Hello World
import java.io.*;
public class GoodFrom {
public static void main (String[] args) {
System.out.println("Hello World");
}
}1. get environment variables
System.getenv("PATH");
System.getenv("JAVA_HOME");
// 2. Get the system property
System.getProperty("pencil color"); // Get the property value
java -Dpencil color= green
System.getProperty("java.specification.version"); // Get the Java version number
Properties p = System.getProperties(); // Get all property values
p.list(System.out);
// 3. String Tokenizer
// Can recognize both, and |
StringTokenizer st = new StringTokenizer("Hello, World|of|Java", ", | ");
while (st.hasMoreElements()) {
st.nextToken();
}
// Treat the separator as a token
StringTokenizer st = new StringTokenizer("Hello, World|of|Java", ", |", true );
// 4. StringBuffer (synchronized) and StringBuilder (non-synchronized)
StringBuilder sb = new StringBuilder();
sb.append("Hello");
sb.append("World");
sb.toString();
new StringBuffer(a).reverse(); // Reverse the string
// 5. Numbers
// Convert between numbers and objects – Integer to int
Integer.intValue();
// rounding of floating point numbers
Math.round()
// number formatting
NumberFormat
// integer -> binary string
toBinaryString() or valueOf()
// integer -> octal string
toOctalString()
// integer -> hex string
toHexString()
// Numbers are formatted as Roman numerals
RomanNumberFormat()
// random number
Random r = new Random();
r.nextDouble();
r.nextInt();
// 6. Date and time
// View the current date
Date today = new Date();
Calendar.getInstance().getTime();
// Format the default locale date output
DateFormat df = DateFormat.getInstance();
df.format(today);
// Format the date output in the designated area
DateFormat df_cn = DateFormat.getDateInstance(DateFormat.FULL, Locale.CHINA);
String now = df_cn.format(today);
// Print the date in the required format
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM- dd hh:mm:ss");
sdf.format(today);
// Set the specific date
GregorianCalendar d1 = new GregorianCalendar(2009, 05, 06); // June 6
GregorianCalendar d2 = new GregorianCalendar(); // Today's
Calendar d3 = Calendar.getInstance(); // Today's
d1.getTime (); // Convert Calendar or GregorianCalendar to Date format
d3.set(Calendar.YEAR, 1999 );
d3.set(Calendar.MONTH, Calendar.APRIL);
d3.set(Calendar.DAY_OF_MONTH, 12 );
// String to date
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM- dd hh:mm:ss");
Date now = sdf.parse(String);
// Date addition and subtraction
Date now = new Date();
long t = now.getTime();
t += 7002460601000 ;
Date then = new Date(t);
Calendar now = Calendar.getInstance();
now.add(Calendar.YEAR, -2 );
// Calculate date interval (convert to long to calculate)
today.getTime() – old.getTime();
// compare dates
Date type, use equals(), before(), after() to calculate
For long type, use ==, <, > to calculate
// day of the day
Use Calendar's get() method
Calendar c = Calendar.getInstance();
c.get(Calendar.YEAR);
// record time
long start = System.currentTimeMillis();
long end = System.currentTimeMillis();
long elapsed = end – start;
System.nanoTime(); // milliseconds
// Convert long integer to seconds
Double.toString(t / 1000D);
// 7. Structured data
// Array copy
System.arrayCopy(oldArray, 0, newArray, 0 , oldArray.length);
// ArrayList
add(Object o) // Add the given element at the end
add( int i, Object o) // Insert the given element at the specified position
clear() // Remove all elements from the collection
Contains(Object o) / / If the Vector contains the given element, return the true value
get( int i) // Return the object handle at the specified position
indexOf(Object o) // If the given object is found, return its index value; otherwise, return -1
remove( Object o) // remove objects by reference
remove( int i) // remove objects by position
toArray() // return an array containing collection objects
// Iterator
List list = new ArrayList();
Iterator it = list.iterator();
while (it.hasNext())
Object o = it.next();
// LinkedList
LinkedList list = new LinkedList();
ListIterator it = list.listIterator();
while (it.hasNext())
Object o = it.next();
// HashMap
HashMap hm = new HashMap();
hm.get(key); // Get value by key
hm.put("No1", "Hexinyu");
hm.put("No2", "Sean");
// Method 1: Get all key values
Iterator it = hm.values().iterator();
while (it.hasNext()) {
String myKey = it.next ();
String myValue = hm.get(myKey);
}
// Method 2: Get all key values
for (String key : hm.keySet()) {
String myKey = key;
String myValue = hm.get(myKey);
}
// Preferences – system-related user settings, like name-value pairs
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);
// The user sets a new value and stores it back
prefs.put("textFontName", " new - bright");
prefs.put("displayFontName", " new - balckletter");
// Properties - similar name-value pairs, between key and value, can be separated by "=", ":" or space, and annotated with "#" and "!"
