Java Programming I on YouTube:
https://www.youtube.com/playlist?list=PLLIqcoGpl73iaXAtS_-V_Xdx3mhTzPwb5

Relational Operators and Flow Control

1. Relational Operators
2. Examples of conditional expressions
3. if-else statement
4. An if without else
5. if-else
6. Another if-else example
7. Comparing Strings
8. Comparing Strings Examples
9. if with else-if
10. The while loop
11. Coding while statements
12. While Statement Example
13. An "Endless" Loop Example
14. Another Loop Example
15. Logical (Boolean) Operators
16. Numeric Ranges in Java
17. Numeric Ranges in Java, cont.
18. Numeric Ranges in Java, cont.

1. Relational Operators

• 
      ----------------------------------
      Operator    Name
      ----------  ----------------------
      ==          Equality 
      !=          Inequality
      >           Greater Than
      <           Less Than
      >=          Greater Than Or Equal
      <=          Less Than Or Equal
      ----------------------------------
  
  

• Relational operators formulate a boolean expression

• The result of a boolean expression is either true or false

• Boolean expressions are also called conditional expressions

   

2. Examples of conditional expressions

• The following boolean expressions can be useful in many control statements such as if that compare numeric data:

  
  discountPercent == 2.3  // equal to a numeric literal
  subtotal != 0           // not equal to a numeric literal
  years > 0               // greater than a numeric literal
  i < months              // less than a numeric variable
  subtotal >= 500         // greater than or equal to a numeric literal
  quantity <= reorderQty  // less than or equal to a numeric variable
  
  

   

3. if-else statement

• The syntax of the if-else statement is

  
      if ( booleanExpression ) { statements }
      [else if ( booleanExpression ) { statements }]<sub>optional</sub> ...
      [else {statements}]<sub>optional</sub>
  
  

• The if statements compare primitive data types in a boolean expression

• Boolean expressions are enclosed in parentheses and use relational operators

   

4. An if without else

• The if statements without else-if or without else clause might looke like this:

  1. With a single statement:

     
         if (subtotal >= 100)
             discountPercent = .2;
     
     

  2. With a block of statements:

     
         if (subtotal >= 100)
         {
             discountPercent = .2;
             status = "Bulk rate";
         }
     
     

   

5. if-else

• An if statement with an else clause:

  
      if ( subtotal >= 100 ) {
          discountPercent = .2;
  
      } else {
          discountPercent = .1;
      }
  
  

   

6. Another if-else example

• 
  if ( age > 17 ) {
      System.out.println("Eligible to vote.");
      System.out.println("No longer a minor.");
  
  } else {
      System.out.println("Not eligible to vote.");
      System.out.println("Still a minor.");
  }
  
  

   

7. Comparing Strings

• Unfortunately, Java does not allow relational operators (>, <, >=, <=, ==, !=) to compare strings.

• In Java, Strings are objects, and objects have two methods for comparing Strings:

  • equals(String) -- case sensitive

  • equalsIgnoreCase(String) -- not case sensitive

• Both methods return a Boolean true or false

   

8. Comparing Strings Examples

• 
  userEntry.equals( "Y" ) // equal to a string literal
  
  userEntry.equalsIgnoreCase( "Y" )
                          // equal to a string literal
  
  ( !lastName.equals( "Jones" ) )
                          // not equal to a string literal
  
  code.equalsIgnoreCase( productCode )
                          // equal to another string variable
  
  

   

9. if with else-if

• An if statement with else-if and else clauses:

  
      if ( customerType.equals( "T" ) ) {
          discountPercent = .4;
  
      } else if ( customerType.equals( "C" ) ) {
          discountPercent = .2;
  
      } else if ( subtotal >= 100 ) {
          discountPercent = .2;
  
      } else {
          discountPercent = .1;
      }
  
  

   

10. The while loop

• The syntax of the while loop is:

  
      while( booleanExpression )
      {
         statements
      }
  
  

   

11. Coding while statements

• A while statement repeats a block of {statements} as long as the boolean expression in the statement yields true.

• When the boolean expression becomes false the block is no longer executed; instead, the statements following the block are then executed

• Any variable defined within a block is visible only within that block. This also means that the variable is no longer reachable after the thread of execution leaves the block

   

12. While Statement Example

• The following demo code displays the total of all the numbers from 10 down to 1:

  
      int x = 10;
      int total = 0
      while( x > 0 ) { 
          total = total + x;
          x = x – 1;
      }
      System.out.println( "Total = " + total );
  
  

   

13. An "Endless" Loop Example

• The user controls whether to continue to execute the block of code by entering "y" or "Y":

  
      Scanner scnr = new Scanner( System.in );
      String response = "y";
  
      while( response.equalsIgnoreCase( "y" ) ) {
          // several statements within the block
          // ...
          System.out.print( "Continue – Y/N?" );
          response = scnr.next();
      }
  
  

   

14. Another Loop Example

• A loop that calculates the sum of the numbers 1 through 4

  
      int idx = 1;
      int sum = 0;
  
      while( idx < 5 ) {
          sum = sum + idx;
          idx = idx + 1;
      }
  
  

   

15. Logical (Boolean) Operators

• 
      !       // logical NOT
  
      &&      // logical AND
  
      ||      // logical OR
  
  

   

16. Numeric Ranges in Java

• Consider:

  
      int value = 7;
  
      if ( 1 < value < 10 ) {
          System.out.println(
              "The value "
              +
              value
              +
              " is greater than 1 and less than 10"
              );
      }
  
  

   

17. Numeric Ranges in Java, cont.

• The previos if statement is illegal in Java!

• The expression

  
      1 < value < 10
  
  

  is interpreted as

  
      ( 1 < 7 ) < 10
  
  

  which then becomes

  
      true < 10
  
  

  which is illegal expression because Java does not allow comparing boolean values with other data types.

   

18. Numeric Ranges in Java, cont.

• The solution is to use the logical AND operator:

  
      int value = 7;
  
      if ( 1 < value && value < 10 ) {
          System.out.println(
              "The value "
              +
              value
              +
              " is greater than 1 and less than 10"
              );
      }