IntroductionJava Programming - Math Class

Standard Classes and Methods:
The Math Class: Java Math class provides several
methods to work on math calculations
Fundamentals of Java 2
Table 4-1: Seven methods in the Math class

Standard Classes and Methods: The
Math Class (cont.)
• Using sqrt() method example:

Math Class (cont.)
 Math class methods example:

Random Class
• Random number generator: Returns numbers
chosen at random from a pre-designated interval
Table 4-2: Methods in the Random class

Defining Methods
8
A method is a collection of statements that are
grouped together to perform an operation.
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Define a method Invoke a method
int z = max(x, y);
actual parameters
(arguments)

Defining Methods
9
A method is a collection of statements that are
grouped together to perform an operation.
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
int z = max(x, y);
actual parameters
(arguments)
method
signature

Method Signature
10
Method signature is the combination of the method name and the
parameter list.
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
int z = max(x, y);
actual parameters
(arguments)
method
signature

Formal Parameters
11
The variables defined in the method header are known as
formal parameters.
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
int z = max(x, y);
actual parameters
(arguments)
method
signature

Actual Parameters
12
When a method is invoked, you pass a value to the parameter. This
value is referred to as actual parameter or argument.
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
int z = max(x, y);
actual parameters
(arguments)
method
signature

Return Value Type
13
A method may return a value. The returnValueType is the data type
of the value the method returns. If the method does not return a
value, the returnValueType is the keyword void. For example, the
returnValueType in the main method is void.
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
modifier
return value
type
method
name
formal
parameters
return value
method
body
method
header
parameter list
int z = max(x, y);
actual parameters
(arguments)
method
signature

Calling Methods
14
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
pass the value of i
pass the value of j

Reuse Methods from Other Classes
NOTE: One of the benefits of methods is for reuse. The max
method can be invoked from any class besides TestMax. If
you create a new class Test, you can invoke the static method
max using ClassName.methodName (e.g., TestMax.max).
15

Pass by Value
18
When you invoke a method with an
argument, the value of the argument is
passed to the parameter. This is referred to
as pass-by-value. If the argument is a
variable rather than a literal value, the value
of the variable is passed to the parameter.
The variable is not affected, regardless of
the changes made to the parameter inside
the method

Overloading Methods
• Overloading methods enables you to define the methods with the
same name as long as their signatures are different.
• The max method that was used earlier works only with the int
data type. But what if you need to determine which of two
floating-point numbers has the maximum value? The solution
is to create another method with the same name but different
parameters, as shown in the following code:
public static double max(double num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
20

Ambiguous Invocation
• The Java compiler determines which method to use
based on the method signature.
• Sometimes there may be two or more possible matches
for an invocation of a method, but the compiler cannot
determine the most specific match. This is referred to as
ambiguous invocation. Ambiguous invocation is a
compile error.
21

Scope of Local Variables
A local variable: a variable defined inside a
method.
Scope: the part of the program where the
variable can be referenced.
The scope of a local variable starts from its
declaration and continues to the end of the
block that contains the variable. A local
variable must be declared before it can be
used.
23

Scope of Local Variables, cont.
You can declare a local variable with the
same name multiple times in different non-
nesting blocks in a method, but you cannot
declare a local variable twice in nested
blocks.
24

A variable declared in the initial action part of a for loop
header has its scope in the entire loop. But a variable
declared inside a for loop body has its scope limited in the
loop body from its declaration and to the end of the block
that contains the variable.
25
public static void method1() {
.
.
for (int i = 1; i < 10; i++) {
.
.
int j;
.
.
.
}
}
The scope of j
The scope of i

26
int x = 1;
int y = 1;
for (int i = 1; i < 10; i++) {
x += i;
}
for (int i = 1; i < 10; i++) {
y += i;
}
}
It is fine to declare i in two
non-nesting blocks
int i = 1;
int sum = 0;
for (int i = 1; i < 10; i++) {
sum += i;
}
}
It is wrong to declare i in
two nesting blocks

Benefits of Methods
27
• Write a method once and reuse it anywhere.
• Information hiding. Hide the implementation
from the user.
• Reduce complexity.

