Java If ... Else 示例

实际应用示例

此示例展示了如何使用 if...else 在用户输入正确密码后"打开门":

int doorCode = 1337;

if (doorCode == 1337) {
  System.out.println("Correct code. The door is now open.");
} else {
  System.out.println("Wrong code. The door remains closed.");
}

这个例子展示了如何使用if...else语句来判断一个数是正数还是负数:

int myNum = 10; // 这是一个正数还是负数?

if (myNum > 0) {
  System.out.println("The value is a positive number.");
} else if (myNum < 0) {
  System.out.println("The value is a negative number.");
} else {
  System.out.println("The value is 0.");
}

查询某人是否达到投票年龄:

int myAge = 25;
int votingAge = 18;

if (myAge >= votingAge) {
  System.out.println("Old enough to vote!");
} else {
  System.out.println("Not old enough to vote.");
}

判断一个人是否达到投票年龄，以及是否是本国公民(使用嵌套的if语句):

int age = 20;
boolean isCitizen = true;

if (age >= 18) {
  System.out.println("Old enough to vote.");
  
  if (isCitizen) {
    System.out.println("And you are a citizen, so you can vote!");
  } else {
    System.out.println("But you must be a citizen to vote.");
  }
} else {
  System.out.println("Not old enough to vote.");
}

判断一个数是奇数还是偶数:

int myNum = 5;

if (myNum % 2 == 0) {
  System.out.println(myNum + " is even");
} else {
  System.out.println(myNum + " is odd");
} 

检查体温:

int temperature = 30;

if (temperature < 0) {
  System.out.println("It's freezing!");
} else if (temperature < 20) {
  System.out.println("It's cool.");
} else {
  System.out.println("It's warm.");
} 

系统访问控制示例 - 您必须先登录，然后必须是管理员，或者拥有高安全级别(1 级或 2 级)才能获得访问权限:

boolean isLoggedIn = true;
boolean isAdmin = false;
int securityLevel = 3; // 1 = highest

if (isLoggedIn && (isAdmin || securityLevel <= 2)) {
  System.out.println("Access granted");
} else {
  System.out.println("Access denied");
}

// Try changing securityLevel to test different outcomes:
//
// securityLevel 1 = Access granted
// securityLevel 2 = Access granted
// securityLevel 3 = Access denied
// securityLevel 4 = Access denied
//
// If isAdmin = true, access is granted.