AP计算机教程4-2：复杂条件

当你希望两个条件同时为true时怎么办？可以使用逻辑与运算符&&来连接两个布尔表达式，这样仅当两表达式均为true时，最终的结果才为true。你妈妈可能会要求你在打扫房间和完成作业以后才能出门，以下是描述这种场景的代码。

public class Test1
{
   public static void main(String[] args)
   {
     boolean cleanedRoom = true;
     boolean didHomework = false;
     if (cleanedRoom && didHomework) System.out.println("You can go out");
     else System.out.println("No, you can't go out");
   }
}

如果两条件成立一个即可呢？可以使用逻辑或运算符||来连接两个布尔表达式，这样只要其中一个为true最终的结果就会是true。爸爸说出门的话可以步行去或选择在他不用车时开家里的车，以下是描述这种场景的代码。

public class Test2
{
   public static void main(String[] args)
   {
     boolean walking = true;
     boolean carIsAvailable = false;
     if (walking || carIsAvailable) System.out.println("You can go out");
     else System.out.println("No, you can't go out");
   }
}

被称为真值表(truth table)的下表展示了逻辑与和逻辑或的所有可能结果。

What is printed when the following code executes and x has been set to 0?

if (x > 0 && (y / x) == 3) System.out.println("first case");
else System.out.println("second case");

What is printed when the following code executes and x has been set to 0?

if (x > 0 || (y / x) == 3) System.out.println("first case");
else System.out.println("second case");

&&和||均使用短路求解(short circuit evaluation)，这意味着如果第一个条件已经能判断整个表达式的真假，就不用求解第二个条件了。对于逻辑与&&而言，如果第一个表达式为false，则整体必为false，不求解第二个条件。对于逻辑或||而言，如果第一个表达式为true，则整体必为true，不求解第二个条件。

What is printed when the following code executes and x has been set to 0 and y is set to 3?

if (x == 0 || (y / x) == 3) System.out.println("first case");
else System.out.println("second case");

What is printed when the following code executes and x has been set to -1?

String message = "help";
if (x >= 0 && message.substring(x).equals("help") System.out.println("first case");
else System.out.println("second case");

What is printed when the following code executes and x has been set to 0 and y is set to 3?

if ((y / x) == 3 || x == 0) System.out.println("first case");
else System.out.println("second case");