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1 ?泛型(Generic)
1.1 說明 增強了java的類型安全，可以在編譯期間對容器內的對象進行類型檢查，在運行期不必進行類型的轉換。而在j2se5之前必須在運行期動態進行容器內對象的檢查及轉換 減少含糊的容器，可以定義什么類型的數據放入容器 ArrayList<Integer> listOfIntegers; // <TYPE_NAME> is new to the syntax
Integer integerObject;
listOfIntegers = new ArrayList<Integer>(); // <TYPE_NAME> is new to the syntax
listOfIntegers.add(new Integer(10)); // 只能是Integer類型
integerObject = listOfIntegers.get(0); // 取出對象不需要轉換 1.2?? 用法 聲明及實例化泛型類： HashMap<String,Float> hm = new HashMap<String,Float>();
//不能使用原始類型
GenList<int> nList = new GenList<int>();? //編譯錯誤 J2SE 5.0目前不支持原始類型作為類型參數(type parameter) 定義泛型接口： public interface GenInterface<T> {
??? void func(T t);
} 定義泛型類： public class ArrayList<ItemType> { ... }
public class GenMap<T, V> { ... } 例1： public class MyList<Element> extends LinkedList<Element>
{
?????? public void swap(int i, int j)
?????? {
????????????? Element temp = this.get(i);
????????????? this.set(i, this.get(j));
????????????? this.set(j, temp);
?????? }
?????? public static void main(String[] args)
?????? {
????????????? MyList<String> list = new MyList<String>();
????????????? list.add("hi");
????????????? list.add("andy");
????????????? System.out.println(list.get(0) + " " + list.get(1));
????????????? list.swap(0,1);
????????????? System.out.println(list.get(0) + " " + list.get(1));
?????? }
} 例2： public class GenList <T>{
?????? private T[] elements;
?????? private int size = 0;
?????? private int length = 0;
?????? public GenList(int size) {
????????????? elements = (T[])new Object[size];
????????????? this.size = size;
?????? }
?????? public T get(int i) {
????????????? if (i < length) {
???????????????????? return elements[i];
????????????? }
????????????? return null;
?????? }
?????? public void add(T e) {
????????????? if (length < size - 1)
???????????????????? elements[length++] = e;
?????? }
} 泛型方法： public class TestGenerics{
?????? public <T> String getString(T obj) { //實現了一個泛型方法
????????????? return obj.toString();
?????? }
?????? public static void main(String [] args){
????????????? TestGenerics t = new TestGenerics();
????????????? String s = "Hello";
????????????? Integer i = 100;
????????????? System.out.println(t.getString(s));
????????????? System.out.println(t.getString(i));
????????????? }
} 1.3 受限泛型 受限泛型是指類型參數的取值范圍是受到限制的. extends關鍵字不僅僅可以用來聲明類的繼承關系, 也可以用來聲明類型參數(type parameter)的受限關系.例如, 我們只需要一個存放數字的列表, 包括整數(Long, Integer, Short), 實數(Double, Float), 不能用來存放其他類型, 例如字符串(String), 也就是說, 要把類型參數T的取值泛型限制在Number極其子類中.在這種情況下, 我們就可以使用extends關鍵字把類型參數(type parameter)限制為數字 示例 public class Limited<T extends Number> {
?????? public static void main(String[] args) {
????????????? Limited<Integer> number;?? //正確
????????????? Limited<String> str;?????? //編譯錯誤
?????? }
} 1.4? 泛型與異常 類型參數在catch塊中不允許出現，但是能用在方法的throws之后。例： import java.io.*;
interface Executor<E extends Exception> {
?????? void execute() throws E;
}
public class GenericExceptionTest {
?????? public static void main(String args[]) {
????????????? try {
???????????????????? Executor<IOException> e = new Executor<IOException>() {
??????????????????????????? public void execute() throws IOException{
?????????????????????????????????? // code here that may throw an
?????????????????????????????????? // IOException or a subtype of
?????????????????????????????????? // IOException
??????????????????????????? }
??????????????????????????? };
???????????????????? e.execute();
????????????? } catch(IOException ioe) {
???????????????????? System.out.println("IOException: " + ioe);
???????????????????? ioe.printStackTrace();
????????????? }
?????? }
} 1.5 泛型的通配符"?" "?"可以用來代替任何類型, 例如使用通配符來實現print方法。 public static void print(GenList<?> list) {})
?
