
variable as part of the variable declaration e.g. the myChar variable
is initially given the value of 'A'.
0 For the char type, there are escape sequences for special characters
e.g. \b for a backspace, \n for a newline, \t for a tab, \\ for a
backslash, \', \“.
0 Generally speaking, a series of digits with no decimal point is typed
as an integer. A long integer can be specified by putting an 'L' or 'l'
after the number. 'L' is preferred as it cannot be confused with the
digit '1'.
0 A series of digits with a decimal point is of type double (e.g. 34.543).
0 But you can specify that the number is a float by putting an 'f' or 'F'
after the number (e.g. 34.543f).
Chapter 2
Comp321: Object Oriented Programming
36
…cont
0 A literal character value is any single Unicode character b/n single
quote marks. The two boolean literals are simply true and false.
0 Java has wrapper classes for each of the integer types – these are
Byte, Short, Integer and Long. These classes define MIN_VALUE and
MAX_VALUE constants that describe the range of each type. They
also have useful static methods that can be used to convert strings
to integer values e.g. Integer.parseInt() to convert a string to an
integer. The exercise below demos these wrapper classes.
0 The primitive types have default values – so if a variable is not
explicitly initialised, it is automatically initialised to its
corresponding default value. These are 0 for int, 0.0 for floatingpoint values, and an empty string for char.
Chapter 2
Comp321: Object Oriented Programming
37
// display them all
System.out.println("The largest byte
value is" + largestByte);
System.out.println("The largest short
value is " + largestShort);
System.out.println("The largest integer
value is " + largestInteger);
System.out.println("The largest long
// integers
value is " + largestLong);
byte largestByte = Byte.MAX_VALUE;
System.out.println("The largest float
short largestShort = Short.MAX_VALUE; value is " + largestFloat);
int largestInteger = integer.MAX_VALUE;
System.out.println("The largest double
value is " + largestDouble);
long largestLong = Long.MAX_VALUE;
* Uses the wrapper classes to get the min
and max values for each number type.
*/
public class MaxVariablesDemo {
public static void main(String args[]) {
if (Character.isUpperCase(aChar)) {
System.out.println("The character "
+ aChar + " is upper case.");
} else {
System.out.println("The character "
+ aChar + " is lower case.");
}
System.out.println("The value of
aBoolean is " + aBoolean);
} Programming
Comp321: Object Oriented
38
}
// real numbers
float largestFloat = Float.MAX_VALUE;
double
largestDouble=Double.MAX_VALUE;
// other primitive types
char aChar = 'S';
boolean aBoolean = true;
Chapter 2
0 The output will be
The largest byte value is127
The largest short value is 32767
The largest short value is 2147483647
The largest float value is 9223372036854775807
The largest float value is 3.4028235E38
The largest float value is 1.7976931348623157E308
The character S is upper case.
The value of aBoolean is true
Chapter 2
Comp321: Object Oriented Programming
39
8.2.Reference Data Types
0 Arrays and classes are reference data types in Java.
0 While a variable that has a primitive data type holds exactly one
value, a variable that has a reference data type is a reference to the
value or set of values represented by the variable.
Classes
0 When a class is defined, it can then be used as a data type. The
variable declaration for a reference data type is the same as for a
primitive data type. For example, if a program has a class named
Customer, a new variable, myCustomer1, to hold an object of type
Customer can be declared like this:
Customer myCustomer1;
Chapter 2
Comp321: Object Oriented Programming
40
Cont…
0 However, to assign a reference to the variable, the new keyword
must be used – new creates a new object from the specified class.
The class name is followed by parentheses in which any
arguments required by the class constructor are passed e.g.
Customer myCustomer1 = new Customer();
0 The above line creates a new object from the Customer class; the
constructor for the Customer class in this case does not take any
arguments.
0 This is the same as the following lines – where the object is
created by a separate assignment:
Customer myCustomer1;
MyCustomer1 = new Customer();
Chapter 2
Comp321: Object Oriented Programming
41
Arrays
0 An array is also a reference type, a structure that can hold multiple values –
0
0
0
0
but all the values must be of the same type. That type can be a primitive
data type or an object type, or it can be other arrays.
In Java, an array is actually an object – but one that has specialised syntax
and behaviour.
An array is declared by specifying the data type of the values it will hold,
followed by [] to indicate that it is an array. For example:
byte[] arrayOfBytes; //array of values of type byte
byte[][] arrayOfArrayOfBytes ; //an array of arrays of byte[] type
Customer[] arrayOfCustomers; //array of objects of the class
//Customer
The C++ syntax for declaring a variable of array type is also supported by
Java i.e.
