Switch Statements

What are switch statements used for?

• Switch statements are often used when you have a menu of options to choose from.
• Algorithms that use switch statements can also be written with a series of nested if..else statements.
• Using switch statements instead of a series of nested if..else statements can make code more readable.
• Default statements are often required to ensure that all cases are addressed, and the switch is exhaustive.

Basic Structure of a Switch Statement

switch(<variable>) {
   case <value1> -> <what you want to do in this case>;
   case <value2> -> <what you want to do in this case>;
   ...
   case <valuex> -> <what you want to do in this case>;
   default -> <what you want to do if none of the other cases match>;
}

Sometimes multiple case values have the same action associated with it. At these times, you can put a comma between the two cases as follows:

switch(<variable>) {
   case <value1>, <value2>, ... <valuex> -> <what you want to do in this case>;
   case <valuey> -> <what you want to do in this case>;
   ...
   case <value> -> <what you want to do in this case>;
   default -> <what you want to do in this case>;
}

Example 1: Training

Consider the following code segment which assigns students in an organization to a specific training session based on which weeks they can volunteer. Students who can volunteer in weeks 1 and 2, will attend training 1; week 3 will attend training 2; week 4 will attend training 3; and all other weeks will attend training 4.

Your Turn

Let's try it in the Java Playground.

• Re-written using a switch statement.
  • use the keyword switch followed by the variable that you want to check the cases against.
  • In the body of the switch, there are a series of case statements.
  • Each case value is followed by the -> operator and what statements to do for that case.
  • The arrow symbol -> is used in switch statements and prevents fall through. This means that once a case statement matches, no other statements will be executed.
  • If the values for a case matches the value of the variable, then the statements associated with that case are executed.
• The re-write is started below.
• Predict the output of the code.
• Run the code to determine if your prediction is correct or not. Modify the weekSelected to have the values 2, 3, and 4 and run each one.
• Add the additional cases that to print Attend training 2 and Attend training 2 for these weeks as well.

Example 2: Calling Methods

Consider a class called Calculator that has methods to calculate the sum and positive difference of two values as follows:

double sum(double value1, double value2);

double posDifference(double value1, double value2);

Based on a string input of + for sum and +- for positive difference, we can call the appropriate method using the conditional operator:

Calculator calc = new Calculator();
double x = //input from user;
double y = //input from user; 
String input = //input from user;
double answer = input.equals("+") ? calc.sum(x, y) : calc.posDifference(x, y);

Expanding on this example, assume Calculator has product and quotient methods as well, the code would be:

Calculator calc = new Calculator();
double x = //input from user;
double y = //input from user; 
String input = //input from user;
double answer = (input.equals("+") ? calc.sum(x, y) 
   : ((input.equals("+-") ? calc.posDifference(x, y)
   : ((input.equals("*") ? calc.product(x, y)
   : calc.quotient(x, y))))));

This code could also be re-written as an if..else statement as follows:

double answer; 
if (input.equals("+")) {
   answer = calc.sum(x, y);
} else {
   if (input.equals("+-")) {
      answer = calc.posDifference(x, y);
   } else {
      if (input.equals("*")) {
         answer = calc.product(x, y);
      } else {
         answer = calc.quotient(x, y);
      }
   }
}

Your Turn

Let's try it in the Java Playground

• The Calculator class is provided for you.
• Predict the output of the code.
• Run the code to determine if your prediction is correct.
• Modify the conditional operator expression to use a switch statement instead.
• Test you code with different x, y, and input values.

Resources

Calculator - Using Conditionals and Lambda Mini-Lab

Next Learn Tutorial

• Learn: Using Switch Expressions