JAVA常用代碼積累
Java是一個(gè)面向?qū)ο蟮恼Z言。對(duì)程序員來說，這意味著要注意應(yīng)中的數(shù)據(jù)和操縱數(shù)據(jù)的方法(method)，而不是嚴(yán)格地用過程來思考。下面是小編整理的關(guān)于JAVA常用代碼積累，希望大家認(rèn)真閱讀!
1.獲取環(huán)境變量
System.getenv(“PATH”);
System.getenv(“JAVA_HOME”);
2.獲取系統(tǒng)屬性
System.getProperty(“pencil color”); // 得到屬性值
java -Dpencil color=green
System.getProperty(“java.specification.version”); // 得到Java版本號(hào)
Properties p = System.getProperties(); // 得到所有屬性值
p.list(System.out);
3.String Tokenizer
// 能夠同時(shí)識(shí)別, 和 |
StringTokenizer st = new StringTokenizer(“Hello, World|of|Java”, “, |”);
while (st.hasMoreElements()) {
st.nextToken();
}
// 把分隔符視為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(); // 反轉(zhuǎn)字符串
5. 數(shù)字
// 數(shù)字與對(duì)象之間互相轉(zhuǎn)換 – Integer轉(zhuǎn)int
Integer.intValue();
// 浮點(diǎn)數(shù)的.舍入
Math.round()
// 數(shù)字格式化
NumberFormat
// 整數(shù) -> 二進(jìn)制字符串
toBinaryString()或valueOf()
// 整數(shù) -> 八進(jìn)制字符串
toOctalString()
// 整數(shù) -> 十六進(jìn)制字符串
toHexString()
// 數(shù)字格式化為羅馬數(shù)字
RomanNumberFormat()
// 隨機(jī)數(shù)
Random r = new Random();
r.nextDouble();
r.nextInt();
6. 日期和時(shí)間
// 查看當(dāng)前日期
Date today = new Date();
Calendar.getInstance().getTime();
// 格式化默認(rèn)區(qū)域日期輸出
DateFormat df = DateFormat.getInstance();
df.format(today);
// 格式化制定區(qū)域日期輸出
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);
// 設(shè)置具體日期
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轉(zhuǎn)成Date格式
d3.set(Calendar.YEAR, 1999);
d3.set(Calendar.MONTH, Calendar.APRIL);
d3.set(Calendar.DAY_OF_MONTH, 12);
// 字符串轉(zhuǎn)日期
SimpleDateFormat sdf = new SimpleDateFormat(“yyyy-MM-dd hh:mm:ss”);
Date now = sdf.parse(String);
// 日期加減
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);
// 計(jì)算日期間隔(轉(zhuǎn)換成long來計(jì)算)
today.getTime() – old.getTime();
// 比較日期
Date類型，就使用equals(), before(), after()來計(jì)算
long類型，就使用==, <, >來計(jì)算
// 第幾日
使用Calendar的get()方法
Calendar c = Calendar.getInstance();
c.get(Calendar.YEAR);
// 記錄耗時(shí)
long start = System.currentTimeMillis();
long end = System.currentTimeMillis();
long elapsed = end – start;
System.nanoTime(); //毫秒
// 長整形轉(zhuǎn)換成秒
Double.toString(t/1000D);
7.結(jié)構(gòu)化數(shù)據(jù)
// 數(shù)組拷貝
System.arrayCopy(oldArray, 0, newArray, 0, oldArray.length);
// ArrayList
add(Object o) // 在末尾添加給定元素
add(int i, Object o) // 在指定位置插入給定元素
clear() // 從集合中刪除全部元素
Contains(Object o) // 如果Vector包含給定元素，返回真值
get(int i) // 返回指定位置的對(duì)象句柄
indexOf(Object o) // 如果找到給定對(duì)象，則返回其索引值;否則，返回-1
remove(Object o) // 根據(jù)引用刪除對(duì)象
remove(int i) // 根據(jù)位置刪除對(duì)象
toArray() // 返回包含集合對(duì)象的數(shù)組
// Iterator
List list = new ArrayList();
Iterator it = list.iterator();
while (it.hasNext())
Object o = it.next();
// 鏈表
LinkedList list = new LinkedList();
ListIterator it = list.listIterator();
while (it.hasNext())
Object o = it.next();
// HashMap
HashMap
hm.get(key); // 通過key得到value
