Mathematical Functions, Characters, and Strings in Java

Introduction to Java Programming and Data Structures

Chapter 4: Mathematical Functions, Characters, and Strings

Mathematical Functions

Java provides many useful methods in the Math class for performing common mathematical functions.
- Definition: A method is a group of statements that performs a specific task.

The Math Class

Class Constants:
- PI
- E
Class Methods:
- Exponent Methods: These methods deal with exponential and logarithmic calculations.
- Rounding Methods: These methods are used for rounding numbers.
- min, max, abs, and random Methods: These methods return minimum or maximum of values, absolute values, and random values, respectively.

Exponent Methods

exp(double a): Returns e raised to the power of a.
log(double a): Returns the natural logarithm of a.
log10(double a): Returns the logarithm of a with base 10.
pow(double a, double b): Returns a raised to the power of b.
sqrt(double a): Returns the square root of a.

Examples of Exponent Methods

Math.exp(1) returns approximately 2.71
Math.log(2.71) returns 1.0
Math.pow(2, 3) returns 8.0
Math.pow(3, 2) returns 9.0
Math.pow(3.5, 2.5) returns approximately 22.91765
Math.sqrt(4) returns 2.0
Math.sqrt(10.5) returns approximately 3.24

Rounding Methods

double ceil(double x): Rounds x up to its nearest integer and returns it as a double value.
double floor(double x): Rounds x down to its nearest integer and returns it as a double value.
double rint(double x): Rounds x to its nearest integer; if x is equally close to two integers, the even one is returned as double.
int round(float x): Returns (int)Math.floor(x + 0.5).
long round(double x): Returns (long)Math.floor(x + 0.5).

Examples of Rounding Methods

Math.ceil(2.1) returns 3.0
Math.ceil(2.0) returns 2.0
Math.ceil(-2.0) returns -2.0
Math.ceil(-2.1) returns -2.0
Math.floor(2.1) returns 2.0
Math.floor(2.0) returns 2.0
Math.floor(-2.0) returns -2.0
Math.floor(-2.1) returns -3.0
Math.rint(2.1) returns 2.0
Math.rint(2.5) returns 2.0
Math.round(2.6f) returns 3

min, max, and abs

max(a, b): Returns the maximum of two parameters.
min(a, b): Returns the minimum of two parameters.
abs(a): Returns the absolute value of the parameter.
random(): Returns a random double value in the range [0.0, 1.0).

Examples of min, max, and abs

Math.max(2, 3) returns 3
Math.max(2.5, 3) returns 3.0
Math.min(2.5, 3.6) returns 2.5
Math.abs(-2) returns 2
Math.abs(-2.1) returns 2.1

The random Method

Generates a random double value greater than or equal to 0.0 and less than 1.0 (0 <= Math.random() < 1.0).
- Example:
  - a + Math.random() * b returns a random number between a and a + b, excluding a + b.
  - a + (int) (Math.random() * b) returns a random integer between a and a + b, excluding a + b.
Examples of Random Method:
- (int) (Math.random() * 10) returns a random integer between 0 and 9.
- 50 + (int) (Math.random() * 50) returns a random integer between 50 and 99.

Characters

Character Data Type: The increment and decrement operators can also be used on char variables to get the next or preceding Unicode character.
- Example:
  - char ch = 'a';
  - System.out.println(++ch); // displays character 'b'.

ASCII Code for Commonly Used Characters

Decimal values and their corresponding Unicode values for characters:
- '0' to '9': Decimal 48 to 57, Unicode 0 to 9
- 'A' to 'Z': Decimal 65 to 90, Unicode 1 to A
- 'a' to 'z': Decimal 97 to 122, Unicode 1 to A

Special Characters

Special Character Display:
- ": Double quotation mark
- \: Backslash
- : Tab
- or : Carriage return

Casting between char and Numeric Types

Example:
- int i = 'a'; // This is equivalent to int i = (int)'a';
- char c = 97; // This is equivalent to char c = (char)97;

Comparing and Testing Characters

Example:
- if (ch >= 'A' && ch <= 'Z') System.out.println(ch + " is an uppercase letter");
- else if (ch >= 'a' && ch <= 'z') System.out.println(ch + " is a lowercase letter");
- else if (ch >= '0' && ch <= '9') System.out.println(ch + " is a numeric character");

Methods in the Character Class

Methods and Descriptions:
- isDigit(ch): Returns true if the specified character is a digit.
- isLetter(ch): Returns true if the specified character is a letter.
- isLetterOrDigit(ch): Returns true if the specified character is a letter or digit.
- isLowerCase(ch): Returns true if the specified character is a lowercase letter.
- isUpperCase(ch): Returns true if the specified character is an uppercase letter.
- toLowerCase(ch): Returns the lowercase of the specified character.
- toUpperCase(ch): Returns the uppercase of the specified character.

Strings

The String Type: The char type only represents one character. To represent a string of characters, use the data type called String.
- Example: String message = "Welcome to Java";
- A String is a predefined class in Java’s library just like the System class and Scanner class.
- The String type is not a primitive type; it is known as a reference type.
- Reference data types will be thoroughly discussed in Chapter 9, “Objects and Classes”.
- Key Points to Know:
  - How to declare a String variable.
  - How to assign a string to the variable.
  - How to concatenate strings.
  - To perform simple operations for strings.

