JAVA BASICS

Java input/output

The Java program shown below is stored in a file named add.java.

import javax.swing.*;
public class add
{ //This program displays a result of an addition
  public static void main(String[] args)
 {
String s;
int n1, n2,sum;
s = JOptionPane.showInputDialog("Enter the first number:");
n1 = Integer.parseInt(s);
s = JOptionPane.showInputDialog("Enter the second number:");
n2 = Integer.parseInt(s);
sum = n1 + n2;
System.out.println(n1 + " + " + n2 + " = " + sum );
JOptionPane.showMessageDialog(null,n1 + " + " + n2 + " = " + sum );
System.exit(0);
 }//end main
}//end class

Run time output
Input

Output

10 + 20 =  30

                                Analyze the program                                                                     

• This program displays a string "Hello World" on the screen
• Files that contain Java programs are named using the .java file extension (for example Hello.java).
• The Java program's file name has to be same as the class name.
• Case-sensitive
• Periods are used to seperate classes, objects, and methods in program code.
• A series of characters that appears between double quotation marks is a literal string.
• Every that you use within a Java program must be part of a class.
• Naming a class
  • Beginning with either an uppercase or lowercase letter (including underscore, _ , and dollar sign, $) followed by letters or digits or both..
  • Containing  only uppercase or lowercase letters, digits, and underscores.
  • No reserved key word
• The key word static in a method header indicates that every member of a class will have an identical, unchanging method.
• void  means the main method does not return a value.
• All Java application programs must have a method named main().
• Three types of comments
  • Line comments begin with two forward slashes (//)
  • Block comments begins with  (/*) and ends with (*/)
• All Java language program ends with a semicolon.
• out is an object. println() is a method to send a string to the screen.
• Compiled Java program file has an extension named class (hello.class)

Note
Double num3;
String s;
S= JOptionPane.showInputDialog(“enter a number”);
Num3=Double.parseDouble (s);

Example:
Algorithm
-         Input 2 integer numbers
-         Calculate the sum
-         Display the input numbers &sum  

import javax.swing.*;       

public class Test

{ public static void main (String [] args)

     String s;                 

            {   int  num1, num2 , sum;

                 s= JOptionPane.showInputDialog(“ENTER THE FIRST NUMBER”);          

num1=Integer.parseInt(s);

s=JOptionPane.showInputDialog(“ENTER THE SECOND NUMBER:”);

num2 = Integer.parseInt(s);

sum=num1+num2;  

            System.out.println(num1);à10

            System.out.println(num2); à20

            System.out.println(“num1”); à num1

            System.out.println( “num1=”+num1); à num1=10

            System.out.println(num1+num2); à30

            System.out.println(sum); à 30

            System.out.println(“num1=”+num1+”\n” + “num2= ”+num2+”\n”+”sum= “ +sum);

                        Output: num1=10

                                    num2=20

                                    sum=30 

            System.out.println(“sum=” + “num1” + “num2”);
                               Output: sum= 10+20
                      
            }
}           

Output to the console window

• Use System.out.println  to output
  println is a Java method - diplay and then move the cursor to the next line
  print - display but does not move the cursor to the next line.

Examples
System.out.println(“Hello”);
Output
Hello
System.out.println(“ABCD”);
System.out.println("EFG");
Output
ABCD
EFG

Escape Sequences

Use escape sequences in a literal string

Example

System.out.print(“AB\nc”);
Output
AB
C
Example
System.out.print(“AB\nC);
System.out.print(“D”);
Output
AB
CD

Example: to display a double quote, put \ before it.
System.out.print(“AB\”DEF”\”);
Output
AB “DEF”

Character code

ASCII code: 1 byte 
Unicode: 2 bytes
ASCII is a subset of Unicode
Example:
Letter A
Unicode -  Dec: 65  Hex: 0041
Example: display a symbol using unicode

System.out.println(“\u00f4”);
Output
ô

Comment statement

Comment - Code explanation 
Compiler skips the comment statement

Two ways to make comment
-  Use // for one single comment statement
or
-Use /*….*/ for multiple comment statement

