Switch Statement ​

In addition to the if statement, there is also a conditional judgment, which is to execute different branches based on the result of a certain expression.

For example, in a game, let the user choose an option:

• Single player mode
• Multiplayer mode
• Exit game

At this time, the switch statement comes in handy.

switch statement jumps to the matching case result based on the result calculated by switch (expression) , and then continues to execute subsequent statements until it encounters break to end execution.

Let's look at an example:

public class Main {
  public static void main(String[] args) {
    int option = 1;
    switch (option) {
      case 1:
        System.out.println("Selected 1");
        break;
      case 2:
        System.out.println("Selected 2");
        break;
      case 3:
        System.out.println("Selected 3");
        break;
    }
  }
}

Modify option values to 1 , 2 , and 3 respectively, and observe the execution results.

If the value of option does not match any case , such as option = 99 , then the switch statement will not execute any statement. At this time, you can add a default to switch statement. When no case is matched, default is executed:

public class Main {
    public static void main(String[] args) {
        int option = 99;
        switch (option) {
        case 1:
            System.out.println("Selected 1");
            break;
        case 2:
            System.out.println("Selected 2");
            break;
        case 3:
            System.out.println("Selected 3");
            break;
        default:
            System.out.println("Selected other");
            break;
        }
    }
}

If the switch statement is translated into an if statement, then the above code is equivalent to:

if (option == 1) {
    System.out.println("Selected 1");
} else if (option == 2) {
    System.out.println("Selected 2");
} else if (option == 3) {
    System.out.println("Selected 3");
} else {
    System.out.println("Selected other");
}

Compared with if ... else if statement, for the case of multiple == judgments, it is clearer to use switch structure.

At the same time, note that the above "translation" can only be matched if the break statement is correctly written for each case in the switch statement.

When using switch , please note that case statement does not have curly braces {} . Moreover, case statement is " penetrating ", and omitting break will lead to unexpected results:

public class Main {
    public static void main(String[] args) {
        int option = 2;
        switch (option) {
        case 1:
            System.out.println("Selected 1");
        case 2:
            System.out.println("Selected 2");
        case 3:
            System.out.println("Selected 3");
        default:
            System.out.println("Selected other");
        }
    }
}

When option = 2 , "Selected 2" , "Selected 3" , and "Selected other" will be output in sequence. The reason is that starting from matching case 2 , all subsequent statements will be executed until the break statement is encountered. Therefore, never forget to write break at any time.

If there are several case statements executing the same set of statement blocks, you can write it like this:

public class Main {
    public static void main(String[] args) {
        int option = 2;
        switch (option) {
        case 1:
            System.out.println("Selected 1");
            break;
        case 2:
        case 3:
            System.out.println("Selected 2, 3");
            break;
        default:
            System.out.println("Selected other");
            break;
        }
    }
}

When using switch statement, as long as there is break , the order of case does not affect the program logic:

switch (option) {
case 3:
    ...
    break;
case 2:
    ...
    break;
case 1:
    ...
    break;
}

However, it is still recommended to arrange them in natural order for easier reading.

The switch statement can also match strings. When string matching, "content equality" is compared. For example:

public class Main {
    public static void main(String[] args) {
        String fruit = "apple";
        switch (fruit) {
        case "apple":
            System.out.println("Selected apple");
            break;
        case "pear":
            System.out.println("Selected pear");
            break;
        case "mango":
            System.out.println("Selected mango");
            break;
        default:
            System.out.println("No fruit selected");
            break;
        }
    }
}

The switch statement can also use enumeration types. We will explain enumeration types later.

Compilation Check ​

When using an IDE, you can automatically check whether break statements and default statements are missing by turning on the IDE's compilation check.

In Idea, select Preferences - Editor - Inspections - Java - Control flow issues and mark the following inspection as Warning:

• 'switch' statement without 'default' branch
• Fallthrough in 'switch' statement

When there is a problem with the switch statement, you can get a warning prompt in the IDE.

Switch Expression ​

When using switch , if break is omitted, serious logical errors will occur, and it is difficult to find errors in the source code.

Starting from Java 12, switch statement is upgraded to a more concise expression syntax, using a method similar to pattern matching (Pattern Matching) to ensure that only one path will be executed and no break statement is required:

public class Main {
    public static void main(String[] args) {
        String fruit = "apple";
        switch (fruit) {
        case "apple" -> System.out.println("Selected apple");
        case "pear" -> System.out.println("Selected pear");
        case "mango" -> {
            System.out.println("Selected mango");
            System.out.println("Good choice!");
        }
        default -> System.out.println("No fruit selected");
        }
    }
}

Note that the new syntax uses -> . If there are multiple statements, they need to be enclosed in {} . Do not write break statements, because the new syntax will only execute matching statements and has no penetration effect.

Many times, we may also use switch statement to assign a value to a variable. For example:

int opt;
switch (fruit) {
case "apple":
    opt = 1;
    break;
case "pear":
case "mango":
    opt = 2;
    break;
default:
    opt = 0;
    break;
}

Using the new switch syntax, not only does break need not be required, but the value can also be returned directly. Rewrite the above code as follows:

public class Main {
    public static void main(String[] args) {
        String fruit = "apple";
        int opt = switch (fruit) {
            case "apple" -> 1;
            case "pear", "mango" -> 2;
            default -> 0;
        }; // 注意赋值语句要以;结束
        System.out.println("opt = " + opt);
    }
}

This results in cleaner code.

yield ​

Most of the time, inside switch expression, we return simple values.

However, if complex statements are needed, we can also write many statements and put them in {...} , and then use yield to return a value as the return value of the switch statement:

public class Main {
    public static void main(String[] args) {
        String fruit = "orange";
        int opt = switch (fruit) {
            case "apple" -> 1;
            case "pear", "mango" -> 2;
            default -> {
                int code = fruit.hashCode();
                yield code; // switch statement return value
            }
        };
        System.out.println("opt = " + opt);
    }
}

Summary ​

The switch statement can make multiple selections, and then execute the subsequent code of the matching case statement;

The calculation result of switch must be an integer, string or enumeration type;

Be careful not to miss out break , it is recommended to turn on fall-through warning;

Always write default , it is recommended to turn on the missing default warning;

Starting from Java 14, the switch statement is officially upgraded to an expression, break is no longer required, and yield return value is allowed.