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();
- }
- }
- 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
- 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===");
- }
- }
- ====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。