Example
import javax.swing.*;
public class Test
{  public static void main (String[]args)
    { // A simple Java displays statement
     System.out.println(“Hello”); //Display “Hello”
     /*Author: Daniel Phan
       CECS 174 */
    }
}

Another Method to input from the keyboard

import java.util.*;
import java.io.*;
public class Test
{
public static void main(String[] args) throws IOException{

//creating an input object of the class Scanner for numeric input
Scanner input = new Scanner (System.in); // System.in is an InputStream
//Create a buffer for string input
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

//declaration of variables
int n1 , n2 = 0;
double n3;
String s;
//Input by the user
System.out.println ("Enter two whole numbers on a line with spaces between them:");
n1 = input.nextInt ();
n2 = input.nextInt ();
System.out.println ("Enter a real number:");
n3 = input.nextDouble();
System.out.println ("Enter a string:");
s = br.readLine();

//Display to the console
System.out.println();
System.out.println ("The sum of those two numbers is " + (n1 + n2));
System.out.println ("number 3: " + n3);
System.out.println ("String s: " + s);
}
}

Binary Numbers

Binary number: two digits only 1 or 0
Decimal Number 10 digits  0-9
Hex Number 16 digits 1-15
0-9 and A-F (10-15)

Example
Convert 1010 to decimal number
1010 =  10 1x2^3 + 0x2^2 + 1x2^1 + 0x2^0

101-->  5

Example Convert Hex to Dec

A6 = Ax16^1 + 6 x16^0
10x16 + 6 = 166

40= 4x16^1+ 0x16^0 =64

Values

Integer values
10, 123, 65000, -65 (no comma or decimal point)
Real values
17.5, 162.783, -65.7, 921.46
Character Values encloses in a single character
‘a’ , ‘b’  or  ‘y’
Logical value - True or False
true or false

Example

6 is an integer value
‘6’ is a character Value
‘yn’  is invalid character value
‘\n’ is a valid character value (escape sequence)

Data type

• Data types
  • Integers - These types are bytes, short, int,  and long, which hold signed, whole-value numbers.
  • Floating-point numbers - These types are float and double, which hold signed floating point numbers.
  • Characters -  This is the char type, which holds the representation of character such as letters and numbers.
  • Boolean - This type is designed to hold only two types of values: true of false.
• Eight primitive data types: boolean(2bytes),  byte(1 bytes), char (2 bytes), double(8 or 4 bytes),   float(4 bytes),  int (4 bytes),  long (8 bytes),  short (1 byte).

Variables
Variable is a memory location.
Naming a Variable

-         1^st character must be a leter a to z

-         OthtersàCombination of numbers and letters

-         Use underscores

-         No special symbols

-         No java keywords (main, public, class….)

-         Case sensitive

Example Naming variables

Variable declaration

Data type  variable name, variable name, ..;

Example
 int x;
double y, z, r ;
char  choice;
double profit,  int abc; //error
double profit;  int abc; // OK
or
double profit;
int abc;

Declare and intialize a variable

data type variablename=____,variablename=_____

Example       int total =0 , sum =10 ;
                    int x=10 , y; // OK
                    int a, b=20 , c ; //OK
                   double revenue = 6000.0 , int profit =10; //ERROR (need semicolon between 6000.0 and int)

• integer variable
  You can create literals in octal format by starting them with a leading zero and in hexadecimal format by starting them  with 0x or 0X.
  Example
  int value;
  value = 16:
  System.out.println("16 decimal = " + value);
  value = 020;
  System.out.println("20 octal = " + value + "in decimal");
  value = 0x10;
  System.out.println("10 hexadecimal = " + value +"in decimal");
  Output
  16 decimal = 16
  20 octal = 16 in decimal
  10 hexadecimal =  16 in decimal
• floating-point variable
  Example
  float value;
  value = 1.5f; //or value= 1.5F    Note The floating-point numbers used as literals are of type double, not of type                                                             float. Adding an f or F to the end of the literal to make it a float or a d or D
                                                            to make it a double.
• Boolean variable
  Example
  boolean value;
  value = true;
  System.out.println("The value = " + value);
  Output
  The value = true
• Character variable
  Example:  
    char x, y;
    x = 'D';
    y = 65;
    ystem.out.println("x = " + x);
  System.out.println("y = " + y);
    Output
    x = D
    y = A

