Java SE 7 新功能與改進:switch 指令支援字串

在 Java SE 7 中,可以在 switch 述句中使用 String 物件:

// Before
Mode mode = null;
String day = getDay();
if ("Monday".equals(day) || "Wednesday".equals(day)) {
    mode = MODE.TEA_TIME;
} else if ("Friday".equals(day)) {
    mode = MODE.BEER;
} else if ("Saturday".equals(day) || "Sunday".equals(day)) {
    mode = MODE.SLEEP;
} else {
    mode = MODE.WORK;
}

// After
Mode mode = null;
String day = getDay();
switch (day) {
    case "Monday":
    case "Wednesday": mode = MODE.TEA_TIME; break;
    case "Friday":    mode = MODE.BEER;     break;
    case "Saturday":
    case "Sunday":    mode = MODE.SLEEP;    break;
    default:          mode = MODE.WORK;
}

switch 述句會使用 String.equals 方法來比較每一個 case 標籤的字串物件,因此在 switch 述句中的字串比對是有大小寫分別的。

// Truth in JVM
Mode mode = null;
String day = getDay();
byte byte0 = -1;
switch(day.hashcode()) {
    case -1984635600:
	    if (day.equals("Monday")) {
		    byte0 = 0;
		}
        break;
    case -897468618:
	    if (day.equals("Wednesday")) {
		    byte0 = 1;
	    }
        break;
    case 2112549247:
	    if (day.equals("Friday")) {
		    byte0 = 2;
	    }
        break;
    // Others...
}

switch(byte0) {
    case 0:
    case 1: mode = MODE.TEA_TIME; break;
    case 2: mode = MODE.BEER;     break;
    // Others...
}

和一連串的 if-then-else 相比,在 switch 述句中使用字串會更有效率,並且容易閱讀。

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