從零自學Java-5.使用條件測試進行判斷

1.使用if語句進行最基本的條件測試；
2.測試一個值大於還是小於另一個值；
3.測試兩個值是否相等；
4.使用與if語句對應的else語句；
5.組合多個條件測試；
6.使用switch語句進行複雜的條件測試；
7.使用三元運算符創建測試；

程式Game：if語句的初步使用

 1 package com.jsample;
 2 
 3 public class Game {
 4     public static void main(String[] args){
 5         int total = 0;
 6         int score = 7;
 7         if(score == 7){
 8             System.out.println("You score a touchdown!");
 9         }
10         if(score == 3){
11             System.out.println("You kick a field goal!");
12         }
13         total = total + score;
14         System.out.println("Total score: " + total);
15     }
16 }

輸出：

You score a touchdown!
Total score: 7

程式Commodity：使用switch語句來購買或銷售東西

 1 package com.jsample;
 2 
 3 public class Commmodity {
 4     public static void main(String[] args){
 5         String command = "BUY";//指令被設定為“BUY”
 6         int balance = 550;
 7         int quantity = 42;
 8 
 9         switch (command) {
10             case "BUY":
11                 quantity += 5;
12                 balance -= 20;
13                 break;
14             case "SELL":
15                 quantity -= 5;
16                 balance += 15;
17         }
18         System.out.println("Balance: " + balance + "\n"
19          + "Quantity: " + quantity);
20     }
21 }

輸出：

Balance: 530
Quantity: 47

程式Clock：使用Java內置的計時功能，跟蹤當前的日期和時間，並將信息用一句話顯示出來

 1 package com.jsample;
 2 
 3 import java.time.*; //讓程式能夠使用類java.time.LocalDateTime,它用於跟蹤當前的日期和時間
 4 import java.time.temporal.*;//讓程式能夠使用java.time.temporalfield.ChronoField
 5 
 6 public class Clock {//開始Clock程式及其main()語句塊
 7     public static void main(String[] args){
 8         //get current time and date
 9         LocalDateTime now = LocalDateTime.now();//創建一個名為now的LocalDateTime對象，該對象包含系統的當前日期和時間
10         int hour = now.get(ChronoField.HOUR_OF_DAY);//創建變數hour,month,day,year，這些變數的值來自LocalDateTime對象
11         int minute = now.get(ChronoField.MINUTE_OF_HOUR);
12         int month = now.get(ChronoField.MONTH_OF_YEAR);
13         int day = now.get(ChronoField.DAY_OF_MONTH);
14         int year = now.get(ChronoField.YEAR);
15 
16         //display greeting
17         if(hour < 12){//顯示三個問候語之一，顯示的內容取決於變數hour的值
18             System.out.println("Good morning.\n");
19         }else if (hour < 17){
20             System.out.println("Good afternoon.\n");
21         }else {
22             System.out.println("Good evening");
23         }
24 
25         //begin time message by showing the minutes
26         System.out.print("It's");//根據變數minute的值來顯示具體的分鐘數
27         if (minute != 0){
28             System.out.print(" " + minute + " ");
29             System.out.print((minute != 1) ? "minutes" : "minute");
30             System.out.print(" past");
31         }
32 
33         //display the hour
34         System.out.print(" ");//顯示十二小時制下的hour值
35         System.out.print((hour > 12) ? (hour - 12) : hour);
36         System.out.print(" o'clock on ");
37 
38         //display the name of the month
39         switch (month){//根據變數month的值來顯示不同的月份名稱
40             case 1:System.out.print("January");break;
41             case 2:System.out.print("February");break;
42             case 3:System.out.print("March");break;
43             case 4:System.out.print("April");break;
44             case 5:System.out.print("May");break;
45             case 6:System.out.print("June");break;
46             case 7:System.out.print("July");break;
47             case 8:System.out.print("August");break;
48             case 9:System.out.print("September");break;
49             case 10:System.out.print("October");break;
50             case 11:System.out.print("November");break;
51             case 12:System.out.print("December");break;
52         }
53 
54         //display the date and year
55         System.out.println(" " + day + "," + year + ".");//顯示當前的日期和年份
56     }//結束main()語句塊
57 }//結束整個clock程式

輸出：

Good morning.

It's 15 minutes past 9 o'clock on March 16,2018.

程式GradeGame：存儲用戶輸入的成績（0-100），自動分等級並輸出評語（分別以if語句和switch語句實現）

 1 package com.jsample;
 2 
 3 public class GradeGame {
 4     public static void main(String[] args){
 5         int grade = Integer.parseInt(args[0]);
 6         char gpa = 'E';
 7 
 8         if (grade > 80)
 9         {
10             System.out.println("A:Perfect");
11             gpa = 'A';
12         }
13         else if (grade > 60)
14         {
15             System.out.println("B Good");
16             gpa = 'B';
17         }
18         else if (grade > 40)
19         {
20             System.out.println("C Not bad");
21             gpa = 'C';
22         }
23         else if (grade > 20)
24         {
25             System.out.println("D You still have lots more to work on");
26             gpa = 'D';
27         }
28         else
29         {
30             System.out.println("F Not even wrong");
31             gpa = 'F';
32         }
33 
34         switch (gpa){
35             case 'A':System.out.println("A:Perfect");break;
36             case 'B':System.out.println("B Good");break;
37             case 'C':System.out.println("C Not bad");break;
38             case 'D':System.out.println("D You still have lots more to work on");break;
39             case 'F':System.out.println("F Not even wrong");break;
40             default:System.out.println("Who's your daddy");break;
41         }
42     }
43 }

輸入：

65

輸出：

B Good
B Good