献礼了~~Java常用代码一筐打尽

Posted 2020-10-05

1.获取环境变量


System.getenv(“PATH”);


System.getenv(“JAVA_HOME”);


//2.获取系统属性


System.getProperty(“pencil color”); // 得到属性值


java -Dpencil color=green


System.getProperty(“java.specification.version”); // 得到Java版本号


Properties p = System.getProperties(); // 得到所有属性值


p.list(System.out);


//3.String Tokenizer


// 能够同时识别, 和 |


StringTokenizer st = new StringTokenizer(“Hello, World|of|Java”, “, |”);


while (st.hasMoreElements()) {


st.nextToken();


}


// 把分隔符视为token


StringTokenizer st = new StringTokenizer(“Hello, World|of|Java”, “, |”, true);


//4.StringBuffer(同步)和StringBuilder(非同步)


StringBuilder sb = new StringBuilder();


sb.append(“Hello”);


sb.append(“World”);


sb.toString();


new StringBuffer(a).reverse(); // 反转字符串


//5. 数字


// 数字与对象之间互相转换 – Integer转int


Integer.intValue();


// 浮点数的舍入


Math.round()


// 数字格式化


NumberFormat


// 整数 -> 二进制字符串


toBinaryString()或valueOf()


// 整数 -> 八进制字符串


toOctalString()


// 整数 -> 十六进制字符串


toHexString()


// 数字格式化为罗马数字


RomanNumberFormat()


// 随机数


Random r = new Random();


r.nextDouble();


r.nextInt();


//6. 日期和时间


// 查看当前日期


Date today = new Date();


Calendar.getInstance().getTime();


// 格式化默认区域日期输出


DateFormat df = DateFormat.getInstance();


df.format(today);


// 格式化制定区域日期输出


DateFormat df_cn = DateFormat.getDateInstance(DateFormat.FULL, Locale.CHINA);


String now = df_cn.format(today);


// 按要求格式打印日期


SimpleDateFormat sdf = new SimpleDateFormat(“yyyy-MM-dd hh:mm:ss”);


sdf.format(today);


// 设置具体日期


GregorianCalendar d1 = new GregorianCalendar(2009, 05, 06); // 6月6日


GregorianCalendar d2 = new GregorianCalendar(); // 今天


Calendar d3 = Calendar.getInstance(); // 今天


d1.getTime(); // Calendar或GregorianCalendar转成Date格式


d3.set(Calendar.YEAR, 1999);


d3.set(Calendar.MONTH, Calendar.APRIL);


d3.set(Calendar.DAY_OF_MONTH, 12);


// 字符串转日期


SimpleDateFormat sdf = new SimpleDateFormat(“yyyy-MM-dd hh:mm:ss”);


Date now = sdf.parse(String);


// 日期加减


Date now = new Date();


long t = now.getTime();


t += 700*24*60*60*1000;


Date then = new Date(t);


Calendar now = Calendar.getInstance();


now.add(Calendar.YEAR, -2);


// 计算日期间隔(转换成long来计算)


today.getTime() – old.getTime();


// 比较日期


Date类型，就使用equals(), before(), after()来计算


long类型，就使用==, <, >来计算


// 第几日


使用Calendar的get()方法


Calendar c = Calendar.getInstance();


c.get(Calendar.YEAR);


// 记录耗时


long start = System.currentTimeMillis();


long end = System.currentTimeMillis();


long elapsed = end – start;


System.nanoTime(); //毫秒


// 长整形转换成秒


Double.toString(t/1000D);


//7.结构化数据


// 数组拷贝


System.arrayCopy(oldArray, 0, newArray, 0, oldArray.length);


// ArrayList


add(Object o) // 在末尾添加给定元素


add(int i, Object o) // 在指定位置插入给定元素


clear() // 从集合中删除全部元素


Contains(Object o) // 如果Vector包含给定元素，返回真值


get(int i) // 返回指定位置的对象句柄


indexOf(Object o) // 如果找到给定对象，则返回其索引值；否则，返回-1


remove(Object o) // 根据引用删除对象


remove(int i) // 根据位置删除对象


toArray() // 返回包含集合对象的数组


// Iterator


List list = new ArrayList();


Iterator it = list.iterator();


while (it.hasNext())


Object o = it.next();


// 链表


LinkedList list = new LinkedList();


ListIterator it = list.listIterator();


while (it.hasNext())


Object o = it.next();


// HashMap


HashMap<String, String> hm = new HashMap<String, String>();


hm.get(key); // 通过key得到value


hm.put(“No1”, “Hexinyu”);


hm.put(“No2”, “Sean”);


