The if Statement

Decides whether or not a section of code executes.

• Uses a boolean to decide

if (boolean expression is true)
    execute next statement

Flowcharts

If statements can be modeled as a flow chart.

One-line Style

This—

if (average > 95)
    grade = 'A';

—is functionally equivalent to:

if (average > 95) grade = 'A';

Block if Statements

Curly braces are used to group conditionally executed statements.

e.g.,

if (expression)
{
    statement1;
    statement2;
}

Note: Some trick questions will leave out the curly braces but indent blocks to look like they are conditionally executed when only the first line is conditional.

Relational Operators and Boolean Expressions

Boolean Expression: Any variable or calculation that results in a true or false condition.

Typically, we’ll use relational operators in boolean expressions because they return true or false.

Reminder: = is the assignment operator, == is the comparison operator!

Flag

Flag: boolean variable that monitors some condition in a program

• When a condition is true, flag is set to true
• e.g.,

boolean isPrime = true;
if (5 % 4 == 0)
    isPrime = false;
if (5 % 3 == 0)
    isPrime = false;
if (5 % 2 == 0)
    isPrime = false;
if (isPrime)
    System.out.println("5 is prime");

Comparing Characters

Characters can be tested with relational operators.

• Characters are stored in memory using the Unicode character format
  • Unicode is stored as a 16-bit number
• Characters a ordinal, they have an order in the Unicode character set
  • Because characters are ordinal, they can be compared to each other.
    • e.g., “A” is represented by 65, “B” is represented by 66, etc. so we don’t even need to reference the Unicode table to compare chars.

char x = 'A';
if (x < 'B')
    System.out.println("A is less than B");

if-else Statements

if (expression)
    statementTrue;
else
    statementFalse;

Note: Curly braces not required if there is only one statement

• But they can help with readability

Nested if statements

Nested if Statement: An if statement inside another if statement (single or block, doesn’t matter)

• Executed only if the outer if statement is true.

if (coldOutside)
{
    if (snowing)
    {
        wearParka();
    }
    else
    {
        wearJacket();
    }
}
else
{
    wearShorts();
}

if-else-if Statements

• Makes certain types of nested decision logic simpler to write.
• Nested if can become very complex.

if (expression)
{
    // a
}
else if (expression)
{
    // b
}
else if (expression)
{
    // c
}
else{
    // catch whatever
}

Note how only one block can be executed

Logical Operators

Two logical (AND, OR) and one unary (NOT) operators are provided in Java.

The System.out.printf Method

Print formatted string.

• Format specifier syntax: %[flags][width][.precision]conversion
  • Flags: Padding with zeros, left-justifying, comma separators
  • Width: Minimum field width for value
  • .precision: Number of decimal places number should be rounded to
  • Conversion: f for floating-point, d for decimal integer, s for string

Examples:

double grossPay = 874.12;
System.out.printf("Your pay is %f \n", grossPay);

• %f: Format specifier. Indicates that a floating-point value will be used.

double grossPay = 874.12111;
System.out.printf("Your pay is %.2f \n", grossPay);

• %.2f: Format specifier. Indicates that a floating-point value will be printed, rounded to two decimal placed.

double grossPay = 5874.12111;
System.out.printf("Your pay is %,.2f \n", grossPay);

• %,.2f: Format specifier. Indicates that a floating-point value will be printed, rounded to two decimal placed, with commas separating thousands

String name = "Ringo";
System.out.printf("Your name is %s", name);

• %s: Format specifier. Indicates that a string will be printed.

int number = 9;
System.out.printf("The value is %6d", number);

• %6d: Format specifier. Indicates that an integer will appear in a field that is 6 spaces wide.

String.format

Works exactly like System.out.printf, except it returns a reference to the formatted string instead of printing to the console.

Comparing String Objects

Common Mistake: Trying to compare two String objects with relational operators (==)

• Objects are just pointers to their location in RAM; you need to use object methods to compare strings.

String name1 = "john", name2 = "John";
String name3 = new String("John");
String name4 = new String("John");

• name1 and name2 share the same memory address; name3 and name4 each have their own unique pointers.

Common methods:

• equals()
• equalsIgnoreCase()
• compareTo()
  • Compares string lexicographically (character by character)
• compareToIgnoreCase()

The Conditional Operator

BooleanExpression ? Value1 : Value2

• Forms a conditional expression
  • Is a ternary (three operand) operator
  • Can be used to simple statements that work like if-else
• If BooleanExpression is true, value of conditional expression is Value1
• If BooleanExpression is false, value of conditional expression is Value2

z = x > y ? 10 : 5;
// functional equivalent:
if (x>y)
    z = 10;
else
    z = 5;

The switch Statement

Allows you to use value of variable/expression to determine how a program will branch.

switch (SwitchExpression)
{
    case CaseExpression;
        break;
    case CaseExpression;
        break;
    case CaseExpression;
        break;
    default:
        // blah
}

• SwitchExpression can be a char, byte, short, int, long, or String
• If SwitchExpressions matched the CaseExpression, the statements between the colon and break will be executed.

The case Statement

• break ends the case
  • break is optional, this allows the program to “fall through” statements
• default section is optional, executed if no CaseExpression matches