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条件语句

学习使用 if、else 以及现代 switch 表达式来控制程序流程，根据特定条件执行代码。

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条件语句

学习使用 if、else 以及现代 switch 表达式来控制程序流程，根据特定条件执行代码。

...
A single condition controls whether a block of code runs or is skipped entirely
1public class TemperatureCheck {
2    public static void main(String[] args) {
3        int currentTemperature = 35;
4        int threshold = 30;
5
6        // Simple if statement
7        if (currentTemperature > threshold) {
8            System.out.println("Warning: It is hot outside!");
9        }
10
11        System.out.println("System check complete.");
12    }
13}
...
A decision splits into two exclusive paths, ensuring that exactly one of the two blocks is executed
1public class PassFailCheck {
2    public static void main(String[] args) {
3        int score = 45;
4
5        if (score >= 50) {
6            System.out.println("Result: Pass");
7        } else {
8            System.out.println("Result: Fail");
9        }
10    }
11}
...
A sequential chain that tests a series of conditions one by one, executing the code for the first true match it finds and ignoring the rest
1public class GradingSystem {
2    public static void main(String[] args) {
3        int score = 75;
4
5        if (score >= 90) {
6            System.out.println("Grade: Distinction");
7        } else if (score >= 70) {
8            System.out.println("Grade: Merit");
9        } else if (score >= 50) {
10            System.out.println("Grade: Pass");
11        } else {
12            System.out.println("Grade: Fail");
13        }
14    }
15}
1public class ScopeCheck {
2    public static void main(String[] args) {
3        boolean accessGranted = true;
4
5        if (accessGranted) {
6            String message = "Welcome, Admin"; // Variable defined inside the block
7            System.out.println(message);
8        }
9
10        // System.out.println(message); // ERROR: 'message' cannot be resolved here
11    }
12}
...
A single input value is matched against multiple labeled options to determine which code path to follow
1public class DayPlanner {
2    public static void main(String[] args) {
3        int dayOfWeek = 3; // 1 = Monday, 7 = Sunday
4        String schedule;
5
6        switch (dayOfWeek) {
7            case 1:
8                schedule = "Gym";
9                break;
10            case 2:
11                schedule = "Team Meeting";
12                break;
13            case 3:
14                schedule = "Coding Focus";
15                break;
16            default:
17                schedule = "Free Time";
18                break;
19        }
20        System.out.println("Today's plan: " + schedule);
21    }
22}
1public class ModernDayPlanner {
2    public static void main(String[] args) {
3        int dayOfWeek = 3;
4
5        switch (dayOfWeek) {
6            case 1 -> System.out.println("Gym");
7            case 2 -> System.out.println("Team Meeting");
8            case 3 -> {
9                System.out.println("Coding Focus");
10                System.out.println("Do not disturb");
11            }
12            default -> System.out.println("Free Time");
13        }
14    }
15}

高级面向对象特性

迭代语句