JAVA常用代码（一）

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);