Java Overview

Comments in a Java Program Comments can be single line comments like C++ Example: //This is a Java Comment Comments can be spread over multiple lines like C Example: /* This is a multiple line comment. */ Comments can be special Java comments that help produce java documentation using the javadoc utitlity program. Example: /** comments for an HTML page goes here */

Identifiers Can be a series of characters consisting of letters, digits, underscores(_) and dollar signs ($). It does not begin with a digit and does not contain spaces and cannot be a reserved word. Examples: WelcomeApplet, $value, _value, my_inputField, button8 Invalid Examples: 8button or input field

Identifier Conventions Class names begin with a capital letter and have a capital letter for every word in the class name ( Ex: WelcomeAppletClass) Variables and Methods begin with lower case letter and have a capital letter for every word in the variable ( Ex: firstNumber, and myFirstMethod) Constants are all caps with an underscore separating individual words ( Ex: RATE_PER_HOUR)

Primitive Data Types and Declaring Variables Integer values (32 bits from to ): int number1, number2; Real values (32 bits): float area; True/False values (8 bits): boolean done=true;

Primitive Data Types Continued Other Integers: byte (8 bits), short (16 bits) and long (64 bits) byte verySmallInteger; short numberInClass; long veryBigInteger; Other real values: double (64 bits) double nationalDebt; –By default, real values are double. For example, 5.0 is considered to be double not float. To make it float, write it as 5.0f

Number Literals int i = 34; long l = ; float f = 100.2f; or float f = 100.2F; double d = 100.2; or double d = 100.2D; or Double d=100.2d;

Primitive Data Types Continued Character data –uses ISO Unicode Character set (16 bits) Established by the Unicode Consortium to support the interchange, processing, and display of texts for the world’s diverse languages. (see –Ranges from ‘\u0000’ to ‘\uFFFF’ –ASCII is a subset of Unicode (‘\u0000’ to ‘\u007F’ corresponds to the 128 ASCII characters Examples: char letter='A'; //A char letter='\u0041'; //A char letterPI='\03A0'; // 

Constants static final datatype CONSTANT_NAME = valueOfConstant; static final double PI = ; static final int SIZE = 3;

Shortcut Operators OperatorExampleEquivalent +=i+=8i = i+8 -=f-=8.0f = f-8.0 *=i*=8i = i*8 /=i/=8i = i/8 %=i%=8i = i%8

Increment and Decrement Operators x = 1; y = 1 + x++; y = x; y = 1 + x--; y = x;

Operator Precedence Casting ++, -- *, /, % +, -, => ==, !=; && || =, +=, -=, *=, /=, %=

Numeric Type Conversion Consider the following statements: byte i = 100; long l = i*3+4; double f = i*3.1+l/2; When performing a binary operation involving operands of different types, Java automatically converts the operand of a smaller range type to a larger range type of the other operand. Example: If one is int and the other is float, the int is converted to float

Numeric Type Conversion Casting is an operation that converts a value of one data type into a value of another data type. Casting a variable of a type with a small range to a variable with a larger range is known as widening a type. Casting a variable of a type with a large range to a variable with a smaller range is known as narrowing a type. Widening a type can be performed automatically. Narrowing a type must be performed explicitly.

Numeric Type Conversion Examples: float f = 10.1; //illegal float f = 10.1f; //legal float f = (float)10.1; //explicit casting

The String Class java.lang.String is a class that models a sequence of characters as a string. The String class contains 11 constructors and > 40 methods Examples: String message = “Welcome to Java!”;

String class String – sequence of character data String objects are immutable – they cannot be changed once they have been created String class has 11 constructors –String () –String (String s)

Strings Instantiating a string object –String name = “Joe”; –String name = new String (“Joe”);

String Methods char charAt (int position) –Return the character located at the specified position char c = name.charAt(2); equals –if (s1.equals(s2))

String Methods int length() –Returns the number of characters in the string int l = name.length(); String substring (int start) String substring (int start, int end) –Used to extract a substring from a larger string –start is the position of the first character to extract –end is NOT the position of the last character to extract but the position following the last character to extract

String Methods Assume: String letters = “abcdef”; System.out.println (letters.substring (3)); –Prints def System.out.println(letters.substring (0,3)); –Prints abc System.out.println (letters.substring (2,4)); –Prints cd

String Methods boolean equals (Object o) –Return true if the object is a string that has the same length and contains the same characters as the string for which the method is called boolean equalsIgnoreCase (Object o)

String Methods int compareTo(String s) –Returns 0 if both strings are the same –Returns a positive number if the argument string comes first –Returns a negative number if the calling string come first

String Methods indexOf and lastIndexOf – searches for a specified character or substring in a String Example: String letters =“abcdefabcd”; –int i=letters.indexOf(‘c’); // i contains 2 –int i=letters.indexOf(‘$’); // i contains -1 –int i=letters.indexOf (‘a’,1); //starts with 1 and searches for ‘a’ i contains 6 –int i=lastIndexOf (‘b’); //i=7 –int i=letters.indexOf(“def”); // i=3

String Methods replace (char,char), toUpperCase(), trim() –String s1 = “hello”; –s1 = s1.replace (‘l’, ‘L’); //returns a string object in which every occurrence of ‘l’ is replaced by ‘L’ –s1 = s1.toUpperCase(); //”HELLO” –s1 = s1.trim() //remove any white spaces at beginning or end