

if ( CONDITION ) ONEstatement 
Explanation:


Here, we will first use a few simple Boolean expressions to learn about the ifstatement

import java.util.Scanner; public class If1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); int a; System.out.print("Enter an integer value: "); a = in.nextInt(); // Read in an integer from keyboard System.out.print("Input value = "); System.out.println(a); if ( a < 0 ) // Ifstatement a = a; // Negate a ONLY if a < 0 System.out.print("It's absolute value = "); System.out.println(a); } } 
Example outputs:
cheung@HOME2(40)> java If1 Enter an integer value: 4 Input value = 4 It's absolute value = 4 cheung@HOME2(41)> java If1 Enter an integer value: 4 Input value = 4 It's absolute value = 4 
How to run the program:


a = read input from keyboard; if ( a < 0 ) a = a; Print a; 
Explanation:

Explanation:

a = read input from keyboard; if ( a < 0 ) a = a; Print a; 
Explanation:

Explanation:



if ( CONDITION ) ONEstatement 
Notes:



import java.util.Scanner; // Import Scanner class (contains methods // for reading keyboard input) public class Abc2 { public static void main(String[] args) { double a, b, c, x1, x2; // Define 5 variable Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b c = in.nextDouble(); // Read in next number into c x1 = ( b  Math.sqrt( b*b  4*a*c ) ) / (2*a); x2 = ( b + Math.sqrt( b*b  4*a*c ) ) / (2*a); System.out.print("x1 = "); System.out.println(x1); System.out.print("x2 = "); System.out.println(x2); } } 
Shortcoming:


Schematically:
import java.util.Scanner; public class If2 { public static void main(String[] args) { double a, b, c, x1, x2; // Define 5 variable Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b c = in.nextDouble(); // Read in next number into c x1 = x2 = 0; // Default solution /*  Only compute solution if determinant b*b  4*a*c >= 0  */ if ( b*b  4*a*c >= 0 ) { x1 = ( b  Math.sqrt( b*b  4*a*c ) ) / (2*a); x2 = ( b + Math.sqrt( b*b  4*a*c ) ) / (2*a); } System.out.print("x1 = "); System.out.println(x1); System.out.print("x2 = "); System.out.println(x2); } } 
How to run the program:

Example:
import java.util.Scanner; public class If3 { public static void main(String[] args) { Scanner in = new Scanner(System.in); int a; System.out.print("Enter an integer value: "); a = in.nextInt(); // Read in an integer from keyboard System.out.print("Input value = "); System.out.println(a); if ( a < 0 ) ; // Bogus ; a = a; System.out.print("It's absolute value = "); System.out.println(a); } } 
Structure diagram of this erroneous program:
Notes:

Example output:
Enter an integer value: 4 Input value = 4 It's absolute value = 4 (Because a = a; is always executed) 
How to run the program:

Example:
import java.util.Scanner; public class If2 { public static void main(String[] args) { double a, b, c, x1, x2; // Define 5 variable Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b c = in.nextDouble(); // Read in next number into c x1 = x2 = 0; // Default solution /*  Only compute solution if determinant b*b  4*a*c >= 0  */ if ( b*b  4*a*c >= 0 ) x1 = ( b  Math.sqrt( b*b  4*a*c ) ) / (2*a); x2 = ( b + Math.sqrt( b*b  4*a*c ) ) / (2*a); System.out.print("x1 = "); System.out.println(x1); System.out.print("x2 = "); System.out.println(x2); } } 
Structure diagram of this erroneous program:
Notes:

Example output: (use: a = 1, b = 1, c = 1)
x1 = 0.0 x2 = NaN < x2 was computed with sqrt(neg. value) 
How to run the program:
