Java 异常处理与输入输出

一、异常

1.1

package exception;

import java.util.Scanner;

public class ArrayIndex {

	public static void main(String[] args) {
		int[] a=new int[10];
		Scanner in=new Scanner(System.in);
		int index=in.nextInt();
		try
		{
			a[index]=25;
			System.out.println("right");
		}
		catch(ArrayIndexOutOfBoundsException b)
		{
		    System.out.println("there is a wrong");	
		}
        in.close();
	}

}

出现异常就会到catch。

1.2 调用的函数中出现异常

package exception;

import java.util.Scanner;

public class ArrayIndex {

	public static void f()
	{
		int[] a=new int[10];
		a[10]=15;
		System.out.println("right");
	}
	public static void main(String[] args) {
		
		try
		{
			f();
		}
		catch(ArrayIndexOutOfBoundsException b)
		{
		    System.out.println("there is a wrong");	
		}
		System.out.println("end");
        
	}

}

1.3

package exception;

import java.util.Scanner;

public class ArrayIndex {

	public static void f()
	{
		int[] a=new int[10];
		a[10]=15;
		System.out.println("hello");
	}
	
	public static void g()
	{
		f();
	}
	public static void h()
	{
		int i=10;
		if(i<100)
			g();
	}
	public static void k()
	{
		try {
			h();
		}
		catch(ArrayIndexOutOfBoundsException e)
		{
			 System.out.println("k()");	
		}
	}
	
	
	public static void main(String[] args) {
		
		try
		{
			k();
		}
		catch(ArrayIndexOutOfBoundsException e)
		{
		    System.out.println("there is a wrong");	
		}
		System.out.println("end");
        
	}

}

try ...catch 里出现嵌套时，只会输出里层嵌套的内容。

catch的存在就是为了处理出现的异常。

相当于在k()函数中发现了异常，并且利用catch已经将异常解决了。所以在main（）中便默认k()没有了异常，所以不会输出“there is a wrong”。

1.4

package exception;

import java.util.Scanner;

public class ArrayIndex {

	public static void f()
	{
		int[] a=new int[10];
		a[10]=15;
		System.out.println("hello");
	}
	
	public static void g()
	{
		f();
	}
	public static void h()
	{
		int i=10;
		if(i<100)
			g();
	}
	public static void k()
	{
		try {
			h();
		}
		catch(ArrayIndexOutOfBoundsException e)
		{
			 System.out.println("k()");	
			 throw e;//抛出异常
		}
	}
	
	
	public static void main(String[] args) {
		
		try
		{
			k();
		}
		catch(ArrayIndexOutOfBoundsException e)
		{
		    System.out.println("there is a wrong");	
		    System.out.println(e.getMessage());//输出e的值，10，这是指数组a[10]处赋值出现问题
		    System.out.println(e);
		    e.printStackTrace();//显示调用路径
		}
		System.out.println("end");
        
	}

}

throw e:相当于将处理的异常又重新添加加进来，这样“there is a wrong ”也便会输出。

或者说对于异常catch没有处理完全。所以还需要再度输出。

1.5

二、流

2.1 二进制数据

package Hello;

import java.io.IOException;
import java.util.Scanner;

public class test {
	
	public static void main(String[] args) {
		System.out.println("Hello world");
		byte[] buffer=new byte[1024];//定义了字节数组
		try {
			int len=System.in.read(buffer);//往buffer中写入数据，并返回长度
			String s=new String(buffer, 0, len);//从buffer中0的位置开始，获取len长度的字节
			System.out.println("读到了"+len+"字节");
			System.out.println(s);
			System.out.println("s的长度是："+s.length());
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

回车和换行算两个字符。

 

2.2 

package Hello;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.Scanner;

public class test {
	
	public static void main(String[] args) {
		System.out.println("Hello world");
		byte[] buffer=new byte[5];
		for(int i=0;i<buffer.length;i++)
		{
			buffer[i]='a';
		}
		try {
			FileOutputStream out=new FileOutputStream("a.txt");
			out.write(buffer);
			out.close();
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

