Java_基础知识回顾

1、ByteArrayInputStream、 ByteArrayOutputStream 

String str = "ZHANGSAN";
      //System.out.println(str.toLowerCase());
      ByteArrayInputStream inputStream = new ByteArrayInputStream(str.getBytes());
      ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
      int b;
      while((b = inputStream.read()) != -1){
          char lowerCase = Character.toLowerCase((char)b);
          outputStream.write(lowerCase);
      }
      byte[] lowerCases = outputStream.toByteArray();
      System.out.println(new String(lowerCases,0, lowerCases.length));

全部在内存中完成byte的转换

2、PrintStream：向目标打印

属于OutputStream的实现类，向目标(可能是文件、标准输出屏幕、输出流、网络等)打印各种样式，不过比一般的输出提供了更多的打印方式，可以打印各种数据类型和样式等

OutputStream outputStream = System.out;
        outputStream.write("helloABC张三".getBytes());
        outputStream.close();

列出当前操作系统的系统参数，输出到文件中

PrintStream printStream = new PrintStream("hello.txt");
      System.getProperties().list(printStream);
      printStream.close();

3、InputStreamReader、OutputStreamWriter， 计算机存储都是字节流的形式，而读取到内存中需要识别一个个的字符(人只能识别字符)，有可能1个字节就代表一个字符(例如英文),也有可能多个字节才能转换成一个字符(例如中文)，如果中间存在丢失或无法转换，则看到的就是一堆?

InputStreamReader:将输入的内容字节变成字符

OutputStreamWriter:将输出的内容从字符变成字节

4、合并流：SequenceInputStream 

File file1 = new File("hello.txt");
   File file2 = new File("test.txt");

   InputStream inputStream1 = new FileInputStream(file1);
   InputStream inputStream2 = new FileInputStream(file2);

   SequenceInputStream sequenceInputStream = new SequenceInputStream(inputStream1,
           inputStream2);
   BufferedInputStream bufferedInputStream = new BufferedInputStream(sequenceInputStream);

   int c;
   while((c = bufferedInputStream.read()) != -1){
       System.out.print((char)c);
   }
   bufferedInputStream.close();
   sequenceInputStream.close();
   inputStream1.close();
   inputStream2.close();

测试结果:helloworld 

在实验中，从bufferedInputStream去取到两个文件大小相加的byte数组中,代码如下，转换出来有问题，有点奇怪，只读到了前一个流中的内容，后面一个流中的内容没读取进来。思考中...

File file1 = new File("hello.txt");
    File file2 = new File("test.txt");

    InputStream inputStream1 = new FileInputStream(file1);
    InputStream inputStream2 = new FileInputStream(file2);

    SequenceInputStream sequenceInputStream = new SequenceInputStream(inputStream1,
            inputStream2);
    BufferedInputStream bufferedInputStream = new BufferedInputStream(sequenceInputStream);

    int length = (int) (file1.length() + file2.length());
    byte[] b = new byte[length];
    bufferedInputStream.read(b, 0, length);
    System.out.println(new String(b,0,length));

    bufferedInputStream.close();
    sequenceInputStream.close();
    inputStream1.close();
    inputStream2.close();

 测试结果如下：hello

5、Zip压缩与解压

压缩程序：

ZipOutputStream zipOutputStream = new ZipOutputStream(new FileOutputStream("hello.zip"));
       zipOutputStream.setLevel(9);

       ZipEntry zipEntry = new ZipEntry("a.txt");
       zipOutputStream.putNextEntry(zipEntry);

       BufferedInputStream bufferedInputStream = new BufferedInputStream(new FileInputStream("test.txt"));
       int content;
       while((content = bufferedInputStream.read()) != -1){
           zipOutputStream.write(content);
       }
       bufferedInputStream.close();
       zipOutputStream.closeEntry();
       zipOutputStream.flush();
       zipOutputStream.close();

 解压程序：

ZipInputStream zipInputStream = new ZipInputStream(new FileInputStream("hello.zip"));
   ZipEntry zipEntry = null;
   while ((zipEntry = zipInputStream.getNextEntry()) != null) {
       BufferedOutputStream bufferedOutputStream = new BufferedOutputStream(
               new FileOutputStream(zipEntry.getName()));
       int content = 0;
       while((content = zipInputStream.read()) != -1){
           bufferedOutputStream.write(content);
       }
       bufferedOutputStream.flush();
       bufferedOutputStream.close();
   }
   zipInputStream.close();

6、zip压缩某目录下的所有文件及子文件

public void zipDirectory(File pathname, ZipOutputStream zipOutputStream) throws Exception {
    if (!pathname.isDirectory()) {
        return;
    }
    File[] files = pathname.listFiles();
    for (File file : files) {
        if (file.isDirectory()) {
            zipDirectory(file, zipOutputStream);
        } else {
            ZipEntry zipEntry = new ZipEntry(pathname.getName() + File.separator
                    + file.getName());
            zipOutputStream.putNextEntry(zipEntry);

