全面了解java异常

throw new 异常类名(参数);

//例:
public class ThrowDemo{
    public static void main(String[]args){
        //创建一个数组
        int[]arr={9,5,2,7};
        //根据索引找对应元素
        int index = 4; 
        int element = getElement(arr,index); 
  		System.out.println(element);
        System.out.println("over!");
    }
    public static int getElement(int arr[],int index){
        //判断索引是否越界
        if(index<0||index>arr.length-1){
            
            throw new ArrayIndexOutOfBoundsException("哥们,角标越界了!!!"); 
        }
        int element=arr[index];
        return element;
    }
}

修饰符 返回值类型 方法名 (参数列表) throws 异常类名1,异常类名2... {  } 

//示例代码
public class ThrowsDemo{
    public static void main(String[]args) throws FileNotFoundException {
        read("a.txt");
    }
    //定义功能时有问题发生需要报告给调用者.可以通过在方法上使用throws关键字进行声明
  public static void read(String path) throws FileNotFoundException{
      if(!path.equals("a.txt")){
          //如果不是a.txt就认为文件不存在是一个异常抛出问题!
          throw new FileNotFoundException("文件不存在");
      }
  }
}

public class ThrowsDemo2 {
    public static void main(String[]args) throwsIOException {
        read("a.txt");
    }
    //定义功能时有问题发生需要报告给调用者.可以通过在方法上使用throws关键字进行声明
  public static void read(String path) throws FileNotFoundException , IOException {
      if(!path.equals("a.txt")) {
          //如果不是a.txt就认为文件不存在是一个异常抛出问题!
          throw new FileNotFoundException("文件不存在");
      }
      if(!path.equals("b.txt")){
          throw new IOException();
      }
  }
}

try {
    编写可能会出现异常的代码
}catch(异常类型 e){
    处理异常的代码
        //记录日志/打印异常信息/继续抛出异常
}

public class TryCatchDemo {
    public static void main (String[]args) {
        try {//当产生异常时,必须有处理方式,要么捕获要么声明
            read("a.txt");
        }catch(FileNotFoundException e){//try中抛出什么异常,括号中就定义什么类型
     	 System.out.println(e);
        }
     System.out.println("OVER!");
    }
    //定义功能时有问题发生需要报告给调用者.可以通过在方法上使用throws关键字进行声明
  public static void read (String path) throws FileNotFoundException {
      if(!path.equals("a.txt")){
          //如果不是a.txt就认为文件不存在是一个异常抛出问题!
          throw new FileNotFoundException("文件不存在");
      }
  }
}

public static void main(String[]args){
    try {
        String str=null;
        System.out.println(str.length());
    }catch(Exceptione){
        e.printStackTrace();//打印异常的堆栈追踪信息
    }
    System.out.println("OVER!");
}

//运行结果:
java.lang.NullPointerException
    at com.langsin.exception.ExceptionDemo.main(ExceptionDemo.java:7)
    OVER!

public class test {
	public int add(int a,int b) {
		try {
			return a+b;
		}catch(Exception e){
			System.out.println("catch语句块");
		}finally {
			System.out.println("finally语句块");
		}
		return 0;
	}
	public static void main(String[] args) {
		test t=new test();
		System.out.println("和是"+t.add(9, 34));
	}
 
}

public class test {
	public int add(int a,int b) {
		try {
			return a+b;
		}catch(Exception e){
			System.out.println("catch语句块");
		}finally {
			System.out.println("finally语句块");
			a=1;
		}
		return 0;
	}
	public static void main(String[] args) {
		test t=new test();
		System.out.println("和是"+t.add(9, 34));
	}
 
}

public class TryTest{
	public static void main(String[] args){
		System.out.println(test());
	}
private static int test(){
	int num = 10;
	try{
		System.out.println("try");
		return num += 80;
	}catch(Exception e){
		System.out.println("error");
	}finally{
		if (num > 20){
			System.out.println("num>20 : " + num);
		}
		System.out.println("finally");
	}
	return num;
 }
}

public class TryTest{
	public static void main(String[] args){
		System.out.println(test());
	}
private static int test(){
	int num = 10;
	try{
		System.out.println("try");
		return num += 80;
	}catch(Exception e){
		System.out.println("error");
	}finally{
		if (num > 20){
			System.out.println("num>20 : " + num);
		}
		System.out.println("finally");
		num = 100;
		return num;
	}
}
}

public class TryTest{
	public static void main(String[] args){
		System.out.println(test());
	}
private static int test(){
	int num = 10;
	try{
		System.out.println("try");
		return num;
	}catch(Exception e){
		System.out.println("error");
	}finally{
		if (num > 20){
			System.out.println("num>20 : " + num);
		}
		System.out.println("finally");
		num = 100;
	}
	return num;
}
}

public class TryTest{
	public static void main(String[] args){
		System.out.println(test().num);
	}
private static Num test(){
	Num number = new Num();
	try{
		System.out.println("try");
		return number;
	}catch(Exception e){
		System.out.println("error");
	}finally{
		if (number.num > 20){
			System.out.println("number.num>20 : " + number.num);
		}
		System.out.println("finally");
		number.num = 100;
	}
	return number;
}
}
class Num{
	public int num = 10;
}

try{
    //编写可能出现异常的代码
}catch(异常类型A e){//当try中出现A类型异常,就用该catch来捕获
    //处理异常的代码
    //记录日志/打印异常信息/继续抛出异常
}catch(异常类型B e){//当try中出现B类型异常,就用该catch来捕获
    //处理异常的代码//记录日志/打印异常信息/继续抛出异常
}

public class RegisterException extends Exception{
    
    public RegisterException (){   
    }
    
    public RegisterException(String message){
        super (message);
    }
}

public class Demo {
    private static String[]names={"lyan","zhyuqi","xdongdong"};
    public static void main(String[]args) {
        try{
            //可能出现异常的代码
            checkUsername("zhyuqi");
            System.out.println("用户名可用!");//没有异常则用户名没有重复,可用
        }catch(RegisterException e){
            //处理异常
            e.printStackTrace();
        }
    }
    //因为RegisterException是编译期异常,又想调用者去处理,所以在方法上声明该异常
  public static boolean checkUsername(String uname) throws RegisterException{for(Stringname:names){
      if(name.equals(uname)){//如果名字在数组里面就抛出注册异常
          throw new RegisterException("亲"+uname+"已经被注册了!");
       }
     }
     return true;
  }
}