Introduction to Java Programming and Data Structures - Chapter 3 Selections

Selections in Java Programming

The boolean Type and Relational Operators

• Purpose: Often, programs require comparing two values (e.g., checking if integer i is greater than integer j).

• Relational Operators: Java provides six comparison operators, also known as relational operators, used to compare two values.

• Result: The outcome of a comparison operation is a boolean value, which can be either true or false.

  • Example: boolean b = (1 > 2); would result in b being false.

• List of Relational Operators:

  • < (less than)

    • Mathematics Symbol: <

    • Example (radius is 5): radius < 0

    • Result: false

  • <= (less than or equal to)

    • Mathematics Symbol: \le

    • Example (radius is 5): radius <= 0

    • Result: false

  • > (greater than)

    • Mathematics Symbol: >

    • Example (radius is 5): radius > 0

    • Result: true

  • >= (greater than or equal to)

    • Mathematics Symbol: \ge

    • Example (radius is 5): radius >= 0

    • Result: true

  • == (equal to)

    • Mathematics Symbol: =

    • Example (radius is 5): radius == 0

    • Result: false

  • != (not equal to)

    • Mathematics Symbol: \ne

    • Example (radius is 5): radius != 0

    • Result: true

One-Way if Statements

• Syntax:
  java if (boolean-expression) { statement(s); }

• Execution Flow: If the boolean-expression evaluates to true, the statement(s) inside the if block are executed. If it evaluates to false, the statement(s) are skipped.

• Flowchart: A diamond shape represents the boolean-expression. If true, flow goes to statements; if false, it bypasses them.

• Example: java if (radius >= 0) { area = radius * radius * PI; // PI represents a constant like 3.14159 System.out.println("The area for the circle of radius " + radius + " is " + area); }

  • Here, if radius is greater than or equal to 0, the area is calculated and printed.

• Notes on Syntax: While braces {} are generally recommended for readability and to avoid potential errors, for a single statement, they are optional.

  • Correct with braces: if (i > 0) { System.out.println("i is positive"); }

  • Equivalent without braces for a single statement: if (i > 0) System.out.println("i is positive");

  • Incorrect (semicolon after if): if (i > 0); { System.out.println("i is positive"); } (This creates an empty statement for the if, and the print statement is always executed, which is a logic error).

• Demo Example (SimpleIfDemo): A program that prompts the user for an integer and prints

Selections in Java Programming

The boolean Type and Relational Operators

• Purpose: Compare two values.

• Relational Operators: Six operators used for comparison.

• Result: A boolean value (true or false).

  • Example: boolean b = (1 > 2); results in b being false.

• List of Relational Operators:

  • < (less than) - Math: <

  • <= (less than or equal to) - Math: \le

  • > (greater than) - Math: >

  • >= (greater than or equal to) - Math: \ge

  • == (equal to) - Math: =

  • != (not equal to) - Math: \ne

One-Way if Statements

• Syntax:

  java if (boolean-expression) { statement(s); }

• Execution Flow:

  • If boolean-expression is true, statement(s) execute.

  • If boolean-expression is false, statement(s) are skipped.

• Key Points on Syntax:

  • Braces {} are recommended for readability, required for multiple statements.

  • Optional for a single statement:

    • if (i > 0) System.out.println("i is positive");

  • Common Error: Do not place a semicolon after the if condition, e.g., if (i > 0); will lead to logic errors.