分支控制

package chapter04.D1分支控制;
​
import java.util.Scanner;
​
public class If {
   public static void main(String[] args) {
     Scanner myscanner = new Scanner(System.in);
     System.out.println("請輸入1-100信用分");
     int grade = myscanner.nextInt();
     //先對輸入的信用分進行範圍有效判斷1-100
     if (grade >= 1 && grade <= 100){
       if (grade == 100){
         System.out.println("信用極好");
       }else if (grade > 80 && grade <= 90){
         System.out.println("信用優秀");
       }else if (grade >=60 && grade <= 80){
         System.out.println("信用一般");
       }else{
         System.out.println("信用不及格");
       }
     }else {
       System.out.println("信用需在1-100，請重新輸入");
     }
     //如果輸入的不是整數，而是字串，可用異常處理機制
   }
}
 

巢狀分支

package chapter04.D2巢狀分支;
​
import java.util.Scanner;
​
public class NestedIf {
   public static void main(String[] args) {
     Scanner myscanner = new Scanner(System.in);
     System.out.println("請輸入成績");
     double score = myscanner.nextDouble();
     if (score>8.0){
       System.out.println("請輸入性別");
       char gender = myscanner.next().charAt(0);//字串轉字元
       if(gender == '男'){
         System.out.println("進入男子組");
       }else if(gender == '女'){
         System.out.println("進入女子組");
       }else {
         System.out.println("性別輸入有誤");
       }
     }else {
       System.out.println("被淘汰了");
     }
   }
}
 

穿透分支

package chapter04.D3穿透分支;
​
import java.util.Scanner;
​
public class Switch01 {
   public static void main(String[] args) {
     Scanner myscanner = new Scanner(System.in);
     System.out.println("請輸入字元a-g");
     char c1=myscanner.next().charAt(0);
     switch (c1){
       case 'a':
         System.out.println("今天星期一");
         break;
       case 'b':
         System.out.println("今天星期二");
         break;
       case 'c':
         System.out.println("今天星期三");
         break;
       default:
         System.out.println("輸入有誤");//無需break也會退出
     }
   }
   }
 

package chapter04.D3穿透分支;
​
public class SwitchDetail {
   public static void main(String[] args) {
     char c = 'a';//表示式資料型別
     switch (c){
       case 'a'://要和case後的常量型別一致，或是可自動轉化的型別 char>int
         System.out.println("ok1");
         break;
       case 20:
         System.out.println("ok2");
         break;
       default:
         System.out.println("ok3");
     }
   }
   /*
   1 表示式型別只能是 byte short int char enum[列舉] String
   case 語句中的值必須是常量或常量表達式，不能是變數如
   char c = 'a';
   char c2 = 'c';
   2 default語句是可選的，沒有匹配的case執行default
   若沒有default語句，沒有輸出
   3 break 跳出switch語句塊
   沒寫break 順序執行switch到結尾，除非遇到break （穿透）
   */
}

package chapter04.D3穿透分支;
​
import java.util.Scanner;
​
public class SwitchExercise {
   public static void main(String[] args) {
     System.out.println("請輸入成績");
     Scanner myscanner = new Scanner(System.in);
     int score=myscanner.nextInt();
​
     if (score>=0&&score<=100){
       switch ((int) (score/60)){
         case 0:
           System.out.println("不及格");
           break;
         case 1:
           System.out.println("及格");
           break;
       }
     }else {
       System.out.println("輸入有誤，請在0-100間");
     }
​
   }
}

package chapter04.D3穿透分支;
​
import java.util.Scanner;
​
public class SwitchExercise02 {
   public static void main(String[] args) {
     System.out.println("輸入月份");
     Scanner myscanner = new Scanner(System.in);
     int month = myscanner.nextInt();
     switch (month){
       case 3:
       case 4:
       case 5:
         System.out.println("春季");
         break;
       case 6:
       case 7:
       case 8:
         System.out.println("夏季");
         break;
       case 9:
       case 10:
       case 11:
         System.out.println("秋季");
         break;
       case 12:
       case 1:
       case 2:
         System.out.println("冬季");
         break;
       default:
         System.out.println("輸入有誤");
     }
​
   }
}

if與switch的比較

switch 和 if 比較
1 如果判斷具體數值不多，符合 byte short int char enum[列舉] String 6型別，建議用switch（雖然if也能用）
2 對於區間和邏輯判斷，用if if使用範圍更廣