JAVA SE学习day_06：字符流、异常处理

一、字符流

java将流按照读写单位划分为字节流与字符流
字节流：超类为InputStream和OutputStream，读写单位为字节
字符流：超类为Reader和Writer，读写单位为cahr
字符流封装了字节与字符的转换，因此字符流仅适合去读写文本数据

1.1 转换流

转换流是字符流一对常用实现类，是一对高级流。在实际开发中我们不会直接操作这个流，但是在流连接中它是非常重要的一环，负责衔接字节流与其他的高级字符流

转换流向文件中写入字符

//向文件中写入文字FileOutputStream fos = new FileOutputStream("osw.txt",true);//加true后变为追加写入操作OutputStreamWriter osw = new OutputStreamWriter(fos,"gbk");/** 第二个参数是用来指定字符集，这样当字符串通过转换流时* 会按照该字符集转换为对应的一组字节。如果不指定则是按照* 系统默认的字符集转换，不推荐不指定的做法。*/osw.write("12345~");osw.write("上山打老虎");System.out.println("over");osw.close();

转换流从文件中读取字符

FileInputStream fis = new FileInputStream("osw.txt");InputStreamReader isr = new InputStreamReader(fis,"gbk");int d=0;while((d = isr.read())!=-1){char c = (char)d;System.out.print(c);}isr.close();

1.2 缓存字符流

java.io.BufferedWriter和BufferedReader

缓存字符流内部有缓冲区，可以块读写文本数据提高读写效率。并且缓冲
字符流还有一个特点就是可以按行读写字符串。
java.io.PrintWriter具有自行行刷新的缓冲字符输出流，内部总是连接 BufferedWriter作为缓冲功能
缓冲字符输出流
java.io.BufferedWriter和 java.io.PrintWriter的连接使用
（单独PrintWriter也可以输出）

     PrintWriter pw = new PrintWriter("pw.txt");pw.println("阿萨德");pw.println("123");System.out.println("over");pw.close();

在流连接中使用PrintWriter

     FileOutputStream fos = new FileOutputStream("pw.txt",true);OutputStreamWriter osw = new OutputStreamWriter(fos,"gbk");BufferedWriter bw = new BufferedWriter(osw);PrintWriter pw = new PrintWriter(bw);pw.println("123");pw.println("asd");System.out.println("over");pw.close();

建立一个简易记事本

     Scanner scan = new Scanner(System.in);System.out.println("请输入文件名：");String name = scan.nextLine();FileOutputStream fos = new FileOutputStream(name+".txt");OutputStreamWriter osw = new OutputStreamWriter(fos,"gbk");BufferedWriter bw = new BufferedWriter(osw);PrintWriter pw = new PrintWriter(bw,true);//加true代表自动行刷新/** PrintWriter构造方法如果第一个参数是一个流（字节流字符流皆可）* 那么就支持第二个boolean型参数，如果该值为true，则具有自动* 行刷新功能，即：每当使用println方法写入一行字符串后会自动* flush*/while(true){System.out.println("请输入想要记载的内容（exit退出）：");String s = scan.nextLine();if(s.equals("exit")){System.out.println("已退出");//println自带flush效果break;}byte[]data = s.getBytes("gdk");pw.println(data);System.out.println("完成记录");}pw.close();

缓冲字符输入流
java.io.BufferedReader

特点：块读，并且可以按行读取字符串

     FileInputStream fis = new FileInputStream("./src/io/BRDemo.java");InputStreamReader isr = new InputStreamReader(fis);BufferedReader br = new BufferedReader(isr);String l = "";while((l = br.readLine())!=null){System.out.println(l);}br.close();

二、异常处理

2.1 try-catch

java异常处理机制

语法：
try{

程序代码片段
}catch（XXXException e）{
try中代码出现XXXException后的处理代码
}

System.out.println("start");try{//String str = null;//String str = "";String str = "a";System.out.println(str.length());System.out.println(str.charAt(0));System.out.println(Integer.parseInt(str));/** try语句块中出错代码以下的内容都不会执行*/System.out.println("!!!!");}catch(NullPointerException e){System.out.println("出现了空指针异常");//catch可以定义多个，针对try中不同异常有不同处理时要分别捕捉}catch(StringIndexOutOfBoundsException e){System.out.println("下标越界异常");//在最后一个catch处捕获Excepthion可防止未知异常导致程序中断//但捕捉Exception一定要放到最后一个catch，以免其他catch失效}catch(Exception e){System.out.println("出现异常");}System.out.println("end");

2.2 finally

finally块

finally块定义在异常处理机制的最后，可以直接跟在try块之后或者最后一个catch块之后
finally可以保证只要程序执行到try语句块中，无论try语句块中的代码是否抛出异常，finally中的代码都必定执行。

