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.
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.
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