Byte arrayOfBytes[]; //array of values of type byte
However, this syntax can be confusing, so it should be avoided in Java.
Chapter 2
Comp321: Object Oriented Programming
42
Arrays(cont)
0 In the above examples, we can say that byte[] and Customer[] are types.
0 After an array has been declared, it must be created. Because an array is
actually an object in Java, the new keyword must be used to create the
array (new is used to create any object). The size of the array i.e. how
many elements it can hold must be specified. For example, to declare an
array called 'arrayOfBytes' that will hold 1024 bytes and to create it:
byte[] arrayOfBytes = new byte[1024];
0 To declare an array of 50 strings:
String[] lines = new String[50];
0 When the array is created in this way, each of the values in the array is
initialised to the corresponding default value for the data type of the
value (the default value for an object type is null, meaning the object has
an empty reference).
0 In Java, arrays have a 0-based index, that is, the first element is at index 0.
The elements are accessed by the array name followed by the index in
square
Chapter 2brackets.
Comp321: Object Oriented Programming
43
Arrays(cont)
0 For example, to declare an array named 'responses', with two
elements whose values are 'Yes' and 'No':
String[] responses = new String[2];
responses[0] = "Yes";
responses[1] = "No";
//to read the elements
System.out.println ("The answer should be " + responses[0] + " or "
+ responses[1] + "!");
0 If the size of an array is n, then the highest index for it is (n-1) e.g.
an array of size 5 holds 5 elements, with the last one being
accessed at index [4].
0 If the code tries to read an element past the end of the array e.g. to
read index [5] in an array of size 5, the interpreter throws an
ArrayIndexOutOfBoundsException at runtime.
Chapter 2
Comp321: Object Oriented Programming
44
Arrays(cont)
0 The size, or length, of an array can be accessed in code by reading the
0
0
0
0
value of the length property of the array object e.g.
int sizeOfResponsesArray = responses.length;
This is commonly used to loop through all the values in an array e.g.
int[] valuesArray; //assume this array is created and initialised
//somewhere else
int totalValue = 0; //store the sum of the array elements
for (int i = 0; i < valuesArray.length; i++)
totalValue += valuesArray [i];
The above example creates an array of values and then uses a for loop to
iterate through the values and add them up. As the index for the array is
0-based, the last element is at the index [array.length-1].
Throwing an exception is part of the runtime error handling of Java. We
will look at how to properly handle exceptions later in the course.
The for syntax includes the initialisation, test and update steps for the
loop – initialise the counter to 0, loop until i reaches the size of the array,
increment i by 1 for each iteration.
Chapter 2
Comp321: Object Oriented Programming
45
Assigning Values to Arrays
0 The null literal can be used to indicate that an object that is of a
reference data type does not yet exist i.e. to initialise an object to
be empty or null. For example:
char[] password = null; //sets the password array of
//characters to be null
0 Literal values can also be used to specify the values of an array, as
in the String array example given above:
String[] responses = new String[2];
responses[0] = "Yes";
responses[1] = "No";
0 An array can also be initialised by the following syntax, where the
array object is created and the elements initialised in one
statement. The element values are separated by commas.
int[] someNumbers = {1, 2, 3, 4}
Chapter 2
Comp321: Object Oriented Programming
46
Assigning Values to Arrays(cont)
0 The new keyword is not used – but the object is implicitly created.
0 The length of this array is now 4 – again, this is implicit from the
specified elements.
0 The statement above could also be written as:
int[] someNumbers = new int[4];
someNumbers[0] = 1;
someNumbers[1] = 2;
someNumbers[2] = 3;
someNumbers[3] = 4;
0 In fact, the Java compiler compiles the single statement into Java byte
codes that are equivalent to the above. The implication of this is that if
your program needs to include a large amount of data into an array, it
may not be most efficient to include the data literally in the array, as the
examples above do. This is because the compiler has to create a lot of
Java byte codes to initialise the array, and then the interpreter has to
execute all that code.
0 It may be more efficient to store the data in an external file and read47it
Comp321: Object Oriented Programming
into the program at runtime.
9.Operators
0 This section covers arithmetic, comparison, conditional and
assignment operators.
0 Java also has a group of operators called shift or bitwise
operators – these operate on the individual bits (1s and 0s) of
integer values. We will not cover these at this point.