1.6? 泛型的一些局限型 不能實例化泛型 T t = new T(); //error
?
不能實例化泛型類型的數組 T[] ts= new T[10];?? //編譯錯誤
?
不能實例化泛型參數數 Pair<String>[] table = new Pair<String>(10); // ERROR
?
類的靜態變量不能聲明為類型參數類型 public class GenClass<T> {
???? private static T t;??? //編譯錯誤
} 泛型類不能繼承自Throwable以及其子類 public GenExpection<T> extends Exception{}??? //編譯錯誤
?
不能用于基礎類型int等 Pair<double> //error
Pair<Double> //right 2 增強循環(Enhanced for Loop) 舊的循環 LinkedList list = new LinkedList();?
list.add("Hi");
list.add("everyone!");
list.add("Was");
list.add("the");
list.add("pizza");
list.add("good?");
for (int i = 0; i < list.size(); i++)
?????? System.out.println((String) list.get(i));
//或者用以下循環
//for(Iterator iter = list.iterator(); iter.hasNext(); ) {
//Integer stringObject = (String)iter.next();
// ... more statements to use stringObject...
//} 新的循環 LinkedList<String> list = new LinkedList<String>();?
list.add("Hi");
list.add("everyone!");
list.add("Was");
list.add("the");
list.add("pizza");
list.add("good?");?
for (String s : list)
?????? System.out.println(s); 很清晰、方便，一看便知其用法 3 可變參數(Variable Arguments) 實現了更靈活的方法參數傳入方式，System.out.printf是個很好的例子 用法：void test(Object … args) 一個很容易理解的例子 public static int add(int ... args){
?????? int total = 0;????
?????? for (int i = 0; i < args.length; i++)
????????????? total += args[i];??????
?????? return total;
}
public static void main(String[] args){
?????? int a;
?????? a = Varargs.add(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
?????? System.out.println(a);
} 4 自動實現裝箱和解箱操作(Boxing/Unboxing Conversions) 說明：實現了基本類型與外覆類之間的隱式轉換。基本類型至外覆類的轉換稱為裝箱，外覆類至基本類型的轉換為解箱。這些類包括 Primitive Type???? Reference Type
boolean?????????? Boolean
byte????????????? Byte
char????????????? Character
short???????????? Short
int?????????????? Integer
long????????????? Long
float????????????? Float
double??????????? Double 例如，舊的實現方式 Integer intObject;
int intPrimitive;
ArrayList arrayList = new ArrayList();
intPrimitive = 11;
intObject = new Integer(intPrimitive);
arrayList.put(intObject); // 不能放入int類型，只能使Integer 新的實現方式 int intPrimitive;
ArrayList arrayList = new ArrayList();
intPrimitive = 11;
//在這里intPrimitive被自動的轉換為Integer類型
arrayList.put(intPrimitive); 5 靜態導入(Static Imports) 很簡單的東西，看一個例子： 沒有靜態導入 Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));
?
