Java 必备面试代码
1. Thread 创建线程
public Class MyThread extends Thread{
public void run(){
System.out.println("aaa");
}
}
public Class Main{
public static void main(String[] args){
MyThread t = new MyThread();
t.start();
}
}2. Runnable 创建线程
public Class ThreadTask implements Runnable{
public void run(){
System.out.println("aaa");
}
}
public Class Main{
public static void main(String[] args){
ThreadTask task = new ThreadTask();
Thread t = new Thread(task);
thread.start();
}
}- 使用线程池创建多线程Callable
public MyCallable implements Callable<String>{
public String call(){
System.out.println("call");
return "call";
}
}
public Class Main{
public static void main(String[] args){
ExecutorService pool = new Executor.newFixedThreadPool(taskSize);
List<Future> list = new ArrayList<Future>();
for(int i=0;i<taskSize;i++){
Callable c = new MyCallable(i + " ");
Future f = pool.submit(c);
list.add(f);
}
// 关闭线程池
pool.shutdown();
//获取并发任务的运行结果
for(Future f :list){
System.out.println("res " + f.get().toString());
}
}
}4. 基于线程池 创建多线程
public Class Main{
public static void main(String[] args){
ExecutorService pool = Executors.newFixedThreadPool(10);
while(true){
Runnable r = new MyRunnable();
pool.execute(r);
}
}
}5. 创建3线程 1 打印 123,2 打印 456， 3 打印 789
public ThreadTask {
// 打印序号
private int no = 1;
// 线程维持的序号状态
private int status = 0;
public synchronized void print(String threadNo){
int threadIndex = Integer.parseInt(threadNo);
// 检验线程的顺序
while(threadIndex != status){
try{
this.wait();
}catch(InterruptedException e){
e.printStackTrace();
}
}
// 每个线程每次打印的个数
for(int count = 0;count < 5;count ++,no ++){
if(count > 0){
System.out.print(",");
}
System.out.print(no)
}
System.out.println()
status = (status + 1) % 3；
// 唤醒其他线程
this.notifyAll();
}
}
public class Problem02{
public static void main(String[] args){
ThreadTask task = new ThreadTask();
for(int i =0;i<3;i++){
new Thread(new Runnable(){
public void run(){
// 每一个线程打印的次数
for(int j=0;j<5;j++){
task.print(Thread.currentThread().getName());
}
}
}) ;
}
}
}6. 创建多线程火车票买票系统
public Class SellTackets implements Runnable{
private int tackets = 100;
private Object lock = new Object();
public void run(){
while(true){
synchronized(lock){
if(tackets > 0){
System.out.println(Thread.currentThread().getName() + " : " + tackets -- + " " );
}else{
break;
}
}
}
}
}// 使用 lock 进行加锁
public Class SellTackets implements Runnable{
private int tackets = 100;
private Lock lock = new ReentrantLock();
public void run(){
while(true){
try{
lock.lock();
if(tackets > 0){
System.out.println(Thread.currentThread().getName() + " : " + tackets -- + " " );
}else{
break;
}
}finally{
lock.unlock();
}
}
}
}7. 生产者与消费者
public class cache {
private static final int MAX_PRODUCT = 20;
private int cacheSzie = 0;
private Object lock = new Object();
public Cache(int size){
this.cacheSize = size;
}
public void produce(){
synchronized(lock){
while(cacheSize >= MAX_PRODUCT){
try{
lock.wait();
}catch(InterruptedException e){
e.printStackTrace();
}
}
cacheSize ++;
lock.notifyAll();
}
}
public void consumer(){
synchronized(lock){
while(cache <= 0){
try{
lock.wait();
}catch(InterruptException e){
e.printStackTrace();
}
}
cacheSize --;
lock.notifyAll();
}
}
}
public Class Produce{
private Cache cache;
public Produce(Cache cache){
this.cache = cache;
}
public void run(){
while(true){
cache.produce();
}
}
}
public Class Consumer{
private Cache cache;
public Consumer(Cache cache){
this.cache = cache;
}
public void run(){
while(true){
cache.consumer();
}
}
}// 使用 ReentrantLock() 实现生产者与消费者
public class cache {
private static final int MAX_PRODUCT = 20;
private int cacheSzie = 0;
private Lock lock = new ReentrantLock();
private Condition consumerList = null;
private Condition produceList = null;
public Cache(int size){
this.cacheSize = size;
consumerList = new Condition();
produceList = new Condition();
}
public void produce(){
lock.lock();
if(cacheSize >= MAX_PRODUCT){
