ControlFlow
文件信息
- 📄 原文件：
ControlFlow.java - 🔤 语言：java
Java 控制流 本文件介绍 Java 中的条件语句、循环、跳转语句等控制流结构。
完整代码
java
/**
* ============================================================
* Java 控制流
* ============================================================
* 本文件介绍 Java 中的条件语句、循环、跳转语句等控制流结构。
* ============================================================
*/
public class ControlFlow {
public static void main(String[] args) {
ifElseStatements();
switchStatements();
switchExpressions();
forLoops();
whileLoops();
breakAndContinue();
exceptionHandling();
}
/**
* ============================================================
* 1. if-else 语句
* ============================================================
*/
public static void ifElseStatements() {
System.out.println("=".repeat(60));
System.out.println("1. if-else 语句");
System.out.println("=".repeat(60));
int score = 85;
// 【基本 if】
if (score >= 60) {
System.out.println("及格了！");
}
// 【if-else】
if (score >= 90) {
System.out.println("优秀");
} else {
System.out.println("继续努力");
}
// 【if-else if-else】
String grade;
if (score >= 90) {
grade = "A";
} else if (score >= 80) {
grade = "B";
} else if (score >= 70) {
grade = "C";
} else if (score >= 60) {
grade = "D";
} else {
grade = "F";
}
System.out.println("分数 " + score + " 的等级: " + grade);
// 【三元运算符】
String result = (score >= 60) ? "通过" : "不通过";
System.out.println("三元运算符: " + result);
// 【嵌套三元运算符】（不推荐，可读性差）
String level = score >= 90 ? "优秀" :
score >= 60 ? "及格" : "不及格";
System.out.println("嵌套三元: " + level);
// 【逻辑运算符】
int age = 25;
boolean hasLicense = true;
if (age >= 18 && hasLicense) {
System.out.println("\n可以开车");
}
if (age < 18 || !hasLicense) {
System.out.println("不能开车");
}
}
/**
* ============================================================
* 2. switch 语句（传统）
* ============================================================
*/
public static void switchStatements() {
System.out.println("\n" + "=".repeat(60));
System.out.println("2. switch 语句（传统）");
System.out.println("=".repeat(60));
// 【基本 switch】
int day = 3;
String dayName;
switch (day) {
case 1:
dayName = "星期一";
break;
case 2:
dayName = "星期二";
break;
case 3:
dayName = "星期三";
break;
case 4:
dayName = "星期四";
break;
case 5:
dayName = "星期五";
break;
case 6:
case 7:
dayName = "周末";
break;
default:
dayName = "无效";
}
System.out.println("day " + day + " = " + dayName);
// 【字符串 switch】（Java 7+）
String fruit = "apple";
String color;
switch (fruit) {
case "apple":
color = "红色";
break;
case "banana":
color = "黄色";
break;
case "grape":
color = "紫色";
break;
default:
color = "未知";
}
System.out.println(fruit + " 是 " + color);
// 【fall-through】（忘记 break 的危险）
System.out.println("\n【警告】忘记 break 会导致 fall-through:");
int month = 2;
switch (month) {
case 1:
System.out.println(" 一月");
// 忘记 break，会继续执行下一个 case
case 2:
System.out.println(" 二月");
break;
default:
System.out.println(" 其他");
}
}
/**
* ============================================================
* 3. switch 表达式（Java 14+）
* ============================================================
*/
public static void switchExpressions() {
System.out.println("\n" + "=".repeat(60));
System.out.println("3. switch 表达式（Java 14+）");
System.out.println("=".repeat(60));
int day = 3;
// 【箭头语法】不需要 break
String dayType = switch (day) {
case 1, 2, 3, 4, 5 -> "工作日";
case 6, 7 -> "周末";
default -> "无效";
};
System.out.println("day " + day + " 是 " + dayType);
// 【yield 返回值】（多行代码时使用）
String description = switch (day) {
case 1 -> "周一，新的开始";
case 5 -> "周五，快乐的一天";
case 6, 7 -> {
String msg = "周末";
msg += "，休息时间";
yield msg; // 使用 yield 返回值
}
default -> "普通的一天";
};
System.out.println(description);
// 【模式匹配】（Java 21+）
Object obj = "Hello";
String result = switch (obj) {
case Integer i -> "整数: " + i;
case String s -> "字符串: " + s;
case Double d -> "浮点数: " + d;
case null -> "空值";
default -> "未知类型";
};
System.out.println("\n模式匹配: " + result);
}
/**
* ============================================================
* 4. for 循环
* ============================================================
*/
public static void forLoops() {
System.out.println("\n" + "=".repeat(60));
System.out.println("4. for 循环");
System.out.println("=".repeat(60));
// 【基本 for 循环】
System.out.println("--- 基本 for 循环 ---");
for (int i = 0; i < 5; i++) {
System.out.print(i + " ");
}
System.out.println();
// 【递减循环】
System.out.println("\n递减:");
for (int i = 5; i > 0; i--) {
System.out.print(i + " ");
}
System.out.println();
// 【步长】
System.out.println("\n步长为2:");
for (int i = 0; i < 10; i += 2) {
System.out.print(i + " ");
}
System.out.println();