// 方法1: 获取全部键值


Iterator<String> it = hm.values().iterator();


while (it.hasNext()) {


String myKey = it.next();


String myValue = hm.get(myKey);


}


// 方法2: 获取全部键值


for (String key : hm.keySet()) {


String myKey = key;


String myValue = hm.get(myKey);


}


// Preferences – 与系统相关的用户设置，类似名-值对


Preferences prefs = Preferences.userNodeForPackage(ArrayDemo.class);


String text = prefs.get(“textFontName”, “lucida-bright”);


String display = prefs.get(“displayFontName”, “lucida-balckletter”);


System.out.println(text);


System.out.println(display);


// 用户设置了新值，存储回去


prefs.put(“textFontName”, “new-bright”);


prefs.put(“displayFontName”, “new-balckletter”);


// Properties – 类似名-值对，key和value之间，可以用”=”，”:”或空格分隔，用”#”和”!”注释


InputStream in = MediationServer.class.getClassLoader().getResourceAsStream(“msconfig.properties”);


Properties prop = new Properties();


prop.load(in);


in.close();


prop.setProperty(key, value);


prop.getProperty(key);


// 排序


1. 数组：Arrays.sort(strings);


2. List：Collections.sort(list);


3. 自定义类：class SubComp implements Comparator


然后使用Arrays.sort(strings, new SubComp())


// 两个接口


1. java.lang.Comparable: 提供对象的自然排序，内置于类中


int compareTo(Object o);


boolean equals(Object o2);


2. java.util.Comparator: 提供特定的比较方法


int compare(Object o1, Object o2)


// 避免重复排序，可以使用TreeMap


TreeMap sorted = new TreeMap(unsortedHashMap);


// 排除重复元素


Hashset hs – new HashSet();


// 搜索对象


binarySearch(): 快速查询 – Arrays, Collections


contains(): 线型搜索 – ArrayList, HashSet, Hashtable, linkedList, Properties, Vector


containsKey(): 检查集合对象是否包含给定 – HashMap, Hashtable, Properties, TreeMap


containsValue(): 主键(或给定值) – HashMap, Hashtable, Properties, TreeMap


indexOf(): 若找到给定对象，返回其位置 – ArrayList, linkedList, List, Stack, Vector


search(): 线型搜素 – Stack


// 集合转数组


toArray();


// 集合总结


Collection: Set – HashSet, TreeSet


Collection: List – ArrayList, Vector, LinkedList


Map: HashMap, HashTable, TreeMap


//8. 泛型与foreach


// 泛型


List<String> myList = new ArrayList<String>();


// foreach


for (String s : myList) {


System.out.println(s);


}


//9.面向对象


// toString()格式化


public class ToStringWith {


int x, y;


public ToStringWith(int anX, int aY) {


x = anX;


y = aY;


}


public String toString() {


return “ToStringWith[” + x + “,” + y + “]”;


}


public static void main(String[] args) {


System.out.println(new ToStringWith(43, 78));


}


}


// 覆盖equals方法


public boolean equals(Object o) {


if (o == this) // 优化


return true;


if (!(o instanceof EqualsDemo)) // 可投射到这个类


return false;


EqualsDemo other = (EqualsDemo)o; // 类型转换


if (int1 != other.int1) // 按字段比较


return false;


if (!obj1.equals(other.obj1))


return false;


return true;


}


// 覆盖hashcode方法


private volatile int hashCode = 0; //延迟初始化


public int hashCode() {


if (hashCode == 0) {


int result = 17;


result = 37 * result + areaCode;


}


return hashCode;


}


// Clone方法


要克隆对象，必须先做两步: 1. 覆盖对象的clone()方法; 2. 实现空的Cloneable接口


public class Clone1 implements Cloneable {


public Object clone() {


return super.clone();


}


}


// Finalize方法


Object f = new Object() {


public void finalize() {


System.out.println(“Running finalize()”);


}


};


Runtime.getRuntime().addShutdownHook(new Thread() {


public void run() {


System.out.println(“Running Shutdown Hook”);


}


});


在调用System.exit(0);的时候，这两个方法将被执行


// Singleton模式


// 实现1


public class MySingleton() {


public static final MySingleton INSTANCE = new MySingleton();


private MySingleton() {}


}


// 实现2


public class MySingleton() {


public static MySingleton instance = new MySingleton();


private MySingleton() {}


public static MySingleton getInstance() {


return instance;


}


}


// 自定义异常


Exception: 编译时检查


RuntimeException: 运行时检查


public class MyException extends RuntimeException {


public MyException() {


super();


}


public MyException(String msg) {


super(msg);