try{
produceList .await();
}catch(InterruptedException e){
e.printStackTrace();
}
}
cacheSize ++;
produceList .signalAll();
lock.unlock();
}
public void consumer(){
lock.lock()
while(cache <= 0){
try{
consumerList .await();
}catch(InterruptException e){
e.printStackTrace();
}
}
cacheSize --;
consumerList .signalAll();
lock.unlock();
}
}
}1. 通过JAVA 反射实现对象的浅拷贝
public static Object copy(Object source) throws Exception{
Object o = null;
Class clazz = source.getClass();
o = clazz.newInstance();
while(clazz != Object.class){
Field fields[] = clazz.getDeclaredFields();
for(Field field : fields) {
if(Modifier.isStatic(field.getModifiers()))
continue;
field.setAccessible(true);
Object value = field.get(source);
// 基本类型
if(field.getType().isPrimitive()) {
field.set(o, value);
}
// 这行就是深度拷贝，否则浅拷贝
// 不为null的对象
else if(value instanceof Object) {
field.set(o, copy(value));
}
field.setAccessible(false);
}
clazz = clazz.getSuperclass();
}
return o;
}// 通过实现序列化和IO流来实现深度copy
public static Object deepCopy(Object o) throws IOException, ClassNotFoundException {
// //先序列化，写入到流里
ByteArrayOutputStream bo = new ByteArrayOutputStream();
ObjectOutputStream oo = new ObjectOutputStream(bo);
oo.writeObject(o);
//然后反序列化，从流里读取出来，即完成复制
ByteArrayInputStream bi = new ByteArrayInputStream(bo.toByteArray());
ObjectInputStream oi = new ObjectInputStream(bi);
return oi.readObject();
}2. 遍历指定目录下的所有文件
public class FileDemo4 {
public static void main(String[] args) {
File srcFolder = new File("D:\\");
getAllClassFilesPath(srcFolder);
}
private static void getAllClassFilesPath(File srcFolder) {
try {
//获得当前目录下的所有文件夹
File[] files = srcFolder.listFiles();
//遍历files对象
for (File file : files) {
//判断当前是否为文件夹
if(file.isDirectory()){
getAllClassFilesPath(file);
}else{
if(file.getName().endsWith(".class")){
file.renameTo(new File("f:/a/b.xlsx"));
System.out.println(file.getAbsolutePath());
}
}
}
} catch (Exception e) {
e.getMessage();
}
}
}3.多级文件夹的复制
public class copy {
public static void main(String[] args) {
//数据源
File srcFolder = new File("H:\\java");
//目的地
File destFolder = new File("F:\\");
//复制多级文件
try {
copyFollder(srcFolder,destFolder);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
private static void copyFollder(File srcFolder, File destFolder) throws Exception {
if(srcFolder.isDirectory()){
String newname = srcFolder.getName();
File newFolder = new File(destFolder,newname);
newFolder.mkdir();
File[] files = srcFolder.listFiles();
for (File file : files) {
copyFollder(file, newFolder);
}
}else{
File newFile = new File(destFolder,srcFolder.getName());
copyFile(srcFolder,newFile);
}
}
private static void copyFile(File srcFolder, File newFile) throws IOException {
//数据源
BufferedInputStream bis = new BufferedInputStream(new FileInputStream(srcFolder));
//目的地
BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(newFile));
//字节数组复制
byte[] bs = new byte[1024];
while(bis.read(bs)!=-1){
bos.write(bs);
bos.flush();
}
//释放资源
bos.close();
bis.close();
}
}System.currentTimeMillis();
4.将文本中的字符排序后加入到另一个文本中
/**
- 1.已知s.txt文件中有一个字符串“ndjdnsnakdapiiisnjvmsdsiajdsailn”;
- 2.读取文件的内容，存储到字符串中
- 3.把字符串转化为字符数组
- 4.对字符数组进行排序
- 5.把字符数组转化为字符串
- 6.通过字符输出流把字符串输出到ss.txt
*/
public class StringArray {
public static void main(String[] args) throws IOException {
//1. 封装路径
File srcFolder = new File("H:\\s.txt");
File destFolder = new File("H:\\ss.txt");
//字符读取流
BufferedReader br = new BufferedReader(new FileReader(srcFolder));
//读取字符串
StringBuilder sb = new StringBuilder();
String line = null;
while((line = br.readLine())!=null){
sb.append(line);
}
//字符串转化为字符数组
char[] arrays = sb.toString().toCharArray();
//将字符数组进行排序
Arrays.sort(arrays);
//将字符数组转化为字符串
String str = String.valueOf(arrays);
System.out.println(str);
//建立输出流输出
BufferedWriter bw = new BufferedWriter(new FileWriter(destFolder));
bw.write(str);
bw.flush();
bw.close();
}
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