Handout 3 - Java Language Basics

8966470.doc

FBE – Computer Science Dept

Mekelle University Faculty of Business & Economics Computer Science Department Comp 262: Internet Programming with Java Handout 3 – Java Language Basics

Reference: You may find it useful to look at the Java tutorial, taken from the Sun Java website, on the intranet – browse to Computers, Course Materials and look under the heading for this course. 1 Overview This handout covers variables, data types and operators in Java. 2 Structure of a Class As we have already seen, all Java code is written inside classes. The structure of a class is as shown below. 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. By convention, class names begin with an upper case letter, and any new words in the name begin with an upper case letter. For example, to define a Circle class: public class Circle { }

Inside the class, the code can be divided into fields, constructors and methods. The fields are the data members of the class. The data members can be class variables or instance variables. 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 and is something that is not possible in C++. 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. 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

Page 1 of 16

8966470.doc

FBE – Computer Science Dept

get around for programmers. Of course, comment blocks should also be used to document the code. public class Circle { //data private double radius; //constructors public Circle () { radius = 1.0;} public Circle (double r) { radius = r; } //methods public double calcArea() { return Math.PI * radius * radius; } public void draw() { … … } }

3 Variables 3.1 Declaration Remember that a variable is like a storage location to store a data value and that a variable can take different values at different times during the execution of a program. Some Java variable declarations we have already seen include: int x; FirstJava myObject = new FirstJava ();

And, from the example above: 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. A convention used in Java programming is that variable names begin 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 and is followed in the examples used in this handout.

Page 2 of 16

8966470.doc

FBE – Computer Science Dept

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 uninitialised variable to be used). Initialisation simply means assigning some initial value to a variable e.g. int i ; i = 0;

to initalise an integer variable to the value 0. However, declaration and initialisation are often combined into one statement e.g. int i = 0;

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 type has a default value of 0. 3.2 Scope A variable also has scope. 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. Generally, Java variables have either class scope or local scope. In Handout 2, we mentioned static fields or variables, which is another way of saying 'class variable'. Such variables have class scope – because they can be referenced within the class and its subclasses without prefixing with the class name. 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. 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. 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. 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: if (...) { int i = 17; ... } System.out.println("The value of i = " + i); // error

Page 3 of 16

8966470.doc

FBE – Computer Science Dept

The scope of parameters to a method or constructor is the entire method or constructor, but the parameters cannot be used outside the method or constructor. 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: • Class variables are used to store data that is common to all objects of the class. • Instance variables are used to store data that persists through the lifetime of an object. • Local variables are often used as temporary storage locations while a method or constructor completes its task. 3.3 Constants 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. This is the equivalent of a constant in C++ or other languages. For example, to declare a constant for the value of the maximum number of students allowed on a course: final int MAX_STUDENTS = 100;

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;

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. 4 Data Types 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. Java has two types of data type – primitive and reference. 4.1 Primitive Data Types 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. 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. We will talk more about these later in the course. The primitive types can be divided into numeric and non-numeric types.

Page 4 of 16

8966470.doc

FBE – Computer Science Dept

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. Integer types

Real number/floatingpoint types

Type byte short int long float

Description Byte-length integer; -128 to 127 Short integer; -32768 to 32767 Integer; -231 to (231 –1) Long integer; -263 to +(263-1) IEEE 7541 floating point; +/- 1.4E-45 to +/- 3.4028235E+38

Storage Size 8 bits (1 byte) 16 bits (2 bytes) 32 bits (4 bytes) 64 bits (8 bytes) 32 bits

double

IEEE 754 floating point; +/- 4.9E-324 to +/1.7976931348623157E+308

64 bits

Table 1 – Numeric data types in Java

The bigger storage size types take up more memory (i.e. short takes more 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. There is no 'unsigned' keyword as in C/C++. The other, 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 working with strings of char values. 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

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'. 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, \', \". 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'. A series of digits with a decimal point is of type double (e.g. 34.543). 1

The real number types adhere to the IEEE 754-1985 standard – this specifies the format of the numbers and the behaviour of the arithmetic of the numbers. Page 5 of 16

8966470.doc

FBE – Computer Science Dept

But you can specify that the number is a float by putting an 'f' or 'F' after the number (e.g. 34.543f). A literal character value is any single Unicode character between single quote marks. The two boolean literals are simply true and false. 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. 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 floating-point values, and an empty string for char. Exercise: try copying or typing the code below into a new .java file named MaxVariablesDemo.java. Compile and run it – it will list the min and max values for the number types and demonstrates the use of the primitive data types. /** * Taken from the Sun Java Tutorial - demo to show the primitive data types. * Uses the wrapper classes to get the min and max values for each number type. */ public class MaxVariablesDemo { public static void main(String args[]) { // integers byte largestByte = short largestShort int largestInteger long largestLong =

Byte.MAX_VALUE; = Short.MAX_VALUE; = Integer.MAX_VALUE; Long.MAX_VALUE;

// real numbers float largestFloat = Float.MAX_VALUE; double largestDouble = Double.MAX_VALUE; // other primitive types char aChar = 'S'; boolean aBoolean = true; // 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 value is " + largestLong); System.out.println("The largest float value is " + largestFloat); Page 6 of 16

8966470.doc

FBE – Computer Science Dept

System.out.println("The largest double value is " + largestDouble); 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); } } Figure 1 – exercise to demonstrate use of the primitive data types

4.2 Reference Data Types Arrays and classes are reference data types in Java. 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 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;

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();

