美好即將發生

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System.out.println(10/0);
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//ArithmeticException 算数异常
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int[] arr = {1,2,3,4,5};
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System.out.println(arr[10]);
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//ArrayIndexOutOfBoundsException 数组越界异常
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int arr = null;
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System.out.println(arr.length);
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//NullPointerException 空指针异常...

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import java.io.*;
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public class Example1 {
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    public static void main(String[] args) throws IOException {
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        FileReader fr = new FileReader("data.txt");  // 假设文件存在
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        BufferedReader br = new BufferedReader(fr);
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        System.out.println("第一行内容是：" + br.readLine());
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        br.close();
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    }
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}

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import java.io.*;
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public class Example2 {
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    public static void main(String[] args) {
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        String filePath = "data.txt";
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        File file = new File(filePath);
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        // ① 防御：检查文件是否存在
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        if (!file.exists()) {
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            System.err.println("错误：文件不存在 → " + filePath);
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            return;
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        }
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        // ② 防御：检查是否是普通文件（防止是目录）
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        if (!file.isFile()) {
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            System.err.println("错误：" + filePath + " 不是一个有效文件");
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            return;
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        }
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        // ③ 使用 try-with-resources，自动关闭流
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        try (BufferedReader br = new BufferedReader(new FileReader(file))) {
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            String firstLine = br.readLine();
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            // ④ 防御：检查文件是否为空
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            if (firstLine == null) {
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                System.err.println("提示：文件为空，没有任何内容。");
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                return;
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            }
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            System.out.println("第一行内容是：" + firstLine);
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        } catch (IOException e) {
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            // ⑤ 防御：捕获所有IO错误并给出友好提示
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            System.err.println("读取文件时出错：" + e.getMessage());
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        }
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    }
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}

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void example(String s){
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    if(s == null){
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        NullPointerException e = new NullPointerException();
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    throw e;
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    //通常写为一条语句，用匿名对象
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    //throw new NullPointerException();
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    }
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}

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void example1(String s) {
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    try {
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        example2();
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    } catch (NullPointerException e) {
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        throw new IllegalArgumentException();
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    }
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}
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void example2(String s) {
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    if (s==null) {
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        throw new NullPointerException();
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    }
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}

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public class TestDemo3 {
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    public static void main(String[] args) {
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        try {
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            example1();
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        } catch (Exception e) {
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            e.printStackTrace();
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        }
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    }
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    static void example1() {
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        try {
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            example2();
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        } catch (NullPointerException e) {
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            throw new IllegalArgumentException();
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        }
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    }
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    static void example2() {
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        throw new NullPointerException();
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    }
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}

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java.lang.IllegalArgumentException
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  at demo2.TestDemo3.example1(TestDemo3.java:23)
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  at demo2.TestDemo3.main(TestDemo3.java:13)

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public class TestDemo3 {
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    public static void main(String[] args) {
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        try {
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            example1();
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        } catch (Exception e) {
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            e.printStackTrace();
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        }
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    }
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    static void example1() {
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        try {
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            example2();
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        } catch (NullPointerException e) {
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            throw new IllegalArgumentException(e);
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        }
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    }
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    static void example2() {
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        throw new NullPointerException();
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    }
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}

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java.lang.IllegalArgumentException: java.lang.NullPointerException
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  at demo2.TestDemo3.example1(TestDemo3.java:23)
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  at demo2.TestDemo3.main(TestDemo3.java:13)
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Caused by: java.lang.NullPointerException
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  at demo2.TestDemo3.example2(TestDemo3.java:28)
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  at demo2.TestDemo3.example1(TestDemo3.java:21)