}


}


//10. 输入和输出


// Stream, Reader, Writer


Stream: 处理字节流


Reader/Writer: 处理字符，通用Unicode


// 从标准输入设备读数据


1. 用System.in的BufferedInputStream()读取字节


int b = System.in.read();


System.out.println(“Read data: ” + (char)b); // 强制转换为字符


2. BufferedReader读取文本


如果从Stream转成Reader，使用InputStreamReader类


BufferedReader is = new BufferedReader(new


InputStreamReader(System.in));


String inputLine;


while ((inputLine = is.readLine()) != null) {


System.out.println(inputLine);


int val = Integer.parseInt(inputLine); // 如果inputLine为整数


}


is.close();


// 向标准输出设备写数据


1. 用System.out的println()打印数据


2. 用PrintWriter打印


PrintWriter pw = new PrintWriter(System.out);


pw.println(“The answer is ” + myAnswer + ” at this time.”);


// Formatter类


格式化打印内容


Formatter fmtr = new Formatter();


fmtr.format(“%1$04d – the year of %2$f”, 1951, Math.PI);


或者System.out.printf();或者System.out.format();


// 原始扫描


void doFile(Reader is) {


int c;


while ((c = is.read()) != -1) {


System.out.println((char)c);


}


}


// Scanner扫描


Scanner可以读取File, InputStream, String, Readable


try {


Scanner scan = new Scanner(new File(“a.txt”));


while (scan.hasNext()) {


String s = scan.next();


}


} catch (FileNotFoundException e) {


e.printStackTrace();


}


}


// 读取文件


BufferedReader is = new BufferedReader(new FileReader(“myFile.txt”));


BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(“bytes.bat”));


is.close();


bos.close();


// 复制文件


BufferedIutputStream is = new BufferedIutputStream(new FileIutputStream(“oldFile.txt”));


BufferedOutputStream os = new BufferedOutputStream(new FileOutputStream(“newFile.txt”));


int b;


while ((b = is.read()) != -1) {


os.write(b);


}


is.close();


os.close();


// 文件读入字符串


StringBuffer sb = new StringBuffer();


char[] b = new char[8192];


int n;


// 读一个块，如果有字符，加入缓冲区


while ((n = is.read(b)) > 0) {


sb.append(b, 0, n);


}


return sb.toString();


// 重定向标准流


String logfile = “error.log”;


System.setErr(new PrintStream(new FileOutputStream(logfile)));


// 读写不同字符集文本


BufferedReader chinese = new BufferedReader(new InputStreamReader(new FileInputStream(“chinese.txt”), “ISO8859_1”));


PrintWriter standard = new PrintWriter(new OutputStreamWriter(new FileOutputStream(“standard.txt”), “UTF-8”));


// 读取二进制数据


DataOutputStream os = new DataOutputStream(new FileOutputStream(“a.txt”));


os.writeInt(i);


os.writeDouble(d);


os.close();


// 从指定位置读数据


RandomAccessFile raf = new RandomAccessFile(fileName, “r”); // r表示已只读打开


raf.seek(15); // 从15开始读


raf.readInt();


raf.radLine();


// 串行化对象


对象串行化，必须实现Serializable接口


// 保存数据到磁盘


ObjectOutputStream os = new ObjectOutputStream(new BufferedOutputStream(new FileOutputStream(FILENAME)));


os.writeObject(serialObject);


os.close();


// 读出数据


ObjectInputStream is = new ObjectInputStream(new FileInputStream(FILENAME));


is.readObject();


is.close();


// 读写Jar或Zip文档


ZipFile zippy = new ZipFile(“a.jar”);


Enumeration all = zippy.entries(); // 枚举值列出所有文件清单


while (all.hasMoreElements()) {


ZipEntry entry = (ZipEntry)all.nextElement();


if (entry.isFile())


println(“Directory: ” + entry.getName());


// 读写文件


FileOutputStream os = new FileOutputStream(entry.getName());


InputStream is = zippy.getInputStream(entry);


int n = 0;


byte[] b = new byte[8092];


while ((n = is.read(b)) > 0) {


os.write(b, 0, n);


is.close();


os.close();


}


}


// 读写gzip文档


FileInputStream fin = new FileInputStream(FILENAME);


GZIPInputStream gzis = new GZIPInputStream(fin);


InputStreamReader xover = new InputStreamReader(gzis);


BufferedReader is = new BufferedReader(xover);


String line;


while ((line = is.readLine()) != null)


System.out.println(“Read: ” + line);

 

社区源码下载：http://www.jinhusns.com/Products/Download/?type=yhq