使用条件语句的细微差别

if1.语句顺序

有时，程序需要根据变量的值或表达式的值执行许多不同的操作。

假设我们的任务是这样的：

• 如果温度大于20度，则穿上衬衫
• 如果温度大于10度且小于（或等于）20，则穿上毛衣
• 如果温度大于0度且小于（或等于）10，则穿上雨衣
• 如果温度低于0度，则穿上外套。

这是如何用代码表示的：

int temperature = 9;

if (temperature > 20)
   System.out.println("put on a shirt");
else // Here the temperature is less than (or equal to) 20
{
   if (temperature > 10)
      System.out.println("put on a sweater");
   else // Here the temperature is less than (or equal to) 10
   {
      if (temperature > 0)
         System.out.println("put on a raincoat");
      else // Here the temperature is less than 0
         System.out.println("put on a coat");
   }
}

If-else语句可以相互嵌套。这使得在程序中实现相当复杂的逻辑成为可能。

但是上面的例子也很有趣，因为我们可以通过省略大括号使代码更简单一些：

int temperature = 9;

if (temperature > 20)
   System.out.println("put on a shirt");
else // Here the temperature is less than (or equal to) 20
   if (temperature > 10)
      System.out.println("put on a sweater");
   else // Here the temperature is less than (or equal to) 10
      if (temperature > 0)
         System.out.println("put on a raincoat");
      else // Here the temperature is less than 0
         System.out.println("put on a coat");

但是，程序员通常会以不同的方式编写此构造：

int temperature = 9;

if (temperature > 20)
   System.out.println("put on a shirt");
else if (temperature > 10) // Here the temperature is less than (or equal to) 20
   System.out.println("put on a sweater");
else if (temperature > 0) // Here the temperature is less than (or equal to) 10
   System.out.println("put on a raincoat");
else // Here the temperature is less than 0
   System.out.println("put on a coat");

所有这三个例子都是等价的。

2.else方块的细微差别

例子：

int age = 65;

if (age < 60)
   if (age > 20)
      System.out.println("You must work");
else
   System.out.println("You don't have to work");

int age = 65;

if (age < 60)
{
   if (age > 20)
     System.out.println("You must work");
   else
     System.out.println("You don't have to work");
}

如果您查看左侧的代码，屏幕输出似乎将是“您不必工作”。但事实并非如此。实际上，else块和“你不必工作”声明与第二个（更接近的）if声明相关联。

在右侧的代码中，关联的if和else以红色突出显示。此外，花括号的位置很明确，清楚地显示了将执行的操作。当字符串You don't have to work never displayed when ageis greater than时60？

3.使用if-else语句的例子

由于我们if-else很好地探索了这个陈述，让我们举一个例子：

import java.util.Scanner;
public class Solution {
   public static void main(String[] args)
   {
     Scanner console = new Scanner(System.in); // Create a Scanner object
     int a = console.nextInt(); // Read the first number from the keyboard
     int b = console.nextInt(); // Read the second number from the keyboard
     if (a < b)                   // If a is less than b
       System.out.println(a);     // we display a
     else                         // otherwise
       System.out.println(b);     // we display b
   }
}