Java基础知识回顾-5 常用基础类

1、排序与查找
使用Arrays可以对数组进行排序与查找操作。Arrays.

sort(Object[] a)可以对实现了Comparable接口的对象进行排序；使用

binarySearch(Object[] a, Object key)将可以对已经排序好了的对象数组进行二分法查找。实例如下：

class Student implements Comparable
{
int num;
String name;
Student(int num,String name)
{
this.num=num;
this.name=name;
}
public String toString()
{
return "number="+num+","+"name="+name;
}
public int compareTo(Object o)
{
Student s=(Student)o;
//return num>s.num ? 1 : (num==s.num ? 0 : -1);
int result=num>s.num ? 1 : (num==s.num ? 0 : -1); 
if(0==result)
{
result=name.compareTo(s.name);
}
return result;
}
}

排序和查找：

Student[] ss=new Student[]{new Student(1,"zhangsan"),
new Student(2,"lisi"),
new Student(3,"wangwu"),
new Student(3,"mybo")};
Arrays.sort(ss);
for(int i=0;i<ss.length;i++)
{
System.out.println(ss[i]);
}
int index=Arrays.binarySearch(ss,new Student(2,"lisi"));
System.out.println("index="+index);
System.out.println(ss[index]);
}

2、封装类和基本类型转换

int i=3;
Integer in=new Integer(i);
int j=in.intValue();
System.out.println("j="+j);
String str=in.toString();
System.out.println("str="+str);

String str2="134";
System.out.println(Integer.valueOf(str2));

3、反射动态获取含参数的构造函数以及创建对象

Class c=Class.forName(args[0]);
Constructor[] cons=c.getDeclaredConstructors();
/*for(int i=0;i<cons.length;i++)
{
System.out.println(cons[i]);
}
Method[] ms=c.getDeclaredMethods();
for(int i=0;i<ms.length;i++)
{
System.out.println(ms[i]);
}*/
Class[] params=cons[0].getParameterTypes();
Object[] paramValues=new Object[params.length];
for(int i=0;i<params.length;i++)
{
if(params[i].isPrimitive())
{
paramValues[i]=new Integer(i+3);
}
}
Object o=cons[0].newInstance(paramValues);
Method[] ms=c.getDeclaredMethods();
ms[0].invoke(o,null);

class Point
{
static
{
System.out.println("Loading Point");
}
int x,y;
void output()
{
System.out.println("x="+x+","+"y="+y);
}
Point(int x,int y)
{
this.x=x;
this.y=y;
}
}

4、获取运行时相关信息、进程相关信息：Runtime类、Process类

Runtime rt=Runtime.getRuntime();
System.out.println(rt.freeMemory());
System.out.println(rt.totalMemory());
try
{
//rt.exec("notepad");
Process p=rt.exec("javac ArrayTest.java");
InputStream is=p.getInputStream();
int data;
while((data=is.read())!=-1)
{
System.out.print((char)data);
}

5、Socket编程
TCP编程：
Server端先启动，并处于阻塞状态。步骤：绑定端口、调用accept方法处于监听状态、输入输出流进行交互、关闭各项资源

public class ServerService {
public static void main(String[] args) throws Exception {
ServerSocket ss = new ServerSocket(6000);
Socket s = ss.accept();
OutputStream os = s.getOutputStream();
os.write("welcome client, this is server!".getBytes());

InputStream is = s.getInputStream();
byte[] b = new byte[100];
int len = is.read(b);

System.out.println(new String(b,0, len));

is.close();
os.close();
s.close();
ss.close();
}
}

Client启动，并连接到Server。步骤：开启到某ip和端口的连接、输入输出流进行交互、关闭各项资源

public class Client {
public static void main(String[] args) throws Exception {
Socket s = new Socket("127.0.0.1", 6000);
OutputStream os = s.getOutputStream();
os.write("hello server, this is client!".getBytes());
InputStream is = s.getInputStream();
byte[] b = new byte[100];
int len = is.read(b);
System.out.println(new String(b, 0, len));

is.close();
os.close();
s.close();
}
}

改进版：多线程server，持续运行

public class ServerService {
public static void main(String[] args) throws Exception {
ServerSocket serverSocket = new ServerSocket(6000);
Server server = new Server(serverSocket);
new Thread(server).start();
}

}

class Server implements Runnable {
private ServerSocket serverSocket;

public Server(ServerSocket serverSocket) {
this.serverSocket = serverSocket;
}

@Override
public void run() {
Socket s = null;
try {
while(true){
s = serverSocket.accept();
OutputStream os = s.getOutputStream();
os.write("welcome client, this is server!".getBytes());

InputStream is = s.getInputStream();
byte[] b = new byte[100];
int len = is.read(b);