InputStream in = MediationServer. class .getClassLoader().getResourceAsStream ("msconfig.properties");
Properties prop = new Properties();
prop.load(in);
in.close();
prop.setProperty(key, value);
prop.getProperty(key);
// sort
Arrays: Arrays.sort(strings);
List: Collections.sort(list);
Custom class: class SubComp implements Comparator
Then use Arrays.sort(strings, new SubComp())
// two interfaces
java.lang.Comparable: Provides natural ordering of objects, built into classes
int compareTo(Object o);
boolean equals(Object o2);
java.util.Comparator: provides specific comparison methods
int compare(Object o1, Object o2)
// To avoid repeated sorting, you can use TreeMap
TreeMap sorted = new TreeMap(unsortedHashMap);
// Exclude duplicate elements
Hashset hs - new HashSet();
// search object
binarySearch(): Quick Search – Arrays, Collections
contains(): Line Search – ArrayList, HashSet, Hashtable, linkedList, Properties, Vector
containsKey(): Checks if the collection object contains the given – HashMap, Hashtable, Properties, TreeMap
containsValue(): primary key (or given value) - HashMap, Hashtable, Properties, TreeMap
indexOf(): If the given object is found, return its position – ArrayList, linkedList, List, Stack, Vector
search(): Linear search – Stack
// collection to array
toArray();
// collection summary
Collection: Set – HashSet, TreeSet
Collection: List – ArrayList, Vector, LinkedList
Map: HashMap, HashTable, TreeMap
// 8. Generics and foreach
// Generic
List myList = new ArrayList();
// foreach
for (String s : myList) {
System.out.println(s);
}
// 9. Object Oriented
// toString() formatting
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 ));
}
}
// Override equals method
public boolean equals(Object o) {
if (o == this ) // optimize
return true ;
if (!(o instanceof EqualsDemo)) // Can be cast to this class
return false ;
EqualsDemo other = (EqualsDemo)o; // Type conversion
if (int1 != other.int1) // Compare by field
return false ;
if (! obj1.equals(other.obj1))
return false ;
return true ;
}
// Override the hashcode method
private volatile int hashCode = 0; // Lazy initialization
public int hashCode() {
if (hashCode == 0 ) {
int result = 17 ;
result = 37 * result + areaCode;
}
return hashCode;
}
// Clone method
To clone an object, you must first do two steps: 1. Override the object's clone() method; 2. Implement the empty Cloneable interface
public class Clone1 implements Cloneable {
public Object clone() {
return super .clone();
}
}
// Finalize method
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");
}
});
When calling System.exit( 0 );, these two methods will be executed
// Singleton mode
// implementation 1
public class MySingleton() {
public static final MySingleton INSTANCE = new MySingleton();
private MySingleton() {}
}
// implementation 2
public class MySingleton() {
public static MySingleton instance = new MySingleton();
private MySingleton() {}
public static MySingleton getInstance() {
return instance;
}
}
// custom exception
Exception: compile time check
RuntimeException: runtime check
public class MyException extends RuntimeException {
public MyException() {
super ();
}
public MyException(String msg) {
super (msg);
}
}
// 10. Input and output
// Stream, Reader, Writer
Stream: process byte streams
Reader / Writer: handles characters, generic Unicode
// read data from standard input device
Read bytes with System.in's BufferedInputStream()
int b = System.in.read();
System.out.println("Read data: " + ( char )b); // Force cast to character
BufferedReader reads text
If converting from Stream to Reader, use InputStreamReader class
BufferedReader is = new BufferedReader( new
InputStreamReader(System.in));
String inputLine;
while ((inputLine = is.readLine()) != null ) {
System.out.println(inputLine);
int val = Integer.parseInt(inputLine); // if inputLine is an integer
}
is.close();
// write data to standard output device
Use System.out's println() to print data
Printing with PrintWriter
PrintWriter pw = new PrintWriter(System.out);
pw.println(“The answer is ” + myAnswer + ” at this time.”);
// Formatter class
format print
Formatter fmtr = new Formatter();
fmtr.format(“ %1$04d – the year of %2$f”, 1951 , Math.PI);
or System.out.printf(); or System.out.format();
// raw scan
void doFile(Reader is) {
int c;
while ((c = is.read()) != -1 ) {
System.out.println(( char )c);
}
}
// Scanner scan
Scanner can read 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();
}
}
// Read file
BufferedReader is = new BufferedReader( new FileReader("myFile.txt"));
BufferedOutputStream bos = new BufferedOutputStream( new FileOutputStream("bytes.bat"));
is.close();
bos.close();
// Copy file
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();
//The file is read into the string
StringBuffer sb = new StringBuffer();
char [] b = new char [8192 ];
int n;
// Read a block, if there are characters, add to the buffer
while ((n = is.read(b)) > 0 ) {
sb.append(b, 0 , n);
}
return sb.toString();
// Redirect standard stream
String logfile = "error.log";
System.setErr( new PrintStream( new FileOutputStream(logfile)));
// Read and write text with different character sets
BufferedReader chinese = new BufferedReader( new InputStreamReader( new FileInputStream("chinese.txt"), "ISO8859_1"));
PrintWriter standard = new PrintWriter( new OutputStreamWriter( new FileOutputStream("standard.txt"), "UTF-8 "));
// Read binary data
DataOutputStream os = new DataOutputStream( new FileOutputStream("a.txt"));
os.writeInt(i);
os.writeDouble(d);
os.close();
// Read data from the specified location
RandomAccessFile raf = new RandomAccessFile(fileName, "r"); // r means read-only open
raf.seek( 15); // Read from 15
raf.readInt();
raf.radLine();
// serialize the object
Object serialization must implement the Serializable interface
// Save data to disk
ObjectOutputStream os = new ObjectOutputStream( new BufferedOutputStream( new FileOutputStream(FILENAME)));
os.writeObject(serialObject);
os.close();
// Read data
ObjectInputStream is = new ObjectInputStream( new FileInputStream(FILENAME));
is.readObject();
is.close();
// Read and write Jar or Zip documents
ZipFile zippy = new ZipFile("a.jar");
Enumeration all = zippy.entries(); // The enumeration value lists all files
while (all.hasMoreElements()) {
ZipEntry entry = (ZipEntry)all.nextElement();
if (entry.isFile())
println("Directory: " + entry.getName());
// Read and write files
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();
}
}
// Read and write gzip files
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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