SCJP 5 : Chapter 2. Flow Control (Part-1)

The syntax of the switch statement is extended ever-so-slightly. The type of the Expression is now permitted to be an enum class. (Note that java.util.Enum is not an enum class.) A new production is added for SwitchLabel:

SwitchLabel:
.....case EnumConst :
.....
EnumConst:
.....Identifier

The Identifier must correspond to one of UNQUALIFIED enumeration constants.

Here is a slightly more complex enum declaration for an enum type with an explicit instance field and an accessor for this field. Each member has a different value in the field, and the values are passed in via a constructor. In this example, the field represents the value, in cents, of an American coin.

public enum Coin { 

            .....PENNY(1), NICKEL(5), DIME(10), QUARTER(25); 

            

            .....Coin(int value) { 

            ..........this.value = value; 

            .....} 

            .....private final int value; 

            .....public int getValue() { return value; } 

            }

Switch statements are useful for simulating the addition of a method to an enum type from outside the type. This example "adds" a color method to the Coin class, and prints a table of coins, their values, and their colors.

import static java.lang.System.out; 

            

            public class CoinTest { 

            .....public static void main(String[] args) { 

            ..........for (Coin c : Coin.values()) { 

            ...............out.println(c + ": \t" + c.getValue() + "c \t" + color(c)); 

            ..........} 

            .....} 

            

            .....private enum CoinColor { 

            ..........COPPER, NICKEL, SILVER 

            .....} 

            

            .....private static CoinColor color(Coin c) { 

            ..........if (c == null) { 

            ...............throw new NullPointerException(); 

            ..........} 

            

            ..........switch (c) { 

            ..........// case Coin.PENNY: {} // Compile error! Must be UNQUALIFIED !!! 

            ...............case PENNY: 

            ....................return CoinColor.COPPER; 

            ...............case NICKEL: 

            ....................return CoinColor.NICKEL; 

            ...............case DIME: 

            ....................return CoinColor.SILVER; 

            ...............case QUARTER: 

            ....................return CoinColor.SILVER; 

            ..........// case 2: {} // Compile error !!! 

            ..........// Type mismatch: cannot convert from int to Coin 

            ..........} 

            ..........throw new AssertionError("Unknown coin: " + c); 

            .....} 

            }

Running the program prints:

PENNY: 1c COPPER
NICKEL: 5c NICKEL
DIME: 10c SILVER
QUARTER: 25c SILVER

______________

Author: Mikalai Zaikin. Please Click Here to visit Authors site for any updates and changes to the study notes.

SCJP 5 : Chapter 2. Flow Control (Part-1)

Develop code that implements an if or switch statement; and identify legal argument types for these statements.

Tags