Arrays
• An array is an ordered list of values
28
0 1 2 3 4 5 6 7 8 9
79 87 94 82 67 98 87 81 74 91
An array of size N is indexed from zero to N-1
scores
The entire array
has a single name
Each value has a numeric index
This array holds 10 values that are indexed from 0 to 9

Arrays
• A particular value in an array is referenced using the array name
followed by the index in brackets
• For example, the expression
scores[2]
refers to the value 94 (the 3rd value in the array)
• That expression represents a place to store a single integer and can be
used wherever an integer variable can be used
29

Arrays
• For example, an array element can be assigned a value, printed, or
used in a calculation:
scores[2] = 89;
scores[first] = scores[first] + 2;
mean = (scores[0] + scores[1])/2;
System.out.println ("Top = " + scores[5]);
30

Arrays
• The values held in an array are called array elements
• An array stores multiple values of the same type (the element
type)
• The element type can be a primitive type or an object reference
• Therefore, we can create an array of integers, or an array of
characters, or an array of String objects, etc.
• In Java, the array itself is an object
• Therefore the name of the array is a object reference variable, and
the array itself must be instantiated
31

Declaring Arrays
• The scores array could be declared as follows:
int[] scores = new int[10];
• The type of the variable scores is int[] (an array of integers)
• Note that the type of the array does not specify its size, but each
object of that type has a specific size
• The reference variable scores is set to a new array object that can
hold 10 integers
32

Declaring Arrays
• Some examples of array declarations:
float[] prices = new float[500];
boolean[] flags;
flags = new boolean[20];
char[] codes = new char[1750];
33

Bounds Checking
• Once an array is created, it has a fixed size
• An index used in an array reference must specify a valid element
• That is, the index value must be in bounds (0 to N-1)
• The Java interpreter throws an
ArrayIndexOutOfBoundsException if an array index is out
of bounds
• This is called automatic bounds checking
34

Bounds Checking
• For example, if the array codes can hold 100 values, it can be
indexed using only the numbers 0 to 99
• If count has the value 100, then the following reference will cause
an exception to be thrown:
System.out.println (codes[count]);
• It’s common to introduce off-by-one errors when using arrays
35
for (int index=0; index <= 100; index++)
codes[index] = index*50 + epsilon;
problem

Bounds Checking
• Each array object has a public constant called length that stores the
size of the array
• It is referenced using the array name:
scores.length
• Note that length holds the number of elements, not the largest
index
36

Alternate Array Syntax
• The brackets of the array type can be associated with the element
type or with the name of the array
• Therefore the following declarations are equivalent:
float[] prices;
float prices[];
• The first format generally is more readable
37

Initializer Lists
• An initializer list can be used to instantiate and initialize an array in
one step
• The values are delimited by braces and separated by commas
• Examples:
int[] units = {147, 323, 89, 933, 540,
269, 97, 114, 298, 476};
char[] letterGrades = {'A', 'B', 'C', 'D', ’F'};
38

Initializer Lists
• Note that when an initializer list is used:
• the new operator is not used
• no size value is specified
• The size of the array is determined by the number of items in the
initializer list
• An initializer list can only be used only in the array declaration
39

Arrays as Parameters
• An entire array can be passed as a parameter to a method
• Like any other object, the reference to the array is passed, making the
formal and actual parameters aliases of each other
• Changing an array element within the method changes the original
• An array element can be passed to a method as well, and follows the
parameter passing rules of that element's type
40

Arrays of Objects
• The elements of an array can be object references
• The following declaration reserves space to store 53 references to
String objects
String[] words = new String[53];
• It does NOT create the String objects themselves
• Each object stored in an array must be instantiated separately
41

Command-Line Arguments
• The signature of the main method indicates that it takes an array of
String objects as a parameter
• These values come from command-line arguments that are provided
when the interpreter is invoked
• For example, the following invocation of the interpreter passes an
array of three String objects into main:
> java delhi colcutta jaipur bangalore
• These strings are stored at indexes 0-3 of the parameter
42

Arrays of Objects
• Objects can have arrays as instance variables
• Many useful structures can be created with arrays and objects
• The software designer must determine carefully an organization of
data and objects that makes sense for the situation
43

Two-Dimensional Arrays
• A one-dimensional array stores a list of elements
• A two-dimensional array can be thought of as a table
of elements, with rows and columns
45
one
dimension
two
dimensions

• To be precise, a two-dimensional array in Java is an
array of arrays
• A two-dimensional array is declared by specifying the
size of each dimension separately:
int[][] scores = new int[12][50];
• A two-dimensional array element is referenced using
two index values
value = scores[3][6]
• The array stored in one row or column can be
specified using one index
46

Expression Type Description
scores int[][] 2D array of integers, or
array of integer arrays
scores[5] int[] array of integers
scores[5][12] int integer
47

Multidimensional Arrays
• An array can have many dimensions
• If it has more than one dimension, it is called a multidimensional array
• Each dimension subdivides the previous one into the specified number
of elements
• Each array dimension has its own length constant
• Because each dimension is an array of array references, the arrays
within one dimension can be of different lengths
• these are sometimes called ragged arrays
48

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