Chapter 2 Introduction to Java Application

Chapter 2: Introduction to Java Application         Our first program in Java: printing a line of text Modifying our first Java program Displaying text with printf Another application: adding integers Memory concepts Arithmetic Decision making Equality and relational operators Our first program in Java: printing a line of text A java application is a computer program that executes when you use the java command to launch the JVM. // Fig. 2.1: Welcome1.java // Text-printing program. public class Welcome1 { // main method begins execution of Java application public static void main( String[] args ) { System.out.println( "Welcome to Java Programming!" ); } // end method main } // end class Welcome1 /******************************************************************* ******************************************************************** / Output: Welcome to Java Programming! Comment statements: Used to document programs and improve their readability. Java compiler ignores comments. A comment begins with: // end of line (single-line) comment, or /* (and ends with */ ) traditional multiple-line comments. 6 White space A white space can be black line, space character or tab, ignored by the compiler. It makes programs easier to read. The basic unit to write a program in Java is a class. There are 2 types of classes: 1. Java class libraries (API) 2. Programmer defined (user defined) classes. Every program in Java consists of at least one class declaration defined by the programmer. A class contains: a- Variables (attributes) b- One or more methods that manipulate the attributes that belong to a certain object of the class. public class Welcome1 public and class are keywords (always lowercase letters) public is an access modifier: An access modifier can be public, private or protected. If not specified it is considered as package. A public class can be accessed from anywhere. When you save the public class declaration in a file, the file name must be: ClassName.java In our example the file must be saved as Welcome1.java So, Welcome1 is the class name: Java is case sensitive – a1 is different than A1 By convention, all class names in Java begin with a capital letter (each subsequent word begins with a capital letter). The class name is an identifier (series of characters consisting of letters, digits, underscores and $ that doesn’t begin with a digit and doesn’t contain spaces.) ◦ Examples of valid identifiers: hello, valid_$1, $23, _Name ◦ Examples of invalid identifiers: 123Here, exam-1, student ID, not@valid, class public static void main (String args[]) In a java application, JVM will not execute the application if the main method does not exist, i.e. for a java application, exactly one of the methods must be called main, otherwise the JVM will not execute the application. A method is a function that performs tasks and returns information. 7 static: a static method can be called without first creating an object of the class in which the method is declared. void: keyword that indicates that this method will perform a task but will not return any information when it completes its task. System.out.println (“Welcome to Java Programming!”); System is a class that is part of java.lang package. A package is a collection of predefined classes that programmers can reuse (examples: java.lang, java.util, javax.swing,…) When you need to reuse a class that is defined in a java package, you can import it first. All import declarations must appear before the first class declaration in the file. Exceptions are classes defined in the package java.lang, where java.lang is imported into all programs by the compiler so the programmer needs not to do so. out is a class variable (attribute). System.out is the standard output object that allows Java application to display sets of characters. println is a method – after printing, it moves the cursor to the next line. “Welcome to Java Programming!” is an argument of the method println, this argument is the string to be printed (without quotation marks). Modifying our first Java program // Fig. 2.3: Welcome2.java // Printing a line of text with multiple statements. public class Welcome2 { // main method begins execution of Java application public static void main( String[] args ) { System.out.print( "Welcome to " ); System.out.println( "Java Programming!" ); } // end method main } // end class Welcome2 Output: Welcome to Java Programming! 8 Another example: // Fig. 2.4: Welcome3.java // Printing multiple lines with a single statement. public class Welcome3 { // main method begins execution of Java application public static void main( String[] args ) { System.out.println( "Welcome\nto\nJava\nProgramming!" ); } // end method main } // end class Welcome3 Output: Welcome to Java Programming! Note: \character : two characters called escape sequence (\ is called escape character) Escape sequence description \n Newline. Position screen cursor at the beginning of the next line \t Horizontal tab. Move screen cursor to the next tab stop \r Carriage return. Position screen curser at the beginning of the current line. Any characters output after the carriage return overrides the characters previously output on that line. \\ Backslash. Used to print a backslash character \” Double quote. Used to print double-quote character \’ Single quote. Used to print single-quote character Displaying text with printf // Fig. 2.6: Welcome4.java // Displaying multiple lines with method System.out.printf. public class Welcome4 { // main method begins execution of Java application public static void main( String[] args ) { System.out.printf( "%s\n%s\n", "Welcome to", "Java Programming!" ); } // end method main } // end class Welcome4 output 9 Welcome to Java Programming! System.out.printf: a method for displaying formatted