            BufferedInputStream bufferedInputStream = new BufferedInputStream(
                    new FileInputStream(file));

            int i;
            while ((i = bufferedInputStream.read()) != -1) {
                zipOutputStream.write(i);
            }

            bufferedInputStream.close();
            zipOutputStream.flush();
            zipOutputStream.closeEntry();
        }
    }

}

 问题：中文编码存在问题，建议选用import org.apache.tools.zip.ZipEntry;

import org.apache.tools.zip.ZipOutputStream，由于其存在方法out.setEncoding("gbk");//指定编码为gbk

6、ThreadLocal

final ThreadLocal<String> threadLocal = new ThreadLocal<String>();
      threadLocal.set("main--");

      Thread thread = new Thread() {
          @Override
          public void run() {
              threadLocal.set("thread--");
              Thread.yield();
              System.out.println(Thread.currentThread().getName() + ":" + threadLocal.get());
          }

      };
      thread.start();
      Thread.yield();
      System.out.println(Thread.currentThread().getName() + ":" + threadLocal.get());

7、数组和List之间的转换

数组->List: Arrays.asList(a)

List->数组:list.toArray()

8、正则表达式

(1)^：在[]内表示取反，在外面表示开头

(2)group

String regex = "[a-z]{3,5}";
        Pattern pattern = Pattern.compile(regex, Pattern.CASE_INSENSITIVE);
        Matcher matcher = pattern.matcher("Abc_defgh_aa_ABCD1");
        while (matcher.find()) {
            System.out.print("位置[左闭右开区间):"+matcher.start() + "_" + matcher.end() + ", 匹配内容:");
            System.out.println(matcher.group());
        }

测试结果:

位置[左闭右开区间):0_3, 匹配内容:Abc
位置[左闭右开区间):4_9, 匹配内容:defgh
位置[左闭右开区间):13_17, 匹配内容:ABCD

(3)邮件的正则表达式

[\w[_.]]+@[\w[_.]]+\.[\w]+

(4)"."点号在正则表达式中表示任何字符， 需要表示点号的时候必须转义\.

(5)group的分组

分组是以正则表达式中的小括号'()'为标准的，当匹配成功后，group或group(0)表示匹配的整个字符串,group(1)代表正则中第一个小括号匹配到的内容,group(2)代表第二个小括号代表的内容，依次类推

(6)匹配require引入文件

"^[\s]*#require[\s]*\("([\w./]+)"\)[\s]*$"含义为：以任意空白字符开头，在#require,再任意空白字符，再(",再任意字母、点号、斜线, 再"),最后任意个空白字符结尾

测试代码：

public static void main(String[] args) {
     FileInputStream fileInputStream = null;
     try {
         fileInputStream = new FileInputStream(new File(
                 "C:/Documents and Settings/***/My Documents/tmp/hello.js"));
     } catch (FileNotFoundException e) {
         e.printStackTrace();
     }

     InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream,
             Charset.defaultCharset());

     BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
     String line = "";
     try {
         while ((line = bufferedReader.readLine()) != null) {
             String requireFile = getRequireFile(line);
             System.out.println(requireFile);
         }
     } catch (IOException e) {
         e.printStackTrace();
     } finally {
         try {
             bufferedReader.close();
             inputStreamReader.close();
             fileInputStream.close();
         } catch (IOException e) {
             e.printStackTrace();
         }
     }

 }

 private static String getRequireFile(String line) {
     String requireFile = "";
     Pattern pattern = Pattern
             .compile("^[\s]*#require[\s]*\("([\w./]+)"\)[\s]*$", Pattern.MULTILINE);
     Matcher matcher = pattern.matcher(line);
     while (matcher.find()) {
         requireFile = matcher.group(1);
     }
     return requireFile;
 }

测试文件内容:

var param;
#require("hello/world_util/alibaba123_utils.js")
    #require("/abc/world_util/alibaba12666_utils.js")

测试结果

hello/world_util/alibaba123_utils.js
/abc/world_util/alibaba12666_utils.js

9、FileReader有待完备的地方，只能使用系统默认的字符集，而没有提供传递字符集的构造函数

FileReader继承了InputStreamReader，但并没有实现父类中带字符集参数的构造函数，所以
FileReader只能按系统默认的字符集来解码

10、阻塞队列：BlockingQueue 

生产者中的 put() 操作会在没有空间可用时阻塞，而消费者的 take() 操作会在队列中没有任何东西时阻塞。

11、信号量：Semaphore, 允许规定数量的线程进入操作，释放之后其他进入执行

Runnable limitedCall = new Runnable() {
        final Random    rand      = new Random();
        final Semaphore available = new Semaphore(3);
        int             count     = 0;

        public void run() {
            int time = rand.nextInt(15);
            int num = count++;

            try {
                available.acquire();

                System.out.println("Executing " + "long-running action for " + time
                        + " seconds... #" + num);

                Thread.sleep(time * 1000);

                System.out.println("Done with #" + num + "!");

                available.release();
            } catch (InterruptedException intEx) {
                intEx.printStackTrace();
            }
        }
    };

    for (int i = 0; i < 10; i++)
        new Thread(limitedCall).start();

