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Switch

Table of contents

  1. Switch Control Flow Statement
    1. Switch Example
  2. Switch Fallthrough Example
  3. Switch Default Example
  4. Switch Expressions
  5. Pattern matching for switch

Switch Control Flow Statement

Consider an application that prints a menu to the console and preforms the action selected by the user.

package demo;

public class App {

  public static void main( final String[] args ) {
    System.out.println( "---------------------" );
    System.out.println( "Menu" );
    System.out.println( "---------------------" );
    System.out.println( " a c    Add Data" );
    System.out.println( " u      Update Data" );
    System.out.println( " x      Delete Data" );
    System.out.println( "---------------------" );

    final char input = 'a';
    System.out.printf( "User selected %s%n", input );
  }

  private static void addData() { /* add */ }

  private static void updateData() { /* update */ }

  private static void deleteData() { /* delete */ }
}

Switch Example

The if/else-if/else can be used to solve this problem, but the switch statement is a better fit

switch ( input ) {
  case 'a':
    addData();
    break;
}

Complete solution

package demo;

public class App {

  public static void main( final String[] args ) {
    System.out.println( "---------------------" );
    System.out.println( "Menu" );
    System.out.println( "---------------------" );
    System.out.println( " a c    Add Data" );
    System.out.println( " u      Update Data" );
    System.out.println( " x      Delete Data" );
    System.out.println( "---------------------" );

    final char input = 'a';

    switch ( input ) {
      case 'a':
        addData();
        break;
      case 'c':
        addData();
        break;
      case 'u':
        updateData();
        break;
      case 'x':
        deleteData();
        break;
    }
  }

  private static void addData() { /* add */ }

  private static void updateData() { /* update */ }

  private static void deleteData() { /* delete */ }
}

Switch Fallthrough Example

Both ā€˜aā€™ and ā€˜cā€™ call the addData() function. The switch statement supports fallthrough which enables the switch to continue from the first match until this is stopped or reaches the end of the switch.

switch ( input ) {
  case 'a':
  case 'c':
    addData();
    break;
}

Complete solution

package demo;

public class App {

  public static void main( final String[] args ) {
    System.out.println( "---------------------" );
    System.out.println( "Menu" );
    System.out.println( "---------------------" );
    System.out.println( " a c    Add Data" );
    System.out.println( " u      Update Data" );
    System.out.println( " x      Delete Data" );
    System.out.println( "---------------------" );

    final char input = 'a';

    switch ( input ) {
      case 'a':
      case 'c':
        addData();
        break;
      case 'u':
        updateData();
        break;
      case 'x':
        deleteData();
        break;
    }
  }

  private static void addData() { /* add */ }

  private static void updateData() { /* update */ }

  private static void deleteData() { /* delete */ }
}

Fallthrough is a discouraged practice by some because it can lead to issues. For example, if we forget the break between the cases, we may end up executing all cases. I use the fallthrough option whenever I see fit as it. With that said, one needs to be aware of the potential problem.

Switch Default Example

When the user input an invalid option, the program simple exits. Improve the program such that it displays a message indicating that the input is invalid.

Invalid input provided!!

The switch statement provides the default block which captures any other cases. The default block needs to be the last block, thus break statement is not required.

switch ( input ) {
  case 'x':
    deleteData();
    break;
  default:
    System.out.println( "Invalid input provided!!" );
}

Complete solution

package demo;

public class App {

  public static void main( final String[] args ) {
    System.out.println( "---------------------" );
    System.out.println( "Menu" );
    System.out.println( "---------------------" );
    System.out.println( " a c    Add Data" );
    System.out.println( " u      Update Data" );
    System.out.println( " x      Delete Data" );
    System.out.println( "---------------------" );

    final char input = 'z';

    switch ( input ) {
      case 'a':
      case 'c':
        addData();
        break;
      case 'u':
        updateData();
        break;
      case 'x':
        deleteData();
        break;
      default:
        System.out.println( "Invalid input provided!!" );
    }
  }

  private static void addData() { /* add */ }

  private static void updateData() { /* update */ }

  private static void deleteData() { /* delete */ }
}

Switch Expressions

Java 14 introduced switch expressions (JEP 361)

Consider the following example

package demo;

public class App {
  public static void main( final String[] args ) {
    final int a = 2;

    switch ( a ) {
      case 1:
      case 3:
        System.out.println( "Options 1 or 3" );
        break;
      case 2:
        System.out.println( "Option 2" );
        break;
      default:
        System.out.println( "Anything other than 1, 2 or 3" );
    }
  }
}

The switch statement is printing a String based on the value if variable a.

This can be converted such that the switch statement becomes and expression that evaluates to a value, String this time.

package demo;

public class App {
  public static void main( final String[] args ) {
    final int a = 2;

    final String result = switch ( a ) {
      case 1, 3 -> "Options 1 or 3";
      case 2 -> "Option 2";
      default -> "Anything other than 1, 2 or 3";
    };

    System.out.println( result );
  }
}

Both examples will output

Option 2

Pattern matching for switch

šŸš§ Pending šŸš§

JEP draft: Pattern matching for switch (Preview)