Chapter 2
Comp321: Object Oriented Programming
48
9.1. Arithmetic
0 Java supports the standard arithmetic operators for all integer
and floating-point numbers. These are:
0 + (addition), - (subtraction), * (multiplication), / (division), %
(modulo).
0 All of these are binary operators i.e. they take two operands and
the operator appears between the two operands (this is called
infix notation), as follows:
Operand1 operator operand2
0 The subtraction operator can also be used as a unary operator
– when it is placed in front of a number e.g. –5. When used in
this way, it effectively multiplies the single operand by –1.
Chapter 2
Comp321: Object Oriented Programming
49
9.1. Arithmetic(cont)
0 The addition operator can also be used to concatenate two
strings. If either one of the two operands is a string, then the
other one is converted to a string and the strings are
concatenated. Try the following code in a main method to see
the addition operator operating on numbers and on strings.
System.out.println (3+4); //prints out 7
System.out.println ("The value is: " + 4); //prints out 'The
value is: 4'
System.out.println ("The value is: " + 3 + 4); //prints out 'The
value is: 34', because there is at least one string operand
System.out.println ("The value is: " + (3+4)); //prints out 'The
value is 7' – because the parentheses indicate that the sum of
3+4 is evaluated first, resulting in the value 7
Chapter 2
Comp321: Object Oriented Programming
50
9.1. Arithmetic(cont)
0 The division operator divides the first operand by the second e.g.
12/4 evaluates to 3.
0 If both operands are integers, the result is an integer and any
remainder is lost. If you want to do division and get the remainder,
at least one of the operands should be typed as a floating-point
number – the result will then be a floating-point.
0 When an integer and a floating-point number are used as operands
to any arithmetic operator, the result is a floating-point number.
This is because the integer is implicitly converted to a floatingpoint value before the operation takes place.
0 The table below summarises the data types for the results of
arithmetic operations, based on what the data types of the
operands are.
Chapter 2
Comp321: Object Oriented Programming
51
9.1. Arithmetic(cont)
Data Type of Result
Data Type of Operands
long
Neither operand is a float or a double (integer
arithmetic); at least one operand is a long.
int
Neither operand is a float or a double (integer
arithmetic); neither operand is a long.
double
At least one operand is a double.
Float
At least one operand is a float; neither operand is
a double.
Chapter 2
Comp321: Object Oriented Programming
52
9.1. Arithmetic(cont)
0 There are also shortcut arithmetic operators, to easily increment
and decrement a value by 1. These are + + and - -. Where the
operator is placed relative to the operand i.e. before or after it
affects the behaviour of the operator.
0 When the operator is placed before the operand, it increments or
decrements the operand and the operation evaluates to the
incremented value of the operand. This is called a pre-increment
operator.
0 When the operator is placed after the operand, it increments or
decrements the operand but the expression evaluates to the value
of the operand before it was incremented/decremented. This is
called a post-increment operator.
Chapter 2
Comp321: Object Oriented Programming
53
9.1. Arithmetic(cont)
0 Consider the code sample below:
int i,j;
i = 1;
j = ++i; //pre-increment - sets j to 2 and i to 2
i = 1;
j = i++; //post-increment sets j to 1 and i to 2
0 These operators are useful as shortcuts for incrementing or
decrementing number values e.g. x++, x-- and are commonly used
to increment/decrement the counter that controls a loop e.g. a for
loop that loops 10 times:
for (int i=0; i<=10; i++;)
{
……
}
Chapter 2
Comp321: Object Oriented Programming
54
9.2.Comparision and Conditional
The comparison operators in Java are much like those in other
languages. These are summarised in the table below. Comparison
operators return a result of data type boolean i.e. true or false.
Operator
Usage
Returns true if…
>
op1 > op2
op1 is greater than op2
>=
op1 >= op2
op1 is greater than or equal to op2
<
op1 < op2
op1 is less than op2
<=
op1 <= op2
op1 is less than or equal to op2
==
op1 == op2
op1 and op2 are equal
!=
op1 != op2
op1 and op2 are not equal
Chapter 2
Comp321: Object Oriented Programming
55
9.2.Comparision and Conditional(cont)
Java also has conditional (logical) operators. These can be used to
combine multiple comparison expressions into a single, more
complex expression. The operands for a conditional operator must
be boolean and the result is also a boolean.
Operator
Name
Usage
&&
Conditional AND
||
Conditional OR
!