2.3 过滤器

package Hello;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.Scanner;

import javax.imageio.stream.FileImageOutputStream;

public class test {
	
	public static void main(String[] args) {
		System.out.println("Hello world");
		byte[] buffer=new byte[10];
		for(int i=0;i<buffer.length;i++)
		{
			buffer[i]=(byte) i;
		}
		try {
			DataOutputStream out=new DataOutputStream(
					new BufferedOutputStream(
							new FileOutputStream("b.dat")));//只能处理字节
			int i=123454;
			out.writeInt(i);
			out.close();
			
			DataInputStream in=new DataInputStream(
					new BufferedInputStream(
							new FileInputStream("b.dat")));
			int j=in.readInt();
			System.out.println(j);
			
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

以16进制存储。

2.4 文本

package Hello;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.Scanner;

import javax.imageio.stream.FileImageOutputStream;

public class test {
	
	public static void main(String[] args) {
		System.out.println("Hello world");
		byte[] buffer=new byte[10];
		for(int i=0;i<buffer.length;i++)
		{
			buffer[i]=(byte) i;
		}
		try {
			PrintWriter out=new PrintWriter(
					new BufferedWriter(
							new OutputStreamWriter(
									new FileOutputStream("c.txt"))));//
			int i=12345;
			out.println(i);
			out.close();
			
			BufferedReader in=new BufferedReader(
					new InputStreamReader(
							new FileInputStream("c.txt")));
			String line;
			while((line=in.readLine())!=null)
			    System.out.println(line);
			
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

2.5编码格式

package Hello;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.Scanner;

import javax.imageio.stream.FileImageOutputStream;

public class test {
	
	public static void main(String[] args) {
		System.out.println("Hello world");
		byte[] buffer=new byte[10];
		for(int i=0;i<buffer.length;i++)
		{
			buffer[i]=(byte) i;
		}
		try {
			PrintWriter out=new PrintWriter(
					new BufferedWriter(
							new OutputStreamWriter(
									new FileOutputStream("c.txt"))));//
			int i=12345;
			out.println(i);
			out.close();
			
			BufferedReader in=new BufferedReader(
					new InputStreamReader(
							new FileInputStream("a.txt"),"GBk"));//设置编码格式
			String line;
			while((line=in.readLine())!=null)
			    System.out.println(line);
			
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

GBK编码时，正确读出

utf8时

2.6 格式化输出

三、流应用

package Hello;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.Socket;

public class hello {
	
	public static void main(String[] args) {
		try {
			Socket socket=new Socket(InetAddress.getByName("localhost"),12345);
			PrintWriter out=new PrintWriter(
					new BufferedWriter(
							new OutputStreamWriter(
									socket.getOutputStream())));//
			out.println("hello");
			
			BufferedReader in=new BufferedReader(
					new InputStreamReader(
							socket.getInputStream()));//设置编码格式
			String line;
			line=in.readLine();
			System.out.println(line);
			out.close();
			socket.close();		
		}catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

四、对象串行化

package Hello;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.Serializable;
import java.net.InetAddress;
import java.net.Socket;

class Student implements Serializable{ //可串行化的类
	private String name;
	private int age;
	private int grade;
	
	public Student(String name,int age,int grade)
	{
		this.name=name;
		this.age=age;
		this.grade=grade;
	}
	
	public String toString()
	{
		return name+" "+age+" "+grade;
	}
}

public class hello {
	
	public static void main(String[] args) {
		try {
			Student s1=new Student("John",18,5);
			System.out.println(s1);
			ObjectOutputStream out=new ObjectOutputStream(
					new FileOutputStream("obj.dat"));
			out.writeObject(s1);
			out.close();
			ObjectInputStream in=new ObjectInputStream(
					new FileInputStream("obj.dat"));
			Student s2=(Student)in.readObject();
			System.out.println(s2);
		}catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

保存在obj.dat中的值