从零自学Java-5.使用条件测试进行判断

1.使用if语句进行最基本的条件测试；
2.测试一个值大于还是小于另一个值；
3.测试两个值是否相等；
4.使用与if语句对应的else语句；
5.组合多个条件测试；
6.使用switch语句进行复杂的条件测试；
7.使用三元运算符创建测试；

程序Game：if语句的初步使用

 package com.jsample;

 public class Game {
     public static void main(String[] args){
         int total = 0;
         int score = 7;
         if(score == 7){
             System.out.println("You score a touchdown!");
         }
         if(score == 3){
             System.out.println("You kick a field goal!");
         }
         total = total + score;
         System.out.println("Total score: " + total);
     }
 }

输出：

You score a touchdown!
Total score: 7

程序Commodity：使用switch语句来购买或销售东西

 package com.jsample;

 public class Commmodity {
     public static void main(String[] args){
         String command = "BUY";//指令被设定为“BUY”
         int balance = 550;
         int quantity = 42;

         switch (command) {
             case "BUY":
                 quantity += 5;
                 balance -= 20;
                 break;
             case "SELL":
                 quantity -= 5;
                 balance += 15;
         }
         System.out.println("Balance: " + balance + "\n"
          + "Quantity: " + quantity);
     }
 }

输出：

Balance: 530
Quantity: 47

程序Clock：使用Java内置的计时功能，跟踪当前的日期和时间，并将信息用一句话显示出来

 package com.jsample;

 import java.time.*; //让程序能够使用类java.time.LocalDateTime,它用于跟踪当前的日期和时间
 import java.time.temporal.*;//让程序能够使用java.time.temporalfield.ChronoField

 public class Clock {//开始Clock程序及其main()语句块
     public static void main(String[] args){
         //get current time and date
         LocalDateTime now = LocalDateTime.now();//创建一个名为now的LocalDateTime对象，该对象包含系统的当前日期和时间
         int hour = now.get(ChronoField.HOUR_OF_DAY);//创建变量hour,month,day,year，这些变量的值来自LocalDateTime对象
         int minute = now.get(ChronoField.MINUTE_OF_HOUR);
         int month = now.get(ChronoField.MONTH_OF_YEAR);
         int day = now.get(ChronoField.DAY_OF_MONTH);
         int year = now.get(ChronoField.YEAR);

         //display greeting
         if(hour < 12){//显示三个问候语之一，显示的内容取决于变量hour的值
             System.out.println("Good morning.\n");
         }else if (hour < 17){
             System.out.println("Good afternoon.\n");
         }else {
             System.out.println("Good evening");
         }

         //begin time message by showing the minutes
         System.out.print("It's");//根据变量minute的值来显示具体的分钟数
         if (minute != 0){
             System.out.print(" " + minute + " ");
             System.out.print((minute != 1) ? "minutes" : "minute");
             System.out.print(" past");
         }

         //display the hour
         System.out.print(" ");//显示十二小时制下的hour值
         System.out.print((hour > 12) ? (hour - 12) : hour);
         System.out.print(" o'clock on ");

         //display the name of the month
         switch (month){//根据变量month的值来显示不同的月份名称
             case 1:System.out.print("January");break;
             case 2:System.out.print("February");break;
             case 3:System.out.print("March");break;
             case 4:System.out.print("April");break;
             case 5:System.out.print("May");break;
             case 6:System.out.print("June");break;
             case 7:System.out.print("July");break;
             case 8:System.out.print("August");break;
             case 9:System.out.print("September");break;
             case 10:System.out.print("October");break;
             case 11:System.out.print("November");break;
             case 12:System.out.print("December");break;
         }

         //display the date and year
         System.out.println(" " + day + "," + year + ".");//显示当前的日期和年份
     }//结束main()语句块
 }//结束整个clock程序

输出：

Good morning.

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

程序GradeGame：存储用户输入的成绩（0-100），自动分等级并输出评语（分别以if语句和switch语句实现）

 package com.jsample;

 public class GradeGame {
     public static void main(String[] args){
         int grade = Integer.parseInt(args[0]);
         char gpa = 'E';

         if (grade > 80)
         {
             System.out.println("A:Perfect");
             gpa = 'A';
         }
         else if (grade > 60)
         {
             System.out.println("B Good");
             gpa = 'B';
         }
         else if (grade > 40)
         {
             System.out.println("C Not bad");
             gpa = 'C';
         }
         else if (grade > 20)
         {
             System.out.println("D You still have lots more to work on");
             gpa = 'D';
         }
         else
         {
             System.out.println("F Not even wrong");
             gpa = 'F';
         }

         switch (gpa){
             case 'A':System.out.println("A:Perfect");break;
             case 'B':System.out.println("B Good");break;
             case 'C':System.out.println("C Not bad");break;
             case 'D':System.out.println("D You still have lots more to work on");break;
             case 'F':System.out.println("F Not even wrong");break;
             default:System.out.println("Who's your daddy");break;
         }
     }
 }

输入：

65

输出：

B Good
B Good