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. This is the same as the following lines – where the object is created by a separate assignment: Customer myCustomer1; MyCustomer1 = new Customer();

Arrays An array is also a reference type, a structure that can hold multiple values – 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: Page 7 of 16

8966470.doc

FBE – Computer Science Dept

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. In the above examples, we can say that byte[] and Customer[] are types. 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];

To declare an array of 50 strings: String[] lines = new String[50];

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). 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 brackets. 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] + "!");

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]. 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 ArrayIndexOutOfBoundsException2 at runtime. The size, or length, of an array can be accessed in code by reading the value of the length property of the array object e.g. int sizeOfResponsesArray = responses.length; 2

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. Page 8 of 16

8966470.doc

FBE – Computer Science Dept

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 the sum of the array elements int totalValue = 0; //store for (int i = 0; i < valuesArray.length; i++) totalValue += values[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]. 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. Assigning Values to Arrays 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

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";

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}

The new keyword is not used – but the object is implicitly created. The length of this array is now 4 – again, this is implicit from the specified elements. 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;

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.

Page 9 of 16

8966470.doc

FBE – Computer Science Dept

It may be more efficient to store the data in an external file and read it into the program at runtime. 5 Operators This section covers arithmetic, comparison, conditional and assignment operators. 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. 5.1 Arithmetic Java supports the standard arithmetic operators for all integer and floating-point numbers. These are: + (addition), - (subtraction), * (multiplication), / (division), % (modulo). 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

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. 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

System.out.println ("The value is: " + (3+4)); //prints out 'The value is 7' – because the parentheses indicate that the

The division operator divides the first operand by the second e.g. 12/4 evaluates to 3. 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. 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 floating-point value before the operation takes place. The table below summarises the data types for the results of arithmetic operations, based on what the data types of the operands are. Data Type of Result

Data Type of Operands Page 10 of 16

8966470.doc

FBE – Computer Science Dept

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.

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. 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. 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. 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

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++;) { …… }

5.2 Comparison 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 op1 > op2

>=

op1 >= op2

< <= == !=

op1 < op2 op1 <= op2 op1 == op2 op1 != op2

Returns true if… op1 is greater than op2 op1 is greater than or equal to op2 op1 is less than op2 op1 is less than or equal to op2 op1 and op2 are equal op1 and op2 are not equal

Page 11 of 16

8966470.doc

FBE – Computer Science Dept

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 Conditional AND

||

Conditional OR

!

Boolean NOT

&

Boolean AND

|

Boolean OR

^

Boolean XOR

Usage 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 Exclusive OR – returns true only if exactly one of the operands is true. Returns false if both are true or both are false

Note the difference between && and &, and between | | and |. 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. 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: someInt int = 8 depositAmt int; depositAmt = 100; …. if (someInt >= 10 && customer1.increaseBalance(depositAmt) ) { …… }

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. Page 12 of 16

8966470.doc

FBE – Computer Science Dept

5.3 Assignment The basic assignment operator is = e.g. int x = 5; //assigns the value of 5 to the integer variable x

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. For example: int I; i = 2; i = i + 3; //the following

statement

is

the

same

as

the

above

two

statements i += 2;

Any of the arithmetic operators can be used in this way i.e. +=, -=, *=, /=, %=. 5.4 Others instanceof 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 "a string" instanceof String //returns true because all strings are instances of the String "a string" instanceof Objectclass //returns true because Strings are also the Object superclass null instances instanceofof String //returns false because null is never an instance of anything

Other language constructs in Java that are sometimes considered as operators are the following. Object Member Access (.) 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. 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 ( ( ) )

Page 13 of 16

8966470.doc

FBE – Computer Science Dept

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, passing 100 as the paramter)

Object Creation (new) 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");

6 Type Conversions and Casting 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

This is called a widening conversion because the value is being converted to a type that has a wider range of legal values. 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. 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. For example: 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. Page 14 of 16

8966470.doc

FBE – Computer Science Dept

A cast is performed by putting the type to convert to in parentheses before the value to be converted. 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

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. For example: int i; i = (int) 13.456; //forces the double literal to the int value 13

Casting can also be used with reference data types, but there are some restrictions. As with primitive types, the Java interpreter automatically carries out widening conversions. Narrowing conversions must be made explicit using a cast. 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()). 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 s = (String) o; //a narrowing conversion, so requires a cast

7 Mathematical Functions 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. For example: double x,y; x = 9; y= Math.sqrt(x); //computes the square root of x Page 15 of 16

8966470.doc

FBE – Computer Science Dept

//use PI to compute the area of a circle double radius; radius = 6; double circleArea = Math.PI * radius * radius;

Some useful Math methods and constants are listed in the table below. Method sin (x) cos (x) tan (x) asin (x) acos (x) atan (x) pow (x, y) exp (x) Log (x) sqrt (x) ceil (x) floor (x) rint (x) abs (a) max (a,b) min (a,b) Constant PI E

Description Trigonometry Sin Trigonometry Cos Trigonometry Tan Trigonometry aSin Trigonometry aCos Trigonometry aTan Returns x to the power of y (xy) Returns e to the power of x (ex) Returns the natural logarithm of x Returns the square root of 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 Returns the absolute value of a Returns the maximum value of a and b Returns the minimum value of a and b Value & Description Double, 3.141592653589793 (access using Math.PI) Double, 2.718281828459045 (access using Math.E)

Note: x and y are parameters of type double; a and b can be of type int, long, float or double Notes prepared by: FBE Computer Science Department.

Page 16 of 16