有了靜態導入 import static java.lang.Math.*;
sqrt(pow(x, 2) + pow(y, 2)); ? 其中import static java.lang.Math.*;就是靜態導入的語法，它的意思是導入Math類中的所有static方法和屬性。這樣我們在使用這些方法和屬性時就不必寫類名。 需要注意的是默認包無法用靜態導入，另外如果導入的類中有重復的方法和屬性則需要寫出類名，否則編譯時無法通過。 6 枚舉類(Enumeration Classes) 用法：public enum Name {types, ….} 簡單的例子： public enum Colors {Red, Yellow, Blue, Orange, Green, Purple, Brown, Black}
public static void main(String[] args){
??? Colors myColor = Colors.Red;
??? System.out.println(myColor);
} 又一個簡單例子： import java.util.*;
enum OperatingSystems {windows, unix, linux, macintosh}
public class EnumExample1 {
??? public static void main(String args[])? {
??????? OperatingSystems os;
??????? os = OperatingSystems.windows;
??????? switch(os) {
??????????? case windows:
??????????????? System.out.println(“You chose Windows!”);
??????????????? break;
??????????? case unix:
??????????????? System.out.println(“You chose Unix!”);
??????????????? break;
??????????? case linux:
??????????????? System.out.println(“You chose Linux!”);
??????????????? break;
??????????? case macintosh:
??????????????? System.out.println(“You chose Macintosh!”);
??????????????? break;
??????????? default:
??????????????? System.out.println(“I don’t know your OS.”);
??????????????? break;
??????? }
??? }
} 應運enum簡寫的例子： import java.util.*;
public class EnumTest
{
?? public static void main(String[] args)
?? {
????? Scanner in = new Scanner(System.in);
????? System.out.print("Enter a size: (SMALL, MEDIUM, LARGE, EXTRA_LARGE) ");
????? String input = in.next().toUpperCase();
????? Size size = Enum.valueOf(Size.class, input);
????? System.out.println("size=" + size);
????? System.out.println("abbreviation=" + size.getAbbreviation());
????? if (size == Size.EXTRA_LARGE)
???????? System.out.println("Good job--you paid attention to the _.");
?? }
}
enum Size
{
?? SMALL("S"), MEDIUM("M"), LARGE("L"), EXTRA_LARGE("XL");
?? private Size(String abbreviation) { this.abbreviation = abbreviation; }
?? public String getAbbreviation() { return abbreviation; }
?? private String abbreviation;
} enum類中擁有方法的一個例子： enum ProgramFlags {
??? showErrors(0x01),
??? includeFileOutput(0x02),
??? useAlternateProcessor(0x04);
??? private int bit;
??? ProgramFlags(int bitNumber) {
??????? bit = bitNumber;
??? }
??? public int getBitNumber()?? {
??????? return(bit);
??? }
}
public class EnumBitmapExample {
??? public static void main(String args[])? {
??????? ProgramFlags flag = ProgramFlags.showErrors;
??????? System.out.println(“Flag selected is: “ +
??????? flag.ordinal() +
??????? “ which is “ +
??????? flag.name());
??? }
} 7? 元數據(Meta data) 請參考 http://www-900.ibm.com/developerWorks/cn/java/j-annotate1/ http://www-900.ibm.com/developerworks/cn/java/j-annotate2.shtml 8 Building Strings(StringBuilder類) 在JDK5.0中引入了StringBuilder類，該類的方法不是同步(synchronized)的，這使得它比StringBuffer更加輕量級和有效。 9 控制臺輸入(Console Input) 在JDK5.0之前我們只能通過JOptionPane.showInputDialog進行輸入，但在5.0中我們可以通過類Scanner在控制臺進行輸入操作 ?例如在1.4中的輸入 String input = JOptionPane.showInputDialog(prompt);
int n = Integer.parseInt(input);
double x = Double.parseDouble(input);
s = input; 在5.0中我們可以 Scanner in = new Scanner(System.in);
System.out.print(prompt);
int n = in.nextInt();
double x = in.nextDouble();
String s = in.nextLine(); 10????? Covariant Return Types(不曉得怎么翻譯，大概是 改變返回類型) JDK5之前我們覆蓋一個方法時我們無法改變被方法的返回類型，但在JDK5中我們可以改變它 例如1.4中我們只能 public Object clone() { ... }
...
Employee cloned = (Employee) e.clone(); 但是在5.0中我們可以改變返回類型為Employee public Employee clone() { ... }
...