     System.out.println("start");try{String str = "";//String str = null;System.out.println("try");/** 就算是有return语句，也要先执行finally中的代码后，* 方法才会返回。*/return;}catch(Exception e){System.out.println("error");}finally{System.out.println("finally");}System.out.println("over");

finally在IO中的应用

finally经常被用来做释放资源操作，比如IO中流的关闭操作

     FileOutputStream fos = null;try {fos = new FileOutputStream("fos.dat");fos.write(1);System.out.println("writer over");} catch (Exception e){e.printStackTrace();} finally {try {if(fos!=null){fos.close();}}catch (IOException e) {e.printStackTrace();}}

2.3 Autocloseable

在JDK7之后。java推出了一个特性：自动关闭

其特性可以让我们用更精简的代码完成IO操作中的异常处以及流的关闭
try（）中定义的流最终都会在finally中被关闭。该特性是编译器认可的，代码会自动转变为FinallyDemo2中的样子并且需要注意的是，不是什么变量都可以定义。只有实现了接口Autocloseable的才可以在这里定义
java.IO中的流和RandonAccessFile都实现了这个接口，可以定义在里面

     try(FileOutputStream fos = new FileOutputStream("fos.dat");  //编译不通过，String没有实现AutoCloseable接口//String str = "";){fos.write(1);System.out.println("writer over");}catch (Exception e) {e.printStackTrace();}

练习

/*5FileOutputStream fos1 = new FileOutputStream("osw.txt",true);OutputStreamWriter osw1 = new OutputStreamWriter(fos1,"gbk");osw1.write("123");osw1.write("sad");System.out.println("end");osw1.close();*//*4 FileOutputStream fos2 = new FileOutputStream("osw.txt",true);OutputStreamWriter osw2 = new OutputStreamWriter(fos2,"gbk");osw2.write("asd");osw2.write("213");System.out.println("over");osw2.close();*//*3 FileOutputStream fos3 = new FileOutputStream("osw.txt",true);OutputStreamWriter osw3 = new OutputStreamWriter(fos3,"gbk");osw3.write("456");osw3.write("wqe");System.out.println("endd");osw3.close();*//*2 FileOutputStream fos4 = new FileOutputStream("osw.txt",true);OutputStreamWriter ows4 = new OutputStreamWriter(fos4,"utf-8");ows4.write("231");ows4.write("asd");System.out.println("over");ows4.close();*//*1 FileOutputStream fos0 = new FileOutputStream("osw.txt",true);OutputStreamWriter osw0 = new OutputStreamWriter(fos0,"utf-8");osw0.write("!@#");osw0.write("asd");System.out.println("end");osw0.close();*/

/*5FileInputStream fis1 = new FileInputStream("osw.txt");InputStreamReader isr1 = new InputStreamReader(fis1,"gbk");int i=0;while((i = isr1.read())!=-1){char c = (char)i;System.out.print(c);}isr1.close();/*4FileInputStream fis2 = new FileInputStream("osw.txt");InputStreamReader isr2 = new InputStreamReader(fis2,"gbk");int a =0;while((a = isr2.read())!=-1){char c = (char)a;System.out.println(c);}isr2.close();/*3FileInputStream fis3 = new FileInputStream("osw.txt");InputStreamReader isr3 = new InputStreamReader(fis3,"utf-8");int s = 0;while((s = isr3.read())!=-1){char c = (char)s;System.out.println(c);}isr3.close();/*2FileInputStream fis4 = new FileInputStream("osw.txt");InputStreamReader isr4 = new InputStreamReader(fis4,"utf-8");int sa = 0;while((sa = isr4.read())!=-1){char c=  (char)sa;System.out.println(c);}isr4.close();*//*1FileInputStream fis0 = new FileInputStream("osw.txt");InputStreamReader isr0 = new InputStreamReader(fis0,"utf-8");int as = 0;while((as = isr0.read())!=-1){char c = (char)as;System.out.print(c);}isr0.close();*/

/*5PrintWriter pw1 = new PrintWriter("pw.txt");pw1.print("123");pw1.print("asd");System.out.println("end ");pw1.close();/*4PrintWriter pw2 = new PrintWriter("pw.txt");pw2.print("asd");pw2.print("213");System.out.println("end ");pw2.close();/*3PrintWriter pw3 = new PrintWriter("pw.txt");pw3.print("!@#");pw3.print("asd");System.out.println("over ");pw3.close();/*2PrintWriter pw4 = new PrintWriter("pw,txt");pw4.print("213");pw4.print("123asd");System.out.println("asdover");pw4.close();/*1PrintWriter pw0 = new PrintWriter("pw.txt");pw0.print("++");pw0.print("");pw0.close();System.out.println("end");*/

