OCP Oracle Certified Professional Java SE 17 Developer Study Guide. Jeanne Boyarsky

FIGURE 3.3 The structure of a switch statement

      Because switch statements can be longer than most decision-making statements, the exam may present invalid switch syntax to see whether you are paying attention.

      Combining case Values

      Notice something new in Figure 3.3? Starting with Java 14, case values can now be combined:

       switch(animal) { case 1,2: System.out.print("Lion"); case 3: System.out.print("Tiger"); }

Prior to Java 14, the equivalent code would have been the following:

       switch(animal) { case 1: case 2: System.out.print("Lion"); case 3: System.out.print("Tiger"); }

      As you see shortly, switch expressions can reduce boilerplate code even more!

      See if you can figure out why each of the following switch statements does not compile:

      int month = 5; switch month { // DOES NOT COMPILE case 1: System.out.print("January"); } switch(month) // DOES NOT COMPILE case 1: System.out.print("January"); switch(month) { case 1: 2: System.out.print("January"); // DOES NOT COMPILE }

      The first switch statement does not compile because it is missing parentheses around the switch variable. The second statement does not compile because it is missing braces around the switch body. The third statement does not compile because a comma (,) should be used to separate combined case statements, not a colon (:).

      One last note you should be aware of for the exam: a switch statement is not required to contain any case statements. For example, this statement is perfectly valid:

      switch(month) {}

      Going back to our printDayOfWeek() method, we can rewrite it to use a switch statement instead of if/else statements:

public void printDayOfWeek(int day) { switch(day) { case 0: System.out.print("Sunday"); break; case 1: System.out.print("Monday"); break; case 2: System.out.print("Tuesday"); break; case 3: System.out.print("Wednesday"); break; case 4: System.out.print("Thursday"); break; case 5: System.out.print("Friday"); break; case 6: System.out.print("Saturday"); break; default: System.out.print("Invalid value"); break; } }

      For simplicity, we just print a message if the value is invalid. If you know about exceptions or have already read Chapter 11, “Exceptions and Localization,” it might make more sense to throw an exception in the default branch if no match is found.

      Exiting with break Statements

      Taking a look at our previous printDayOfWeek() implementation, you'll see a break statement at the end of each case and default section. A break statement terminates the switch statement and returns flow control to the enclosing process. Put simply, it ends the switch statement immediately.

      The break statements are optional, but without them the code will execute every branch following a matching case statement, including any default statements it finds. Without break statements in each branch, the order of case and default statements is now extremely important. What do you think the following prints when printSeason(2) is called?

      public void printSeason(int month) { switch(month) { case 1, 2, 3: System.out.print("Winter"); case 4, 5, 6: System.out.print("Spring"); default: System.out.print("Unknown"); case 7, 8, 9: System.out.print("Summer"); case 10, 11, 12: System.out.print("Fall"); } }

      It prints everything!

      WinterSpringUnknownSummerFall

It matches the first case statement and executes all of the branches in the order they are found, including the default statement. It is common, although certainly not required, to use a break statement after every case statement.

       The exam creators are fond of switch examples that are missing break statements! When evaluating switch statements on the exam, always consider that multiple branches may be visited in a single execution.

      Selecting switch Data Types

      As shown in Figure 3.3, a switch statement has a target variable that is not evaluated until runtime. The type of this target can include select primitive data types (int, byte, short, char) and their associated wrapper classes (Integer, Byte, Short, Character). The following is a list of all data types supported by switch statements:

      For this chapter, you just need to know that an enumeration, or enum, represents a fixed set of constants, such as days of the week, months of the year, and so on. We cover enums in more detail in Chapter 7, including showing how they can define variables, methods, and constructors.

       Notice that boolean, long, float, and double are excluded from switch statements, as are their associated Boolean, Long, Float, and Double classes. The reasons are varied, such as boolean having too small a range of values and floating-point numbers having quite a wide range of values. For the exam, though, you just need to know that they are not permitted in switch statements.

      Determining Acceptable Case Values

Not just any variable or value can be used in a case statement. First, the values in each case statement must be compile-time constant values of the same data type as the switch value. This means you can use only literals, enum constants, or final constant variables of the same data type. By final constant, we mean that the variable must be marked with the final modifier and initialized with a literal value in the same expression in which it is declared. For example, you can't have a case statement value that requires executing a method at runtime, even if that method always returns the same value. For