[Upgrade to Java 17] switch expression

Mentioned  switch, should be familiar to all Java developers. switch Different logic is executed according to the value of the variable, which is used to replace multi-branch if/else.

However,  switch there are some common problems with using statements.

The first is   the execution of the branch  switch in the statement  , which is carried out downward by default, and needs to be added  to jump out   of the execution. This behavior is prone to bugs.casebreakswitch

trafficControl For example, the method  in the following code  RED will execute all  case branches when the parameter value is , because no  break statement is added.

public class SwitchStatement {

  enum TrafficColor {
    RED, YELLOW, GREEN
  }

  public void trafficControl(TrafficColor color) {
    switch (color) {
      case RED:
        this.stop();
      case GREEN:
        this.go();
      default:
        this.watchOut();
    }
  }

  private void go() {
    System.out.println("Go!");
  }

  private void stop() {
    System.out.println("Stop!");
  }

  private void watchOut() {
    System.out.println("Be careful!");
  }
}

switch Another common use of statements is to assign values ​​to the same variable in different branches. As shown in the code below. This way of writing is more cumbersome.

public class SwitchStatementAssignment {

  enum TrafficColor {
    RED, YELLOW, GREEN
  }

  public String getMessage(TrafficColor color) {
    String message;
    switch (color) {
      case RED:
        message = "Stop!";
        break;
      case GREEN:
        message = "Go!";
        break;
      default:
        message = "Be careful!";
    }
    return message;
  }
}

To solve these two problems,  switch expressions and arrow labels can be used.

switch Expressions were introduced as a preview feature in Java 12, previewed again in Java 13, and made official in Java 14.

switch Can be used as an expression and thus can have its own value. This simplifies the assignment operation. switch The value of the expression is determined by the branch. The following code shows  switch the basic usage of expressions.

public class SwitchExpression {

  public String formatGifts(int number) {
    return switch (number) {
      case 0 -> "no gifts";
      case 1 -> "only one gift";
      default -> number + " gifts";
    };
  }
}

In the above code, switch the expressions use arrows instead of traditional colons. After using the arrow, code execution does not go to the next branch, which is equivalent to adding  break. In the code below, the same  trafficControl method, after using the arrow label, is no longer needed  break.

public class SwitchLabel {

  enum TrafficColor {
    RED, YELLOW, GREEN
  }

  public void trafficControl(TrafficColor color) {
    switch (color) {
      case RED -> System.out.println("Stop!");
      case YELLOW -> System.out.println("Be careful!");
      case GREEN -> System.out.println("Go!");
    }
  }
}

In most cases, a single expression after the arrow label will suffice. If you have complex logic, you can use code blocks. At this time, it needs yield to be used to provide a value.

public class YieldValue {

  public String formatGifts(int number) {
    return switch (number) {
      case 0 -> "no gifts";
      case 1 -> "only one gift";
      default -> {
        if (number < 0) {
          yield "no gifts";
        } else {
          yield number + " gifts";
        }
      }
    };
  }
}

switch It can also be used in statements  using traditional labels  yield.

public class YieldValue2 {

  public String formatGifts(int number) {
    return switch (number) {
      case 0:
        yield "no gifts";
      case 1:
        yield "only one gift";
      default: {
        if (number < 0) {
          yield "no gifts";
        } else {
          yield number + " gifts";
        }
      }
    };
  }
}

The content of the expression  switch is introduced here. See my B station for the corresponding video.

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