 12、死锁

public class Demo06 {
    public static void main(String[] args) {
        DeadLock deadLock1 = new DeadLock();
        DeadLock deadLock2 = new DeadLock();
        deadLock1.setFlag(true);
        deadLock2.setFlag(false);

        new Thread(deadLock1).start();
        new Thread(deadLock2).start();
    }
}

class DeadLock implements Runnable {
    private boolean flag = false;

    public boolean isFlag() {
        return flag;
    }

    public void setFlag(boolean flag) {
        this.flag = flag;
    }

    private static Object object1 = new Object();
    private static Object object2 = new Object();

    public void run() {
        if (flag) {
            synchronized (object1) {
                System.out.println(Thread.currentThread().getName() + " get object1.");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (object2) {
                    System.out.println(Thread.currentThread().getName() + " get object2.");

                }
            }

        } else {
            synchronized (object2) {
                System.out.println(Thread.currentThread().getName() + " get object2.");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (object1) {
                    System.out.println(Thread.currentThread().getName() + " get object1.");

                }
            }

        }
    }

}

13、反射：通过classloader加载类，标准做法如下：

ClassLoader cl = Thread.currentThread().getContextClassLoader();
if (cl == null) cl = MyClass.class.getClassLoader(); // fallback
Class clazz = cl.loadClass(name);

14、文件大小限制

错误做法：

public int getFileSize(File f) {
  long l = f.length();
  return (int) l;
}

正确做法如下：

不支持传递超过2GB的文件. 最好的做法是对长度进行检查, 溢出时抛出异常

public int getFileSize(File f) {
  long l = f.length();
  if (l > Integer.MAX_VALUE) throw new IllegalStateException("int overflow");
  return (int) l;
}

 15、线程sleep中断

try {
        Thread.sleep(1000);
} catch (InterruptedException e) {
        Thread.currentThread().interrupt();
}
or
while (true) {
        if (Thread.currentThread().isInterrupted()) break;
}

16、开发中常用术语解释

java的几种对象(PO,VO,DAO,BO,POJO)解释  
   一、PO:persistant object 持久对象,可以看成是与数据库中的表相映射的java对象。最简单的PO就是对应数据库中某个表中的一条记录，多个记录可以用PO的集合。PO中应该不包含任何对数据库的操作。

   二、VO:value object值对象。通常用于业务层之间的数据传递，和PO一样也是仅仅包含数据而已。但应是抽象出的业务对象,可以和表对应,也可以不,这根据业务的需要.个人觉得同DTO(数据传输对象),在web上传递。 

   三、DAO:data access object 数据访问对象，此对象用于访问数据库。通常和PO结合使用，DAO中包含了各种数据库的操作方法。通过它的方法,结合PO对数据库进行相关的操作。 

   四、BO:business object 业务对象,封装业务逻辑的java对象,通过调用DAO方法,结合PO,VO进行业务操作。 

   五、POJO:plain ordinary java object 简单无规则java对象,我个人觉得它和其他不是一个层面上的东西,VO和PO应该都属于它。 

17、多线售票系统：

class TicketSeller implements Runnable {
    private int ticketCount = 10;
    @Override
    public void run() {
        while (ticketCount > 0) {
            synchronized (this) {
                if (ticketCount > 0) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName()
                            + " sell ticket: " + ticketCount--);
                }

            }
        }
    }

}

public class Demo01 {
    public static void main(String[] args) {
        TicketSeller ticketSeller = new TicketSeller();
        new Thread(ticketSeller, "Thread A").start();
        new Thread(ticketSeller, "Thread B").start();
        new Thread(ticketSeller, "Thread C").start();
        new Thread(ticketSeller, "Thread D").start();
    }
}

 测试结果：

Thread A sell ticket: 10
Thread A sell ticket: 9
Thread D sell ticket: 8
Thread D sell ticket: 7
Thread D sell ticket: 6
Thread C sell ticket: 5
Thread C sell ticket: 4
Thread C sell ticket: 3
Thread B sell ticket: 2
Thread B sell ticket: 1

18、中断处理

class TicketSeller implements Runnable {

    @Override
    public void run() {
        try {
            System.out.println("线程启动");
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            System.out.println("线程被中断");
            // e.printStackTrace();
        }
    }

}

public class Demo01 {
    public static void main(String[] args) throws InterruptedException {
        TicketSeller ticketSeller = new TicketSeller();
        Thread thread = new Thread(ticketSeller, "Thread A");
        thread.start();

        System.out.println("====主线程执行===");
        Thread.sleep(1000);
        // thread.interrupt();
        System.out.println("线程被中断否：" + thread.isInterrupted());
        thread.interrupt();
        System.out.println("线程被中断否：" + thread.isInterrupted());
        System.out.println("线程被中断否2：" + thread.isInterrupted());
        System.out.println("主线程是否被中断：" + Thread.interrupted());

        System.out.println("====主线程结束===");

    }
}

 测试结果：

====主线程执行===
线程启动
线程被中断否：false
线程被中断否：true
线程被中断否2：true
主线程是否被中断：false
线程被中断
====主线程结束===

结论： 

 interrupt中断该线程， isInterrupted检查该线程是否被中断， interrupted检查当前线程是否被中断。