Employee cloned = e.clone(); 11 格式化I/O(Formatted I/O) 增加了類似C的格式化輸入輸出，簡單的例子： public class TestFormat{
??? public static void main(String[] args){
??????? int a = 150000, b = 10;
??????? float c = 5.0101f, d = 3.14f;
??????? System.out.printf("%4d %4d%n", a, b);
??????? System.out.printf("%x %x%n", a, b);
??????? System.out.printf("%3.2f %1.1f%n", c, d);
??????? System.out.printf("%1.3e %1.3e%n", c, d*100);
??? }
} 輸出結果為： 150000?? 10 249f0 a 5.01 3.1 5.010e+00 3.140e+02 下面是一些格式化參數說明(摘自Core Java 2 Volume I - Fundamentals, Seventh Edition)
Table 3-5. Conversions for printf
Conversion Character | Type | Example | d | Decimal integer | 159 | x | Hexadecimal integer | 9f | o | Octal integer | 237 | f | Fixed-point floating-point | 15.9 | e | Exponential floating-point | 1.59E+01 | g | General floating-point (the shorter of e and f) |
| a | Hexadecimal floating point | 0x1.fccdp3 | s | String | Hello | c | Character | H | b | Boolean | TRUE | h | Hash code | 42628b2 | tx | Date and time | See Table 3-7 | % | The percent symbol | % | n | The platform-dependent line separator |
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Table 3-7. Date and Time Conversion Characters Conversion Character | Type | Example | C | Complete date and time | Mon Feb 09 18:05:19 PST 2004 | F | ISO 8601 date | 2004-02-09 | D | U.S. formatted date (month/day/year) | 02/09/2004 | T | 24-hour time | 18:05:19 | r | 12-hour time | 06:05:19 pm | R | 24-hour time, no seconds | 18:05 | Y | Four-digit year (with leading zeroes) | 2004 | y | Last two digits of the year (with leading zeroes) | 04 | C | First two digits of the year (with leading zeroes) | 20 | B | Full month name | February | b or h | Abbreviated month name | Feb | m | Two-digit month (with leading zeroes) | 02 | d | Two-digit day (with leading zeroes) | 09 | e | Two-digit day (without leading zeroes) | 9 | A | Full weekday name | Monday | a | Abbreviated weekday name | Mon | j | Three-digit day of year (with leading zeroes), between 001 and 366 | 069 | H | Two-digit hour (with leading zeroes), between 00 and 23 | 18 | k | Two-digit hour (without leading zeroes), between 0 and 23 | 18 | I | Two-digit hour (with leading zeroes), between 01 and 12 | 06 | l | Two-digit hour (without leading zeroes), between 1 and 12 | 6 | M | Two-digit minutes (with leading zeroes) | 05 | S | Two-digit seconds (with leading zeroes) | 19 | L | Three-digit milliseconds (with leading zeroes) | 047 | N | Nine-digit nanoseconds (with leading zeroes) | 047000000 | P | Uppercase morning or afternoon marker | PM | p | Lowercase morning or afternoon marker | pm | z | RFC 822 numeric offset from GMT | -0800 | Z | Time zone | PST | s | Seconds since 1970-01-01 00:00:00 GMT | 1078884319 | E | Milliseconds since 1970-01-01 00:00:00 GMT | 1078884319047 |
Table 3-6. Flags for printf Flag | Purpose | Example | + | Prints sign for positive and negative numbers | +3333.33 | space | Adds a space before positive numbers | | 3333.33| | 0 | Adds leading zeroes | 003333.33 | - | Left-justifies field | |3333.33 | | ( | Encloses negative number in parentheses | (3333.33) | , | Adds group separators | 3,333.33 | # (for f format) | Always includes a decimal point | 3,333. | # (for x or o format) | Adds 0x or 0 prefix | 0xcafe | ^ | Converts to upper case | 0XCAFE | $ | Specifies the index of the argument to be formatted; for example, %1$d %1$x prints the first argument in decimal and hexadecimal | 159 9F | < | Formats the same value as the previous specification; for example, %d %<x prints the same number in decimal and hexadecimal |
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這里是一些簡單的介紹，更詳細的說明請參考： Core Java 2 Volume I - Fundamentals, Seventh Edition Core Java 2 Volume II - Advanced Features, Seventh Edition 里面都有一些很精彩的描述，中文名稱就是《Java核心技術》。只有第七版才有J2SE5.0的介紹，但是第七版好像還沒有中文版。本文還參考了Professional Java JDK - 5th Edition.
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