     /*5FileOutputStream fos1 = new FileOutputStream("pw.txt");OutputStreamWriter osw1 = new OutputStreamWriter(fos1,"gbk");BufferedWriter bw1 = new BufferedWriter(osw1);PrintWriter pw1 = new PrintWriter(bw1);pw1.println("asd");pw1.print("asd");pw1.close();/*4FileOutputStream fos2 = new FileOutputStream("pw.txt");OutputStreamWriter osw2 = new OutputStreamWriter(fos2,"utf-8");BufferedWriter bw2 = new BufferedWriter(osw2);PrintWriter pw2 = new PrintWriter(bw2);pw2.print("asd");pw2.close();/*3FileOutputStream fos3 = new FileOutputStream("pw.txt");OutputStreamWriter osw3 = new OutputStreamWriter(fos3,"utf-8");BufferedWriter bw3 = new BufferedWriter(osw3);PrintWriter pw3 = new PrintWriter(bw3);pw3.print("213");pw3.close();/*2FileOutputStream fos4 = new FileOutputStream("pw.txt");OutputStreamWriter osw4 = new OutputStreamWriter(fos4,"gbk");BufferedWriter bw4 = new BufferedWriter(osw4);PrintWriter pw4 = new PrintWriter(bw4);pw4.print("Asd");pw4.close();/*1FileOutputStream fos0 = new FileOutputStream("pw.txt");OutputStreamWriter osw0 = new OutputStreamWriter(fos0,"gbk");BufferedWriter bw0 = new BufferedWriter(osw0);PrintWriter pw0 = new PrintWriter(bw0);pw0.print("wsd");pw0.close();*/

     /*5FileInputStream fis1 = new FileInputStream("./src/io/BRDemo.java");InputStreamReader isr1 = new InputStreamReader(fis1);BufferedReader br1 = new BufferedReader(isr1);String s1 = "";while((s1 = br1.readLine())!=null){System.out.println(s1);}br1.close();/*4FileInputStream fis2 = new FileInputStream("./src/io/BRDemo.java");InputStreamReader isr2 = new InputStreamReader(fis2);BufferedReader br2 = new BufferedReader(isr2);String s2 = "";while((s2 = br2.readLine())!=null){System.out.println(s2);}br2.close();/*3FileInputStream fis3 = new FileInputStream("./src/io/BRDemo.java");InputStreamReader isr3 = new InputStreamReader(fis3);BufferedReader br3 = new BufferedReader(isr3);String s3 = "";while((s3=br3.readLine())!=null){System.out.println(s3);}br3.close();/*2FileInputStream fis4 = new FileInputStream("./src/io/BRDemo.java");InputStreamReader isr4 = new InputStreamReader(fis4);BufferedReader br4 = new BufferedReader(isr4);String s4 = "";while((s4=br4.readLine())!=null){System.out.println(s4);}br4.close();/*1FileInputStream fis5 = new FileInputStream("./src/io/BRDemo.java");InputStreamReader isr5 = new InputStreamReader(fis5);BufferedReader br5 = new BufferedReader(isr5);String s5 = "";while((s5=br5.readLine())!=null){System.out.println(s5);}br5.close();*/

FileOutputStream fos = null;try {fos = new FileOutputStream("fos.dat");fos.write(1);System.out.println("writer over");} catch (Exception e){e.printStackTrace();} finally {try {if(fos!=null){fos.close();}}catch (IOException e) {e.printStackTrace();}}/*4FileOutputStream fos1 = null;try{fos1=new FileOutputStream("fos.dat") ;fos1.write(5);System.out.println("writer over");return;}catch(Exception e){e.printStackTrace();} finally {try{if(fos1!=null){fos1.close();}}catch(IOException e) {e.printStackTrace();}}/*3FileOutputStream fos2 = null;try {fos2 = new FileOutputStream("fos.txt");fos2.write(4);System.out.println("writer end");return;}catch(Exception e){e.printStackTrace();} finally {try {if(fos2!=null){fos2.close();}}catch(IOException e){e.printStackTrace();}}/*2FileOutputStream fos3 = null;try{fos3 = new FileOutputStream("fos.dat");fos3.write(4);System.out.println("asdas");return;}catch(Exception e){e.printStackTrace();}finally{try{if(fos3!=null){fos3.close();}}catch(IOException e){e.printStackTrace();}}/*1FileOutputStream fos4 = null;try{fos3 = new FileOutputStream("fos.dat");fos3.write(4);System.out.println("over");return;}catch(Exception e){e.printStackTrace();}finally{try{if(fos4!=null){fos4.close();}}catch(IOException e){e.printStackTrace();}}*/

     try(FileOutputStream fos = new FileOutputStream("fos.dat");  //编译不通过，String没有实现AutoCloseable接口//String str = "";){fos.write(1);System.out.println("writer over");}catch (Exception e) {e.printStackTrace();}/*4try(FileOutputStream fos1 = new FileOutputStream("fos.txt");   ){fos1.write(4);System.out.println("over");}catch(Exception e){e.printStackTrace();}/*3try(FileOutputStream fos2 = new FileOutputStream("fos.dat");    ){fos2.write(7);System.out.println("over");}catch(Exception e){e.printStackTrace();}/*2try(FileOutputStream fos3 = new FileOutputStream("fos.dat");    ){fos3.write(23);System.out.print("over");}catch(Exception e){e.printStackTrace();}/*1try(FileOutputStream fos4 = new FileOutputStream("fos.dat"); ){fos4.write(23);System.out.print("over");}catch(Exception e){e.printStackTrace();}*/

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