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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 |
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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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这里是一些简单的介绍，更详细的说明请参考： 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，中文名称就是《Java核心技术》。只有第七版才有J2SE5.0的介绍，但是第七版好像还没有中文版。本文还参考了Professional Java JDK - 5th Edition.
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