data, this method takes multiple arguments separated by commas. Format string may consist of: 1. Fixed text 2. Format specifiers: place holders for the values and it specifies the type of data to ouput. % character (the character represents the data type) for example: %s : place holder for a String %d : place holder for integer value %f : place holder for floating-point numbers %b : place holder for boolean Variables: A variable is a location in the computer's memory where a value can be stored for use later in a program. We have two kinds of variables: 1- Local variables Syntax: dataType variableName [= value]; 2- Field variables Syntax: [AccessModifier] [static] dataType variableName [= value]; Variables must be declared with a name and a type:   A variable name can be any valid identifier. By convention, variable-name identifiers begin with a lowercase letter, and every word in the name after the first begins with a capital letter. E.g. firstNumber Variable's type specifies the kind of information stored at that location in memory. Data Types can be: 1- Primitive types 2- Reference types (Example: String) 10 Primitive Data Types Category Description JAVA Size 8 bits byte whole numbers (without decimal points) short 2 bytes int 4 bytes numbers real numbers characters single characters booleans boolean values Values 7 -2147483648 to 2147483647 long 8 bytes -2 to 2 -1 float 4 bytes e.g. 17.345f double 8 bytes e.g. 12452.212 (more accurate) char 2 bytes e.g. 'a', '1' and '?' boolean 1 byte true or false 63 63 Note: All keywords must appear in lowercase letters Declaration statements end with a semicolon (;) Variables of the same type may be declared in one declaration or in multiple declarations. For examples, if you want to declare n1 and n2 as integer variables you can use any of the following: int n1; int n2; or int n1, n2; Local variables (those declared in the body of a particular method) must be initialized before being used. For example: int n1; n1=15; or int n1=15; (or their values can be obtained from the user) char examples: char a = 'z'; // the letter z (a actually stores the integer value of the letter z) char b = '\n'; // new line Strings A string is a sequence of characters that are treated as a single unit. A string may be assigned to a String reference, the declaration: 11 7 -128 to 127 (i.e. -2 to 2 1) -32768 to 32767 String color="blue"; Initializes string variable color to refer to a string object that contains the string "blue". The class String is used to represent strings in Java. This class provides constructors for initializing String objects in different ways. For example: String s = new String ("hello"); Instantiates a new string object using class String's constructor using the sequence of characters "hello" as its argument and assigns its reference to s. 12 Addition Example: // Fig. 2.7: Addition.java // Addition program that displays the sum of two numbers. import java.util.Scanner; // program uses class Scanner public class Addition { // main method begins execution of Java application public static void main( String args[] ) { // create Scanner to obtain input from command window Scanner input = new Scanner( System.in ); int number1; // first number to add int number2; // second number to add int sum; // sum of number1 and number2 System.out.print( "Enter first integer: " ); // prompt number1 = input.nextInt(); // read first number from user System.out.print( "Enter second integer: " ); // prompt number2 = input.nextInt(); // read second number from user sum = number1 + number2; // add numbers System.out.printf( "Sum is %d\n", sum ); // display sum } // end method main } // end class Addition /********************************************************************************************************( The statement: import java.util.Scanner; is an import declaration that helps the compiler locate a class that is used in this program. The above declaration indicates that the program uses java's predefined Scanner class from package (java.util). Notes:  All import declarations must appear before the first class declaration in the file.  By default, the package java.lang is imported in every java program. So, classes in java.lang don’t require an import declaration (e.g. class System). 13 The statement: Scanner input = new Scanner (System.in); is a variable declaration statement: variable name is input, type is Scanner. [A Scanner enables a program to read data (e.g numbers) for use in a program, the data can come from a file or the keyboard]. The above statement creates an object of class Scanner (using the keyword new) and the source of data is specified by System.in (standard input object), which is the keyboard. The statement: number1 = input.nextInt(); uses the method nextInt to obtain an integer from the user. Other Scanner methods:         next(): read individual words, until a white space character nextDouble(): read double value nextLine(): read characters typed by the user until the new line character is encountered. nextByte() nextFloat() nextLong() nextShort() hasNext(): determine whether there's more data to input (returns Boolean value) Operators: Arithmetic operators Java operation Arithmetic operator Algebraic expression Java expression Addition + f+7 f +7 Subtraction - p-c p–c Multiplication * bm b*m Division (integer or real division) Remainder / x/y or x÷y x/y % r mod s r%s Integer division yields integer quotient (provided that both operands are integers). Fractions in the result are truncated (no rounding occurs). Examples: int x = 7/4; //x= 1 int y= 17/5; //y= 3 14 In case at least one of the operands was a floating point number, the division will be a real division (result should be stored in a variable of type double). For example: double x = 5/2.0; //x= 2.5 Remainder operator yields the remainder after division. For example: 7%4 = 3 17%5 = 2 2%5=2 12%2=0 (note: the sign of the result is the sign of the first operand) Equality and Relational operators Standard algebraic equality or relational operator Java equality or relational operator Sample java condition Meaning of java condition = == x == y x is equal to y ≠ != x != y x is not equal to y > > x>y x is greater than y < < x= x>=y x is greater than or equal to y ≤ <= X<=y x is less than or equal to y Equality operators Relational operators Assignment operator (=) Used to assign values to variables. Everything to the right of the assignment operator is always evaluated before the assignment is performed. 