Chapter 3:Decision Structures

 3.1 The if Statement  3.2 The if-else Statement  3.3 The if-else-if Statement  3.4 Nested if Statements  3.5 Logical Operators  3.6 Comparing String Objects  3.7 More About Variable Declaration and Scope  3.9 The switch Statement  3.10 Creating Objects with the DecimalFormat Class  3.12 Common Errors to Avoid

Variable Scope  In Java, a local variable does not have to be declared at the beginning of the method.  The scope of a local variable begins at the point it is declared and terminates at the end of the method.  When a program enters a section of code where a variable has scope, that variable has come into scope, which means the variable is visible to the program.

VariableScope.java // Get the user's first name. String firstName; firstName = JOptionPane.showInputDialog( "Enter your " + "first name."); String lastName; lastName = JOptionPane.showInputDialog( "Enter your " + "last name."); JOptionPane.showMessageDialog(null, "Hello " + firstName + " " + lastName);

The Conditional Operator  The conditional operator is a ternary (three operand) operator.  The conditional operator allows a programmer to write a simple if-else type statement.  The format of the operators is: expression1 ? expression2 : expression3  The conditional operator can also return a value.

The Conditional Operator  The conditional operator can be used as a shortened if-else statement: x > y ? z = 10 : z = 5;  This line is functionally equivalent to: if(x > ) z = 10; else z = 5;

The Conditional Operator  Many times, the conditional operator is used to supply a value. number = x > y ? 10 : 5;  This is functionally equivalent to: if(x > y) number = 10; else number = 5;

ConsultantCharges.java double hours, // To hold the hours worked charges; // To hold the charges String input; // To hold user input input = JOptionPane.showInputDialog( "How many hours were worked? "); hours = Double.parseDouble(input); hours = hours < 5 ? 5 : hours; charges = 50.0 * hours; JOptionPane.showMessageDialog(null, "The charges are $" + charges);

The switch Statement  The if-else statements allow the programmer to make true / false branches.  The switch statement allows the programmer to use an ordinal value to determine how a program will branch.  The switch statement can evaluate an integer type or character type variable and make decisions based on the value.

The switch Statement  The switch statement takes the form: switch (SwitchExpression) { case CaseExpression: // place one or more statements here break; case CaseExpression: // place one or more statements here break; default: // place one or more statements here }

The switch Statement  The switch statement takes an ordinal value (byte, short, int, long, char) as the SwitchExpression. switch (SwitchExpression) { … }  The switch statement will evaluate the expression.  If there is an associated case statement that matches that value, program execution will be transferred to that case statement.

The switch Statement  Each case statement will have a corresponding CaseExpression that must be unique. case CaseExpression: // place one or more statements here break;  If the SwitchExpression matches the CaseExpression, the Java statements between the colon and the break statement will be executed.

The switch Case  The break statement ends the case statement.  The break statement is optional.  If a case does not contain a break, then program execution continues into the next case.

NoBreaks.java switch (number) { case 1: System.out.println("You entered 1."); case 2: System.out.println("You entered 2."); case 3: System.out.println("You entered 3."); default: System.out.println("That's not 1, 2, or 3!"); }

PetFood.java switch(foodGrade) { case 'a': case 'A': System.out.println("30 cents per lb."); break; case 'b': case 'B': System.out.println("20 cents per lb."); break; case 'c': case 'C': System.out.println("15 cents per lb."); break; default: System.out.println("Invalid choice."); }

The switch Case  The default case is optional and will be executed if no CaseExpression matches the SwitchExpression.

SwitchDemo.java // Determine the number entered. switch (number) { case 1: System.out.println("You entered 1."); break; case 2: System.out.println("You entered 2."); break; case 3: System.out.println("You entered 3."); break; default: System.out.println("That's not 1, 2, or 3!"); }

The DecimalFormat Class  When printing out double and float values, the full fractional value will be printed.  The DecimalFormat class can be used to format these values.  In order to use the DecimalFormat class, the Java import statement must be used.  At the top of the program the statement import java.text.DecimalFormat; must be used.

Format1.java double number1 = ; double number2 = ; double number3 = ; double number4 = ; DecimalFormat formatter = new DecimalFormat("#0.00"); // Display the formatted variable contents. System.out.println(formatter.format(number1)); System.out.println(formatter.format(number2)); System.out.println(formatter.format(number3)); System.out.println(formatter.format(number4));

Format2.java double number1 = ; double number2 = ; double number3 = ; double number4 = ; // Create a DecimalFormat object. DecimalFormat formatter = new DecimalFormat("000.00"); // Display the formatted variable contents. System.out.println(formatter.format(number1)); System.out.println(formatter.format(number2)); System.out.println(formatter.format(number3)); System.out.println(formatter.format(number4));

Format3.java double number1 = ; double number2 = ; double number3 = ; double number4 = ; double number5 = ; DecimalFormat formatter = new DecimalFormat("#,##0.00"); System.out.println(formatter.format(number1)); System.out.println(formatter.format(number2)); System.out.println(formatter.format(number3)); System.out.println(formatter.format(number4)); System.out.println(formatter.format(number5));

Format4.java double number1 = 0.12; double number2 = 0.05; DecimalFormat formatter = new DecimalFormat("#0%"); // Display the formatted variable contents. System.out.println(formatter.format(number1)); System.out.println(formatter.format(number2));