Figure 1.1 lists some common Java operators and their functions.

int x=5, y=z, z;

z=x/y; //zà 2

int a, b=5, c=2;

Precedence prefers to the order in which the compiler will execute the operations. Operators have different precedences associated with them. Operators also have associativity, which refers to the order in which the compiler will execute operations that have the same precedence. For example, the expression num1 + num2 - num3 includes two operators that have the same precedence--the addition and the subtraction operator. The two operator have left to right associativity, so the computer will begin with the leftmost operator and then move to the right. In this case, the subtraction operator will follow the addition operator. Figure 1.2 shows the precedence and associativity of the operators discussed in this lecture.

Logical Operator

Relational operators

You can use relational operators to construct a relational expressions that compare two values. The result of comparison will be true or false.

Relational expressions

A simple relational expression consists of two operands and one relational operator.
Example:  x > y
                The result of this comparison is true or false value. If the test is true, a value of 1 is returned; otherwise, a value of
                zero is returned.

In Java, the relational expression are not bounded by data type. A relational operator can be used to compare two values of the same data type or the different data type. The operands in a relational expression can include any of the following:

• Variables as operands
  Example:  x = = y
                  if x is equal to y, then the test result is true (1) ;otherwise, the result is zero (0).
• Constants as operands
  Example: x  < = 10
                 if x is less than or equal to 10, the test result is true (1); otherwise, the test result is false (0).
• Arithmetic expressions as operands
  Example: x > 40 - y
                Test to determine if x is greater than the evaluation expression 40 - y. If it is, the test result is true (1). If x
                x is less than (40 - y), the test result will false (0).

In Java, all boolean values (as true or false sometimes are called) are represented by integers. The value 0 is interpreted as false when it's used with relational or logical operators, and any other integer (positive or negative)   is regarded as true.

          Example:    x = y > 1 + n;
                          In this statement, the variable x would be assigned a value of 0 or 1 depending on whether y is greater than,
                          less than, or equal to 1 + n.

Single alternative decision structure

if STATEMENT 

Pseudocode

if  condition is true then
        true body
endif

Java structure

if ( Condition )
{
        true Body

}

Notes

• If condition is true, Java will perform true body.

• The {  and  } is not required if the true body has only one Java statement to execute when the condition is true.
   

Example:
Pseudocode                                                                     Java

if x is greater than y then                                    if ( x > y )
    display x                                                                   System.out.println(x);
endif

if x is greater than y then                                    if (  x > y )
    z = x + y;                                                          {     z = x + y;
    Display z                                                                System.out.println(z);
endif                                                                     }
Display "Bye"                                                          System.out.println("Bye")

Dual-alternative decision structure

if ... else  STATEMENT

Pseudocode

if  condition is true then
        True body
else
        False body
endif

Java structure

if ( Condition )
{
       true body
}
else
{
       false body
}

 

Notes

• If condition is true, Java will perform true body.

• The {  and  } is not required if the true body has only one Java statement to execute when the condition is true.
   

Example:
Pseudocode                                                                     Java

if x is greater than y then                                    if ( x > y )
    display x                                                                   System.out.println(x);
else                                                                      else
   display y                                                                    System.out.println(y);
endif

if x is greater than y then                                    if (  x > y )
    z = x + y;                                                          {     z = x + y;
    Display z                                                                System.out.println(z);
else                                                                       }
    z = x - y;                                                            else
    Display z                                                           {   z = x - y;
endif                                                                           System.out.println(z);
Display "Bye"                                                     }
                                                                                  System.out.println("Bye");