Simple Methods for String Objects (1 of 2)

Method Descriptions:
- length(): Returns the number of characters in this string.
- charAt(index): Returns the character at the specified index from this string.
- concat(s1): Returns a new string that concatenates this string with string s1.
- toUpperCase(): Returns a new string with all letters in uppercase.
- toLowerCase(): Returns a new string with all letters in lowercase.
- trim(): Returns a new string with whitespace characters trimmed on both sides.

Simple Methods for String Objects (2 of 2)

Strings are objects in Java. The methods described can only be invoked from a specific string instance (called instance methods).
- Static Method: A non-instance method that can be invoked without using an object.
- Math Methods: All the methods defined in the Math class are static methods. They are not tied to a specific object instance.
- Syntax for Instance Method: referenceVariable.methodName(arguments)
- Syntax for Static Method: ClassName.methodName(arguments) (e.g., Math.pow(a,b)).

Getting String Length

Example:
- String message = "Welcome to Java";
- System.out.println("The length of " + message + " is " + message.length());

Getting Characters from a String

Example:
- String message = "Welcome to Java";
- System.out.println("The first character in message is " + message.charAt(0));

Converting Strings

Example:
- "Welcome".toLowerCase() returns a new string, "welcome".
- "Welcome".toUpperCase() returns a new string, "WELCOME".
- " Welcome ".trim() returns a new string, "Welcome".

String Concatenation

Example:
- String s3 = s1.concat(s2);
- String s3 = s1 + s2; // Three strings are concatenated
- String message = "Welcome " + "to " + "Java";
- To concatenate string Chapter with number 2: String s = "Chapter" + 2; // s becomes "Chapter2".
- To concatenate string Supplement with character B: String s1 = "Supplement" + 'B'; // s1 becomes "SupplementB".

Reading a String From the Console

Example:
- Scanner input = new Scanner(System.in);
- System.out.print("Enter three words separated by spaces: ");
- String s1 = input.next();
- String s2 = input.next();
- String s3 = input.next();
- System.out.println("s1 is " + s1);
- System.out.println("s2 is " + s2);
- System.out.println("s3 is " + s3);

Reading a Character From the Console

Example:
- Scanner input = new Scanner(System.in);
- System.out.print("Enter a character: ");
- String s = input.nextLine();
- char ch = s.charAt(0);
- System.out.println("The character entered is " + ch);

Comparing Strings

Methods Description:
- equals(s1): Returns true if this string is equal to string s1.
- equalsIgnoreCase(s1): Returns true if this string is equal to string s1 (case insensitive).
- compareTo(s1): Returns an integer that indicates whether this string is greater than, equal to, or less than s1.
- compareToIgnoreCase(s1): Same as compareTo but case insensitive.
- startsWith(prefix): Returns true if this string starts with the specified prefix.
- endsWith(suffix): Returns true if this string ends with the specified suffix.

Obtaining Substrings

Methods and Descriptions:
- substring(beginIndex): Returns this string’s substring that begins with the character at the specified beginIndex and extends to the end of the string.
- substring(beginIndex, endIndex): Returns this string’s substring that begins at the specified beginIndex and extends to the character at index endIndex - 1. Note that the character at endIndex is not part of the substring.

Finding a Character or a Substring in a String (1 of 2)

Methods and Descriptions:
- indexOf(ch): Returns the index of the first occurrence of ch in the string; returns -1 if not matched.
- indexOf(ch, fromIndex): Returns the index of the first occurrence of ch after fromIndex in the string; returns -1 if not matched.
- indexOf(s): Returns the index of the first occurrence of string s in this string; returns -1 if not matched.
- indexOf(s, fromIndex): Returns the index of the first occurrence of string s in this string after fromIndex; returns -1 if not matched.
- lastIndexOf(ch): Returns the index of the last occurrence of ch in the string; returns -1 if not matched.
- lastIndexOf(ch, fromIndex): Returns the index of the last occurrence of ch before fromIndex in this string; returns -1 if not matched.
- lastIndexOf(s): Returns the index of the last occurrence of string s; returns -1 if not matched.
- lastIndexOf(s, fromIndex): Returns the index of the last occurrence of string s before fromIndex; returns -1 if not matched.

Finding a Character or a Substring in a String (2 of 2)

Example:
- int k = s.indexOf(' ');
- String firstName = s.substring(0, k);
- String lastName = s.substring(k + 1);

Conversion between Strings and Numbers

Examples:
- int intValue = Integer.parseInt(intString);
- double doubleValue = Double.parseDouble(doubleString);
- String s = number + "";

Formatting Output

Use the printf statement.
- Syntax: System.out.printf(format, items);
- Where format is a string that may consist of substrings and format specifiers.
- A format specifier specifies how an item should be displayed and begins with a percent sign.

Frequently-Used Specifiers

Specifiers and Outputs:
- %b: a boolean value (true or false)
- %c: a character (e.g., 'a')
- %d: a decimal integer (e.g., 200)
- %f: a floating-point number (e.g., 45.460000)
- %e: a number in standard scientific notation (e.g., 4.556000e+01)
- %s: a string (e.g., "Java is cool")
Example:
- int count = 5;
- double amount = 45.56;
- System.out.printf("count is %d and amount is %f", count, amount); displays count is 5 and amount is 45.560000 items.

Format Decimal Places

Example:
public class Specifier { public static void main(String[] args) { System.out.printf("The number is %10.2f", 23.3658); } }
This example specifies the number format:
- The first value defines the total width of the output.
- The second value specifies the number of decimal places.