1           泛型(Generic)

1.1          说明

增强了java的类型安全，可以在编译期间对容器内的对象进行类型检查，在运行期不必进行类型的转换。而在j2se5之前必须在运行期动态进行容器内对象的检查及转换

减少含糊的容器，可以定义什么类型的数据放入容器

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类型

integerObject = listOfIntegers.get(0); // 取出对象不需要转换

1.2          用法

声明及实例化泛型类：

HashMap<String,Float> hm = new HashMap<String,Float>();

//不能使用原始类型

GenList<int> nList = new GenList<int>();  //编译错误

J2SE 5.0目前不支持原始类型作为类型参数(type parameter)

定义泛型接口：

public interface GenInterface<T> {

    void func(T t);

}

定义泛型类：

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) { //实现了一个泛型方法

              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

　　受限泛型是指类型参数的取值范围是受到限制的. extends关键字不仅仅可以用来声明类的继承关系, 也可以用来声明类型参数(type parameter)的受限关系.例如, 我们只需要一个存放数字的列表, 包括整数(Long, Integer, Short), 实数(Double, Float), 不能用来存放其他类型, 例如字符串(String), 也就是说, 要把类型参数T的取值泛型限制在Number极其子类中.在这种情况下, 我们就可以使用extends关键字把类型参数(type parameter)限制为数字

示例

public class Limited<T extends Number> {

       public static void main(String[] args) {

              Limited<Integer> number;   //正确

              Limited<String> str;       //编译错误

       }

}

1.4          泛型与异常

类型参数在catch块中不允许出现，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'?'

'?'可以用来代替任何类型, 例如使用通配符来实现print方法。

public static void print(GenList<?> list) {})

1.6          泛型的一些局限型

不能实例化泛型

T t = new T(); //error

不能实例化泛型类型的数组

T[] ts= new T[10];   //编译错误

不能实例化泛型参数数

Pair<String>[] table = new Pair<String>(10); // ERROR

类的静态变量不能声明为类型参数类型

public class GenClass<T> {

     private static T t;    //编译错误

}

泛型类不能继承自Throwable以及其子类

public GenExpection<T> extends Exception{}    //编译错误

不能用于基础类型int等

Pair<double> //error

Pair<Double> //right

2           增强循环(Enhanced for Loop)

旧的循环

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));

//或者用以下循环

//for(Iterator iter = list.iterator(); iter.hasNext(); ) {

//Integer stringObject = (String)iter.next();

// ... more statements to use stringObject...

//}

新的循环

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           可变参数(Variable Arguments)

实现了更灵活的方法参数传入方式，System.out.printf是个很好的例子

用法：void test(Object … args)

一个很容易理解的例子

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           自动实现装箱和解箱操作(Boxing/Unboxing Conversions)

说明：实现了基本类型与外覆类之间的隐式转换。基本类型至外覆类的转换称为装箱，外覆类至基本类型的转换为解箱。这些类包括

Primitive Type     Reference Type

boolean           Boolean

byte              Byte

char              Character

short             Short

int               Integer

long              Long

float              Float

double            Double

例如，旧的实现方式

Integer intObject;

int intPrimitive;

ArrayList arrayList = new ArrayList();

intPrimitive = 11;

intObject = new Integer(intPrimitive);

arrayList.put(intObject); // 不能放入int类型，只能使Integer

新的实现方式

int intPrimitive;

ArrayList arrayList = new ArrayList();

intPrimitive = 11;

//在这里intPrimitive被自动的转换为Integer类型

arrayList.put(intPrimitive);

5           静态导入(Static Imports)

很简单的东西，看一个例子：

没有静态导入

Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));

有了静态导入

import static java.lang.Math.*;

sqrt(pow(x, 2) + pow(y, 2));

 

其中import static java.lang.Math.*;就是静态导入的语法，它的意思是导入Math类中的所有static方法和属性。这样我们在使用这些方法和属性时就不必写类名。

需要注意的是默认包无法用静态导入，另外如果导入的类中有重复的方法和属性则需要写出类名，否则编译时无法通过。

6          枚举类(Enumeration Classes)

用法：public enum Name {types, ….}

简单的例子：

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);

}

又一个简单例子：

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;

        }

    }

}

应运enum简写的例子：

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类中拥有方法的一个例子：

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          元数据(Meta data)

请参考

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类)

在JDK5.0中引入了StringBuilder类，该类的方法不是同步(synchronized)的，这使得它比StringBuffer更加轻量级和有效。

9          控制台输入(Console Input)

在JDK5.0之前我们只能通过JOptionPane.showInputDialog进行输入，但在5.0中我们可以通过类Scanner在控制台进行输入操作

    例如在1.4中的输入

    String input = JOptionPane.showInputDialog(prompt);

int n = Integer.parseInt(input);

double x = Double.parseDouble(input);

s = input;

在5.0中我们可以

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(不晓得怎么翻译，大概是 改变返回类型)

JDK5之前我们覆盖一个方法时我们无法改变被方法的返回类型，但在JDK5中我们可以改变它

例如1.4中我们只能

public Object clone() { ... }

...

Employee cloned = (Employee) e.clone();

但是在5.0中我们可以改变返回类型为Employee

public Employee clone() { ... }

...

Employee cloned = e.clone();

11      格式化I/O(Formatted I/O)

增加了类似C的格式化输入输出，简单的例子：

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);

    }

}

输出结果为：

150000   10

249f0 a

5.01 3.1

5.010e+00 3.140e+02

下面是一些格式化参数说明(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