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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(nèi)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(tài)進行容器內(nèi)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(shù)據(jù)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(shù)(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(xiàn)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(shù)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(shù)(type parameter)的受限關系.例如, 我們只需要一個存放數(shù)字的列表, 包括整數(shù)(Long, Integer, Short), 實數(shù)(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(shù)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(shù)(type parameter)限制為數(shù)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(shù)在catch塊中不允許出現(xiàn)，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(xiàn)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(shù)組 T[] ts= new T[10]; //編譯錯誤 不能實例化泛型參數(shù)數(shù) Pair<String>[] table = new Pair<String>(10); // ERROR 類的靜態(tài)變量不能聲明為類型參數(shù)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(huán)(Enhanced for Loop) 舊的循環(huán) 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(huán) //for(Iterator iter = list.iterator(); iter.hasNext(); ) { //Integer stringObject = (String)iter.next(); // ... more statements to use stringObject... //} 新的循環(huán) 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(shù)(Variable Arguments) 實現(xiàn)了更靈活的方法參數(shù)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(xiàn)裝箱和解箱操作(Boxing/Unboxing Conversions) 說明：實現(xiàn)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(xiàn)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(xiàn)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(tài)導入(Static Imports) 很簡單的東西，看一個例子： 沒有靜態(tài)導入 Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2)); 有了靜態(tài)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(tài)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(tài)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(shù)據(jù)(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(shù)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 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 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 |
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