Switch

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layout: default title: switch Statements: Multi-Way Decision

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Sometimes we'll compare a value with several different values, and follow a different execution path depending on which value matches.

A switch statement compares a single value to a list of possibilities.

int answer;
//...
switch (answer) {
  case 0:
    statements;
    statements;
    break;
  case 1:
    statements;
    statements;
    break;
  case 2:
    statements;
    statements;
    break;
  default:
    statements;
    statements;
}

This switch compares answer to each case value, in order.

When it finds a matching case, it executes the statements in that case.

char grade = 'A';
String message;

switch (grade) {
case 'A':
  message = "Keep it up";
  break;
case 'B':
  message = "Good job";
  break;
case 'C':
  message = "Doing alright";
  break;
case 'D':
  message = "Let's talk";
  break;
case 'F':
  message = "You should come to class";
  break;
default:
  message = "That's not a grade";
}

Of the data types we know, we can use int, char, and String data types in switch statements.

Practice Exercise

Cases must be constant values, like literals: "A", 12, 'C'. They cannot be variables or anything else Java has to determine at runtime.

switch(num) {
  case 1:
    break;
  case variable: //NO
    break;
}

Drill

Switch/src/drills/SwitchInvestigation.java

We're going to figure out what break; does in a switch. This drill switches on a String value.

• Run the program with a letter grade.
• Comment out the break; statements for case "A" and "B".
• Run the program and enter A. What output do you notice? Uncomment the break in case: "B". Now what happens?
• Move the entire default case before case "A":. Your code will look like:

  switch (grade) {
    default:
      System.out.println("That's not a grade");
    case "A":
      System.out.println("Keep it up");
      //break;
    case "B":
      System.out.println("Good job");
      break;
    //...
  }
  

• Run the program again with the grade Z. What output do you see?

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