System.out.println(new String(b, 0, len));

is.close();
os.close();
s.close();
}
} catch (IOException e) {
e.printStackTrace();
}

}

}

UDP编程：
接收端编程：UDPReceiver

public class UDPReceiver {
public static void main(String[] args) {
try {
DatagramSocket ds = new DatagramSocket(6000);
byte[] buf = new byte[100];
DatagramPacket dp = new DatagramPacket(buf, 100);
ds.receive(dp);

System.out.println(new String(buf, 0, dp.getLength()));
ds.close();
} catch (SocketException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}

发送端编程：UDPSender

public class UDPSender {
public static void main(String[] args) {
try {
DatagramSocket ds = new DatagramSocket();
String str = "hello, this is zhangsan";
DatagramPacket dp = new DatagramPacket(str.getBytes(), str.length(),
InetAddress.getByName("localhost"), 6000);

ds.send(dp);
ds.close();
} catch (SocketException e) {
e.printStackTrace();
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}

6、文件操作
创建与操作系统无关的文件

File fDir=new File(File.separator);
String strFile="JavaLesson"+File.separator+"Lesson7"+
File.separator+"1.txt";
File f=new File(fDir,strFile);
f.createNewFile();

【注意】File fDir=new File(File.separator); 表示根目录， 在windows下面表示所在目录结构的根目录'D:\'，
在linux或unix下，代表根目录'/'。

7、setToString和mapToString方法
setToString

private <T> String setToString(Set<T> set) {

StringBuffer sb = new StringBuffer();

for (T o : set) {

sb.append(o.toString());

}

return sb.toString();

}

mapToString

private <K, V> String mapToString(Map<K, V> configMap) {

StringBuffer sb = new StringBuffer();

for (K k : configMap.keySet()) {

sb.append(k + "->" + configMap.get(k));

}

return sb.toString();

}

8、多线程程序的停止方式
在多线程程序中，由于Thread类的stop方法已经标记为depracated， 所以不能采取stop方法来停止。需要自己写程序来控制程序的退出，方式有二种：
（1）使用私有boolean flag变量进行控制

private boolean flag = true;

@Override
public void run(){
while(flag){
//...
}
}

public void stopThread(){
flag = false;
}

（2）使用break语句退出循环

@Override
public void run(){
while(true){
if(...)
break; //退出循环
}
}

9、序列化对象
前提：对象必须实现Serializable接口，含有不能被序列化的对象时申明为transient，静态成员变量不能被序列化

ObjectOutputStream os = new ObjectOutputStream();
os.writeObject(new SerObject());

例如，要写入可通过 ObjectInputStream 中的示例读取的对象，请执行以下操作：

FileOutputStream fos = new FileOutputStream("t.tmp");
ObjectOutputStream oos = new ObjectOutputStream(fos);

oos.writeInt(12345);
oos.writeObject("Today");
oos.writeObject(new Date());

oos.close();

例如，要从由 ObjectOutputStream 中的示例写入的流读取：

FileInputStream fis = new FileInputStream("t.tmp");
ObjectInputStream ois = new ObjectInputStream(fis);

int i = ois.readInt();
String today = (String) ois.readObject();
Date date = (Date) ois.readObject();

ois.close();

10、多线程下，同步方法使用的是this监视器，可以与this同步块进行同步，其他对象(如obj)则不可以。

public class Demo01 {

public static void main(String[] args) throws Exception {

MyThread mt = new MyThread();
new Thread(mt).start();
Thread.sleep(10); //必须等待，否则主线程会在自己的时间片内完成flag值的修改，则启动的时候执行的都是obj中的代码，达不到验证同步的目的
mt.flag = true;
new Thread(mt).start();

}
}

class MyThread implements Runnable {
private Logger logger;
private Object object = new Object();
public boolean flag   = false;

public MyThread() {
super();
DOMConfigurator.configure(this.getClass().getClassLoader()
.getResource("com/alibaba/thread/log4j.xml"));
logger = LoggerFactory.getLogger(this.getClass().getName());
}

private int ticket = 100;

/*
* (non-Javadoc)
* @see java.lang.Runnable#run()
*/
public void run() {

if (flag == false) {
while (true) {
sell();
}
} else {
while (true) {
 synchronized (this) { 
if (ticket > 0) {
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (logger.isInfoEnabled()) {
logger.info("obj: " + Thread.currentThread().getName() + " sell: "
+ ticket--);
}
} else {
return;
}
}
}
}

}

public synchronized void sell() { 
if (ticket > 0) {
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (logger.isInfoEnabled()) {
logger.info("syn: " + Thread.currentThread().getName() + " sell: " + ticket--);
}
} else {
return;
}
}

}