Java异常

Java异常

​ 异常的超类为Throwable，类似于类中的Object，异常分为两大类：Error和Exception，Error会导致程序终止，问题很严重，但无法处理，一般处理Exception。

异常关键字

try catch finally：用于捕获异常

举例1：try catch绑定在一起，捕获异常时必须存在，finally可有可无，无论有没有发生异常，finally中的代码块一定会执行，finally中的代码块的作用是善后，如IO流的关闭。

public void num() {
        // try catch
        try {
            int i = 10 / 0;
        } catch (Exception e) {
            System.out.println("try catch");
        }
        
        // try catch finally
        try {
            int i = 10 / 0;
        } catch (Exception e) {
            System.out.print("try catch ");
        } finally {
            System.out.println("finally");
        }

        // finally
        try {
            int i = 10 / 1;
        } catch (Exception e) {
            System.out.println("try catch");
        } finally {
            System.out.println("finally");
        }
    }

举例2：可以存在多个catch，但捕获异常的顺序从小到大，并且只会被一个异常捕获

public  void num() {
        // ArithmeticException
        try {
            int i = 10 / 0;
        } catch(ArithmeticException e) {
            System.out.println("ArithmeticException");
        } catch (Exception e) {
            System.out.println("Exception");
        } catch (Throwable e) {
            System.out.println("Throwable");
        }
    }

throw：用于在方法块中抛出异常

public class Application {
    public static void main(String[] args) {
        divide(1, 0);
    }
    public static int divide(int num1, int num2) {
        if (num2 == 0) {
            throw new ArithmeticException();
        }
        return num1 / num2;
    }
}

throws：用于方法抛出异常

public class Application {
    public static void main(String[] args) {
        System.out.println(divide(1, 0 ));
    }
    public static int divide(int num1, int num2) throws ArithmeticException{
        return num1 / num2;
    }
}

自定义异常

​ 自定义异常需要继承异常

举例1：自定义异常文件需重写toString方法，e.printStackTrace()：会自动调用自定义类的toString方法

// MyException.java文件
public class MyException extends Exception {
    private String msg;
    MyException() {
    }
    MyException(String msg) {
        this.msg = msg;
    }
    @Override
    public String toString() {
        return "MyException{" +
                "msg='" + msg + '\'' +
                '}';
    }
}
// Application.java文件
public class Application {
    public static void main(String[] args) {
        //MyException{msg='151'}
        getAge(151);
    }
    public static void exceptionHandle(int age) throws MyException {
        if (age < 0 || age > 150) {
            throw new MyException(age + "");
        }
    }
    public static void getAge(int age) {
        try {
            exceptionHandle(age);
            System.out.println(age);
        } catch (MyException e) {
            e.printStackTrace();
        }
    }

}

举例2：继承异常为RuntimeException及其子类，不会被强制要求处理，继承除RuntimeException及其子类，会被强制要求处理，要么抛出异常，要么捕获异常，不然会报错。

// MyException.java文件
public class MyException extends Exception {
    private String msg;
    MyException() {
    }
    MyException(String msg) {
        this.msg = msg;
    }
    @Override
    public String toString() {
        return "MyException{" +
                "msg='" + msg + '\'' +
                '}';
    }
}
// MyRuntimeException.java文件
public class MyRuntimeException extends RuntimeException{
    private String msg;
    MyRuntimeException() {
    }
    MyRuntimeException(String msg) {
        this.msg = msg;
    }

    @Override
    public String toString() {
        return "MyRuntimeException{" +
                "msg='" + msg + '\'' +
                '}';
    }
}
// Application.java文件
public class Application {
    public static void main(String[] args) {
        getAge(151);
        getName("");
    }
    public static void exceptionHandle(int age) throws MyException {
        if (age < 0 || age > 150) {
            throw new MyException(age + "");
        }
    }
    public static void runtimeExceptionHandle(String name) {
        if (name.equals("")) {
            throw new MyRuntimeException("请输入姓名");
        }
    }
    public static void getAge(int age) {
        try {
            exceptionHandle(age);
            System.out.println(age);
        } catch (MyException e) {
            e.printStackTrace();
        }
    }
    public static void getName(String name) {
        runtimeExceptionHandle(name);
    }
}