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。