15 Compound assignment operator Any statement of the form: variable = variable operator expression; Where operator is one of the binary operators +, -, *, / or % can be written in the form: Variable operator= expression; e.g. c=c+3 can be written as Assignment operator c+=3; Sample expression Explanation assigns Assume: int c=3, d=5, e=4, f=6, g=12; += c+=7 c=c+7 10 to c -= d-=4 d=d-4 1 to d *= e*=5 e=e*5 20 to e /= f/=3 F=f/3 2 to f %= g%=9 g=g%9 3 to g Increment and decrement operators Java provides two unary operators for adding 1 to or subtracting 1 from the value of a numeric variable. The unary increment operator, ++, and the unary decrement operator, -Operator Operator name ++ Prefix increment Sample expression ++a ++ Postfix increment a++ -- Prefix decrement --b -- Postfix decrement b-- Explanation Increment a by 1, then use the new value of a in the expression in which a resides. Use the current value of a in the expression in which a resides, then increment a by 1. Decrement b by 1, then use the new value of b in the expression in which b resides. Use the current value of b in the expression in which b resides, then decrement b by 1. 16 Increment operator example // Fig. 4.16: Increment.java // Prefix increment and postfix increment operators. public class Increment { public static void main( String args[] ) { int c; // demonstrate postfix increment operator c = 5; // assign 5 to c System.out.println( c ); // prints 5 System.out.println( c++ ); // prints 5 then postincrements System.out.println( c ); // prints 6 System.out.println(); // skip a line // demonstrate prefix increment operator c = 5; // assign 5 to c System.out.println( c ); // prints 5 System.out.println( ++c ); // preincrements then prints 6 System.out.println( c ); // prints 6 } // end main } // end class Increment /************************************************************************** Parentheses for grouping subexpressions To multiply a times the quantity b+c, we write: a*(b+c) if an expression contains nested parentheses, such as ((a+b)*c) The expression in the innermost set of parentheses is evaluated first. 17 Rules of operator precedence: Operators Associativity Type ++ Left to right Increment and decrement operators Left to right Multiplicative + - Left to right Additive < <= > >= Left to right Relational == != Left to right Equality Right to left Conditional Right to left assignment -- * ?: / % (will be explained later) = += -= *= /= %= Examples: Algebra java m=(a+b+c+d+e)/5; m=a+b+c+d+e/5; y = mx +b y= m*x+b; z = pr mod q+ w/y z = p * r % q + w / x - y; y = ax2+bx+c y = a * x * x + b * x + c; z= p * r % q + w / x - y; 6 1 2 4 3 5 y = a * x * x + b * x + c; 6 1 2 4 3 5 suppose a=2, b=3, c=7, x=5, then y=72 18 Comparison example: // Fig. 2.15: Comparison.java // Compare integers using if statements, relational operators and equality operators. import java.util.Scanner; // program uses class Scanner public class Comparison { // main method begins execution of Java application public static void main( String args[] ) { // create Scanner to obtain input from command window Scanner input = new Scanner( System.in ); int number1; // first number to compare int number2; // second number to compare System.out.print( "Enter first integer: " ); // prompt number1 = input.nextInt(); // read first number from user System.out.print( "Enter second integer: " ); // prompt number2 = input.nextInt(); // read second number from user if ( number1 == number2 ) System.out.printf( "%d == %d\n", number1, number2 ); if ( number1 != number2 ) System.out.printf( "%d != %d\n", number1, number2 ); if ( number1 < number2 ) System.out.printf( "%d < %d\n", number1, number2 ); if ( number1 > number2 ) System.out.printf( "%d > %d\n", number1, number2 ); if ( number1 <= number2 ) System.out.printf( "%d <= %d\n", number1, number2 ); if ( number1 >= number2 ) System.out.printf( "%d >= %d\n", number1, number2 ); } // end method main } // end class Comparison /********************************************************************************************************( 19 Conversion between primitive types Promotion: implicit conversion, conversion without losing data. Promotions allowed for primitive types: Type Valid promotions double None float double long float or double int long, float or double char int, long, float or double short int, long, float or double (but not char) byte short, int, long, float or double (but not char) Boolean None (Boolean values are not considered to be numbers in java. For example, the following statements are correct: int y = 4; char c = 'A'; double x = y; //x=4.0 int d = c; // d=65 but: double a = 4.5; int b = a; //ERROR int m=65; char c=m; //Error Casting: explicit conversion, conversion may cause loss of data. For example: int a = (int) 3.5; //a=3 long b = 5; int a = (int) b; //a=5 (note that the value of b is not changed) Note: when you declare a float variable and initialize it to some value, you should cast this value because it is considered as double. For example: float x=4.5; //Error float x=(float) 4.5; //correct float x=4.5f; //correct float c=5.6f; long b=c; //Error float c =5.6f; long b=(long) c; //b=5 20