switch文

中級 Java

Jim White

Java Developer

switchとは？

switchはif-elseと同様の目的で使う
- 条件に応じて処理を行う
- 複数のif-then-elseの代替として使用
複数のif-then-elseより簡潔なことが多い

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の構文

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

switchの例

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

Javaが残りのcaseへ進むのを止める
- breakがないと、次のcaseの文をbreakまで実行する

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で複数のcaseをまとめられる:

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

任意で末尾にdefaultケースを追加できる
- defaultは式と比較する値を持たない
- どのcaseにも一致しない場合、defaultの文が実行される
- defaultはswitchの最後に置く必要がある
defaultは「その他すべて」のケースに相当

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.

if文とswitchの比較

条件分岐でも同じ処理は可能
あらゆるデータ型で使用可

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は読みやすい場合がある
一部の型でのみ使用可（int、byte、short、char、Stringなど）

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");
}

Ayo berlatih!

中級 Java