hm.put(“No1”, “Hexinyu”);
hm.put(“No2”, “Sean”);
// 方法1: 獲取全部鍵值
Iterator
while (it.hasNext()) {
String myKey = it.next();
String myValue = hm.get(myKey);
}
// 方法2: 獲取全部鍵值
for (String key : hm.keySet()) {
String myKey = key;
String myValue = hm.get(myKey);
}
// Preferences – 與系統(tǒng)相關(guān)的用戶設(shè)置，類似名-值對(duì)
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);
// 用戶設(shè)置了新值，存儲(chǔ)回去
prefs.put(“textFontName”, “new-bright”);
prefs.put(“displayFontName”, “new-balckletter”);
// Properties – 類似名-值對(duì)，key和value之間，可以用”=”，”:”或空格分隔，用”#”和”!”注釋
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. 數(shù)組：Arrays.sort(strings);
2. List：Collections.sort(list);
3. 自定義類：class SubComp implements Comparator
然后使用Arrays.sort(strings, new SubComp())
// 兩個(gè)接口
1. java.lang.Comparable: 提供對(duì)象的自然排序，內(nèi)置于類中
int compareTo(Object o);
boolean equals(Object o2);
2. java.util.Comparator: 提供特定的比較方法
int compare(Object o1, Object o2)
// 避免重復(fù)排序，可以使用TreeMap
TreeMap sorted = new TreeMap(unsortedHashMap);
// 排除重復(fù)元素
Hashset hs – new HashSet();
// 搜索對(duì)象
binarySearch(): 快速查詢 – Arrays, Collections
contains(): 線型搜索 – ArrayList, HashSet, Hashtable, linkedList, Properties, Vector
containsKey(): 檢查集合對(duì)象是否包含給定 – HashMap, Hashtable, Properties, TreeMap
containsValue(): 主鍵(或給定值) – HashMap, Hashtable, Properties, TreeMap
indexOf(): 若找到給定對(duì)象，返回其位置 – ArrayList, linkedList, List, Stack, Vector
search(): 線型搜素 – Stack
// 集合轉(zhuǎn)數(shù)組
toArray();
// 集合總結(jié)
Collection: Set – HashSet, TreeSet
Collection: List – ArrayList, Vector, LinkedList
Map: HashMap, HashTable, TreeMap
8. 泛型與foreach
// 泛型
List
// foreach
for (String s : myList) {
System.out.println(s);
}
9.面向?qū)ο?/strong>
// 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));
}
}
// 覆蓋equals方法
public boolean equals(Object o) {
if (o == this) // 優(yōu)化
return true;
if (!(o instanceof EqualsDemo)) // 可投射到這個(gè)類
return false;
EqualsDemo other = (EqualsDemo)o; // 類型轉(zhuǎn)換
if (int1 != other.int1) // 按字段比較
return false;
if (!obj1.equals(other.obj1))
return false;
return true;
}
// 覆蓋hashcode方法
private volatile int hashCode = 0; //延遲初始化
public int hashCode() {
if (hashCode == 0) {
int result = 17;
result = 37 * result + areaCode;
}
return hashCode;
}
// Clone方法
要克隆對(duì)象，必須先做兩步: 1. 覆蓋對(duì)象的clone()方法; 2. 實(shí)現(xiàn)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”);
}
});
在調(diào)用System.exit(0);的時(shí)候，這兩個(gè)方法將被執(zhí)行
// Singleton模式
// 實(shí)現(xiàn)1
public class MySingleton() {
public static final MySingleton INSTANCE = new MySingleton();
private MySingleton() {}
}
// 實(shí)現(xiàn)2
public class MySingleton() {
public static MySingleton instance = new MySingleton();
private MySingleton() {}
public static MySingleton getInstance() {
return instance;
}
}
// 自定義異常
Exception: 編譯時(shí)檢查
RuntimeException: 運(yùn)行時(shí)檢查