// 【增强 for 循环（for-each）】
System.out.println("\n--- 增强 for 循环 ---");
int[] numbers = {1, 2, 3, 4, 5};
for (int num : numbers) {
System.out.print(num + " ");
}
System.out.println();
String[] fruits = {"apple", "banana", "cherry"};
for (String fruit : fruits) {
System.out.println(" " + fruit);
}
// 【嵌套循环】
System.out.println("\n--- 嵌套循环（乘法表）---");
for (int i = 1; i <= 3; i++) {
for (int j = 1; j <= 3; j++) {
System.out.print(i + "×" + j + "=" + (i * j) + "\t");
}
System.out.println();
}
// 【无限循环】
// for (;;) { } // 等同于 while (true)
}
/**
* ============================================================
* 5. while 和 do-while 循环
* ============================================================
*/
public static void whileLoops() {
System.out.println("\n" + "=".repeat(60));
System.out.println("5. while 和 do-while 循环");
System.out.println("=".repeat(60));
// 【while 循环】
System.out.println("--- while 循环 ---");
int count = 0;
while (count < 5) {
System.out.print(count + " ");
count++;
}
System.out.println();
// 【do-while 循环】至少执行一次
System.out.println("\n--- do-while 循环 ---");
int n = 0;
do {
System.out.print(n + " ");
n++;
} while (n < 5);
System.out.println();
// 【区别】
System.out.println("\n--- while vs do-while ---");
int x = 10;
// while: 条件为 false，不执行
while (x < 5) {
System.out.println("while: " + x);
}
// do-while: 条件为 false，也执行一次
do {
System.out.println("do-while: " + x + " (至少执行一次)");
} while (x < 5);
}
/**
* ============================================================
* 6. break 和 continue
* ============================================================
*/
public static void breakAndContinue() {
System.out.println("\n" + "=".repeat(60));
System.out.println("6. break 和 continue");
System.out.println("=".repeat(60));
// 【break】跳出循环
System.out.println("--- break ---");
for (int i = 0; i < 10; i++) {
if (i == 5) {
System.out.println("遇到 5，跳出循环");
break;
}
System.out.print(i + " ");
}
System.out.println();
// 【continue】跳过本次迭代
System.out.println("\n--- continue ---");
for (int i = 0; i < 10; i++) {
if (i % 2 == 0) {
continue; // 跳过偶数
}
System.out.print(i + " ");
}
System.out.println();
// 【带标签的 break/continue】用于嵌套循环
System.out.println("\n--- 带标签的 break ---");
outer:
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
if (i == 1 && j == 1) {
System.out.println("跳出外层循环");
break outer;
}
System.out.println(" i=" + i + ", j=" + j);
}
}
}
/**
* ============================================================
* 7. 异常处理
* ============================================================
*/
public static void exceptionHandling() {
System.out.println("\n" + "=".repeat(60));
System.out.println("7. 异常处理");
System.out.println("=".repeat(60));
// 【try-catch】
System.out.println("--- try-catch ---");
try {
int result = 10 / 0; // ArithmeticException
System.out.println(result);
} catch (ArithmeticException e) {
System.out.println("捕获异常: " + e.getMessage());
}
// 【多个 catch】
System.out.println("\n--- 多个 catch ---");
try {
String s = null;
System.out.println(s.length()); // NullPointerException
} catch (NullPointerException e) {
System.out.println("空指针异常");
} catch (Exception e) {
System.out.println("其他异常: " + e);
}
// 【多异常捕获】（Java 7+）
System.out.println("\n--- 多异常捕获 ---");
try {
int[] arr = {1, 2, 3};
System.out.println(arr[10]); // ArrayIndexOutOfBoundsException
} catch (ArrayIndexOutOfBoundsException | NullPointerException e) {
System.out.println("数组或空指针异常: " + e.getClass().getSimpleName());
}
// 【try-catch-finally】
System.out.println("\n--- try-catch-finally ---");
try {
System.out.println("try 块");
// int x = 1 / 0;
} catch (Exception e) {
System.out.println("catch 块");
} finally {
System.out.println("finally 块（总是执行）");
}
// 【try-with-resources】（Java 7+）
System.out.println("\n--- try-with-resources ---");
// 自动关闭资源
try (var scanner = new java.util.Scanner("Hello World")) {
System.out.println("读取: " + scanner.next());
} // 自动调用 scanner.close()
// 【抛出异常】
System.out.println("\n--- 抛出异常 ---");
try {
validateAge(-5);
} catch (IllegalArgumentException e) {
System.out.println("捕获: " + e.getMessage());
}
// 【自定义异常】
System.out.println("\n--- 自定义异常 ---");
try {
throw new CustomException("自定义错误信息");
} catch (CustomException e) {
System.out.println("捕获自定义异常: " + e.getMessage());
}
}
// 抛出异常的方法
public static void validateAge(int age) {
if (age < 0) {
throw new IllegalArgumentException("年龄不能为负: " + age);
}
}
// 自定义异常类
static class CustomException extends Exception {
public CustomException(String message) {
super(message);
}
}
}
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