Switch 陳述式

Java 中級

Jim White

Java Developer

什麼是 switch？

switch 與 if-else 用途相近。
- 依條件執行動作。
- 可取代多個 if-then-else。
相較多重 if-then-else，更為精簡

if (bonus == 10000) {
    System.out.println("Let's buy a car!");
} else if (bonus == 5000) {
    System.out.println("Let's take a trip!");
} else if (bonus == 1000) {
    System.out.println("Let's save!");
}

Switch 語法

switch (expression) {
    case value1:  // if (expression == value1)
        // statements
    case value2:  // if (expression == value2)
        // statements
    // use as many case statements as needed
}

Switch 範例

char direction = 'N';

switch (direction) {
    case 'E':  
        System.out.println("We are headed east.");
    case 'S':  
        System.out.println("We are headed south.");
    case 'W':  
        System.out.println("We are headed west.");
    case 'N':   
        System.out.println("We are headed north."); // This would be printed
}

break

阻止 Java 繼續執行後續的 case。
- 若無 break，Java 會往下個 case 繼續執行，直到遇到 break。

char direction = 'W';

switch (direction) {
    case 'E':
        System.out.println("We are headed east.");
    case 'S':
        System.out.println("We are headed south.");
    case 'W':
        System.out.println("We are headed west."); // This would be printed
    case 'N':
        System.out.println("We are headed north."); // This would also be printed
}

break 可將多個 case 合併處理：

char direction = 'N';

switch (direction) {
    case 'E':     
    case 'W':  
        System.out.println("We are headed east or west.");
        break;
    case 'S':  
    case 'N':
        System.out.println("We are headed south or north.");
        break;       
}

We are headed south or north.

Default

default 可選，通常放在 switch 的最後。
- default 不與任何值比較
- 若沒有 case 符合，會執行 default 的敘述
- default 必須是 switch 的最後一個 case
default 相當於「其他所有值」的處理

char direction = 'z';

switch (direction) {
  case 'E':     
  case 'W':  
    System.out.println("To east or west.");
    break;
  case 'S':  
  case 'N':
    System.out.println("To south or north.");
    break; 
  default:
    System.out.println("We are lost.");
}

We are lost.

條件判斷 vs. switch

使用條件判斷也能完成同樣工作。
適用於所有資料型別。

char character = 'A';
if (character == 'A') {
    System.out.println("It's A");
} else if (direction =='B') {
    System.out.println("It's B");
} else {
    System.out.println("Unknown");
}

switch 較易閱讀。
僅適用於部分型別，如 int、byte、short、char、String。

char character = 'A';
switch (character) {
  case 'A':
    System.out.println("It's A");
    break;
  case 'B':  
    System.out.println("It's B");
    break;
  default:
    System.out.println("Unknown");
}

一起來練習吧！

Java 中級