public class MyException extends RuntimeException {
public MyException() {
super();
}
public MyException(String msg) {
super(msg);
}
}
10. 輸入和輸出
// Stream, Reader, Writer
Stream: 處理字節(jié)流
Reader/Writer: 處理字符，通用Unicode
// 從標(biāo)準(zhǔn)輸入設(shè)備讀數(shù)據(jù)
1. 用System.in的BufferedInputStream()讀取字節(jié)
int b = System.in.read();
System.out.println(“Read data: ” + (char)b); // 強(qiáng)制轉(zhuǎn)換為字符
2. BufferedReader讀取文本
如果從Stream轉(zhuǎn)成Reader，使用InputStreamReader類
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為整數(shù)
}
is.close();
// 向標(biāo)準(zhǔn)輸出設(shè)備寫數(shù)據(jù)
1. 用System.out的println()打印數(shù)據(jù)
2. 用PrintWriter打印
PrintWriter pw = new PrintWriter(System.out);
pw.println(“The answer is ” + myAnswer + ” at this time.”);
// Formatter類
格式化打印內(nèi)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();
// 原始掃描
void doFile(Reader is) {
int c;
while ((c = is.read()) != -1) {
System.out.println((char)c);
}
}
// Scanner掃描
Scanner可以讀取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();
}
}
// 讀取文件
BufferedReader is = new BufferedReader(new FileReader(“myFile.txt”));
BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(“bytes.bat”));
is.close();
bos.close();
// 復(fù)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();
// 文件讀入字符串
StringBuffer sb = new StringBuffer();
char[] b = new char[8192];
int n;
// 讀一個(gè)塊，如果有字符，加入緩沖區(qū)
while ((n = is.read(b)) > 0) {
sb.append(b, 0, n);
}
return sb.toString();
// 重定向標(biāo)準(zhǔn)流
String logfile = “error.log”;
System.setErr(new PrintStream(new FileOutputStream(logfile)));
// 讀寫不同字符集文本
BufferedReader chinese = new BufferedReader(new InputStreamReader(new FileInputStream(“chinese.txt”), “ISO8859_1”));
PrintWriter standard = new PrintWriter(new OutputStreamWriter(new FileOutputStream(“standard.txt”), “UTF-8”));
// 讀取二進(jìn)制數(shù)據(jù)
DataOutputStream os = new DataOutputStream(new FileOutputStream(“a.txt”));
os.writeInt(i);
os.writeDouble(d);
os.close();
// 從指定位置讀數(shù)據(jù)
RandomAccessFile raf = new RandomAccessFile(fileName, “r”); // r表示已只讀打開
raf.seek(15); // 從15開始讀
raf.readInt();
raf.radLine();
// 串行化對(duì)象
對(duì)象串行化，必須實(shí)現(xiàn)Serializable接口
// 保存數(shù)據(jù)到磁盤
ObjectOutputStream os = new ObjectOutputStream(new BufferedOutputStream(new FileOutputStream(FILENAME)));
os.writeObject(serialObject);
os.close();
// 讀出數(shù)據(jù)
ObjectInputStream is = new ObjectInputStream(new FileInputStream(FILENAME));
is.readObject();
is.close();
// 讀寫Jar或Zip文檔
ZipFile zippy = new ZipFile(“a.jar”);
Enumeration all = zippy.entries(); // 枚舉值列出所有文件清單
while (all.hasMoreElements()) {
ZipEntry entry = (ZipEntry)all.nextElement();
if (entry.isFile())
println(“Directory: ” + entry.getName());
// 讀寫文件
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();
}
}
// 讀寫gzip文檔
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);
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