Boolean NOT
&
Boolean AND
|
Boolean OR
^
Boolean XOR
op1 & op2
Returns true if op1 and op2 are both true – only evaluates op2 if
op1 is true
op1 | | op2
returns true if op1 or op2 is true – only evaluates op2 if op1 is
false
!op
Returns true if op is false i.e. changes the boolean value of the
operand
op1 & op2
Like && but always evaluates both operands
op1 | op2
Like || but always evaluates both operands, even if the first one is
true
op1 ^op2
56
Exclusive OR – returns true only if exactly one of the operands
is
true. Returns false if both are true or both are false
Chapter 2
9.2.Comparision and Conditional(cont)
0 Note the difference between && and &, and between | | and |.
0 If the right-hand operand in a && or | | operation carries out some
action, such as reading in some input or updating a value,
remember that if the operand is not evaluated, the action will not
be carried out.
0 For example, suppose there is an object named customer1 which
has a method increaseBalance() which returns a Boolean – true if
the increase is successful and false if it is not. Take a statement that
checks for the value of a variable (beingInt) being greater than a
given value AND the increaseBalance() method being successfully
invoked. The statement can be written as follows, using a
conditional AND:
Chapter 2
Comp321: Object Oriented Programming
57
9.2.Comparision and Conditional(cont)
someInt int = 8
depositAmt int;
depositAmt = 100;
….
if (someInt >= 10 && customer1.increaseBalance(depositAmt) )
{
……
}
0 In this case, the value of someInt is 8, which is less than 10, so the
expression 'someInt>=10' is false. This means that the second
expression, customer1.increaseBalance() will not be evaluated. So,
the balance will not be increased. This could be a problem, if it is
necessary to ensure that the balance is increased. In that case, a
boolean AND operator (&) should be used – as it will evaluate both
expressions, even if the first one is false.
Chapter 2
Comp321: Object Oriented Programming
58
9.3. Assignment
0 The basic assignment operator is = e.g.
int x = 5; //assigns the value of 5 to the integer variable x
0 There are also several short-hand assignment operators. These
are used to perform some operation on an operand and to assign
the result to a variable, all in one go.
0 For example:
int i;
i = 2;
i = i + 3;
0 //the following statement is the same as the above two
statements
i += 2;
0 Any of the arithmetic operators can be used in this way i.e.
+=, -=, *=, /=, %=.
Chapter 2
Comp321: Object Oriented Programming
59
9.4.
Others
instanceof
0 The instanceof operator is a special operator that is used to check
if a given object is an instance of a particular class. The left
operand is the object being tested and the right operand is the
class to check for. It returns true if the object is an instance of the
class and false if not. For example:
person instanceof student //returns true if the object
//person belongs to the class Student
0 "a string" instanceof String //returns true because all strings are
instances of the String class
0 "a string" instanceof Object //returns true because Strings are
also instances of the Object superclass
0 null instanceof String //returns false because null is never an
instance of anything .
Chapter 2
Comp321: Object Oriented Programming
60
9.4. Others(cont)
Object Member Access (.)
0 The dot (.) operator is used to access the data and methods of an object.
The data fields and methods of an object are also known as members of
the object.
0 For example:
Person aPerson = new Person();
…
String theName = new String();
theName = aPerson.name //evaluates to the value of the name data field
of the Person object
Method Invocation (( ))
0 A method can be accessed using the dot operator, and it is invoked by
using the ( ) operator after the method name. Any arguments or
parameters to the method are placed inside the brackets. For example:
Person aPerson = new Person();
…
aPerson.increaseSalary(100); //invokes the method increaseSalary, 61
passing 100 as the paramter)
9.4. Others(cont)
Object Creation (new)
0 As already seen, new is used to create a new object or array. The new
keyword is followed by the class name and a list of arguments to be
passed to the object constructor. The arguments are placed inside
brackets. For example, if the constructor for the Person class takes
the name and father's name as arguments:
Person aPerson = new Person("Firstname", "Fathersname");
Chapter 2
Comp321: Object Oriented Programming
62
10. Type Conversions and Casting
0 Java carries out implicit conversions between number types. For
example, if a short literal value is assigned to an int data type, Java
automatically converts the value to int:
int x = 32767; //32767 is a literal value of type short, but is
converted to int
0 This is called a widening conversion because the value is being
converted to a type that has a wider range of legal values.
0 A narrowing conversion occurs when a value is converted to a
type that is not wider than it. Sometimes this type of conversion is
not safe e.g. it is safe to convert the integer value 13 to a byte, but
not to convert 13000 to a byte, because the byte type can only hold
numbers between –128 and 127.
0 Because of this, the Java compiler produces a compile error when
the code attempts a narrowing conversion. This happens even if
the actual value being converted would fit into the narrower range.
Chapter 2
Comp321: Object Oriented Programming
63
10. Type Conversions and Casting(cont)
0 For example:
0
0
0
0
int i = 13;
byte b = i; //not allowed by the compiler because byte is a narrower type
(even though 13 is an allowed value for byte)
There is, however, one exception to this rule: an integer literal (an int
value) can be assigned to a byte or a short variable, but only if the literal
falls inside the range of the byte/short variable. So this line would be
allowed:
byte b = 13; //allowed because 13 is an integer literal
The error produced by the compiler for a narrowing conversion includes
the message 'possible loss of precision‘.
The above are examples of implicit conversion – Java carries out the
conversion automatically, if it is ok to do so. However, a conversion from
one type to another can be forced using a cast. This can be used when a
narrowing conversion would occur, and if the programmer knows that
data will not be lost.
A cast is performed by putting the type to convert to in parentheses
before
value to be converted.
Chapterthe
2
Comp321: Object Oriented Programming
64
10. Type Conversions and Casting(cont)
0 Taking the example above, there the narrowing conversion of the
int value to a byte is not allowed by the compiler, this can be forced
using a cast as follows:
int i = 13;
byte b = (byte) i; //force the int value 13 to be converted to a byte
0 A cast is often used to convert a floating-point value to an integer.
When this occurs, the fractional part of the floating-point value is
truncated to leave an integer.
0 For example:
int i;
i = (int) 13.456; //forces the double literal to the int value 13
0 Casting can also be used with reference data types, but there are
some restrictions.
0 As with primitive types, the Java interpreter automatically carries
outChapter
widening
conversions.
Narrowing conversions must be made
2
Comp321: Object Oriented Programming
65
explicit using a cast.
10. Type Conversions and Casting(cont)
0 When converting reference data types, the following rules apply:
 An object can be converted to the type of its superclass, or any ancestor
class in its class hierarchy. This is a widening conversion.
 An object can be converted to the type of its own subclass. This is a
narrowing conversion, so it requires a cast.
 An object cannot be converted to an unrelated type i.e. to a class that the
object is not a subclass or superclass of.
 All Java classes automatically inherit from the Object superclass (this
gives them special methods such as toString(), clone() and equals()).
0 For example, the String class is a subclass of Object. So a string value can
be assigned to a variable of type Object. If assigning the value to a String
variable, a cast is required.
Object o = " a string"; //a widening conversion
//later in the code, the value of o can be cast back to a String
String
conversion,
Chapters2 = (String) o; //a narrowing
Comp321: Object Oriented
Programming so requires a cast
66
11. Mathematical Functions
0 In Java, mathematical functions such as square root and trigonometric
functions are implemented as methods on the Math class (equivalent of
the C <math.h> functions). This class is part of the built-in java.lang
package – because it is a built-in class, it does not have to be imported to
use it. The Math class also provides constants for the mathematical
values PI and E.
0 For example:
double x,y;
x = 9;
y= Math.sqrt(x); //computes the square root of x
//use PI to compute the area of a circle
double radius;
radius = 6;
double circleArea = Math.PI * radius * radius;
Chapter
2
Comp321:
Oriented Programming
67
0 Some
useful
Math methods
andObject
constants
are listed in the table below.
Note: x and y are parameters of type double; a and b can be of type int, long, float or double
Method
Description
Method
Description
sin (x)
Trigonometry Sin
Log (x)
Returns the natural logarithm of x
cos (x)
Trigonometry Cos
sqrt (x)
Returns the square root of x
tan (x)
Trigonometry Tan
ceil (x)
asin (x)
Trigonometry aSin
floor (x)
acos (x)
Trigonometry aCos
rint (x)
Returns the smallest whole number
greater than or equal to x (rounding
up)
Returns the greatest whole number
less than or equal to x (rounding
down)
Truncated value of x
atan (x)
Trigonometry aTan
abs (a)
Returns the absolute value of a
pow (x, y)
Returns x to the power of y (xy)
exp (x)
Returns e to the power of x (ex)
max (a,b) Returns the maximum value of a
and b
min (a,b) Returns the minimum value of a
and b
Constant
Value & Description
PI
E
Double, 3.141592653589793 (access
using Math.PI)
Double, 2.718281828459045 (access
Chapter 2 using Math.E)
Comp321: Object Oriented Programming
68
Questions?
End of chapter 2
Chapter 2
Comp321: Object Oriented Programming
69