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
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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：
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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
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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：
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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
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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
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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
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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
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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：
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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：
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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：
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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：
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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
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Scanner in = new Scanner(System.in); System.out.print(prompt); int n = in.nextInt(); double x = in.nextDouble(); String s = in.nextLine(); |
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
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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
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public Employee clone() { ... } ... Employee cloned = e.clone(); |
增加了类似C的格式化输入输出，简单的例子：
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public class TestFormat{ public static void main(String[] args){ int a = 150000, b = 10; float c = System.out.printf("%4d %4d%n", a, b); System.out.printf("%x %x%n", a, b); System.out.printf("% System.out.printf("%1.3e %1.3e%n", c, d*100); } } |
150000 10
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
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Conversion Character |
Type |
Example |
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d |
Decimal integer |
159 |
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x |
Hexadecimal integer |
|
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o |
Octal integer |
237 |
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f |
Fixed-point floating-point |
15.9 |
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e |
Exponential floating-point |
1.59E+01 |
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g |
General floating-point (the shorter of e and f) |
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a |
Hexadecimal floating point |
0x1.fccdp3 |
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s |
String |
Hello |
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c |
Character |
H |
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b |
Boolean |
TRUE |
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h |
Hash code |
42628b2 |
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tx |
Date and time |
See Table 3-7 |
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% |
The percent symbol |
% |
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n |
The platform-dependent line separator |
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Table 3-7. Date and Time Conversion Characters
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Conversion Character |
Type |
Example |
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C |
Complete date and time |
Mon Feb 09 18:05:19 PST 2004 |
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F |
ISO 8601 date |
|
|
D |
|
|
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T |
24-hour time |
18:05:19 |
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r |
12-hour time |
06:05:19 pm |
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R |
24-hour time, no seconds |
18:05 |
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Y |
Four-digit year (with leading zeroes) |
2004 |
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y |
Last two digits of the year (with leading zeroes) |
04 |
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C |
First two digits of the year (with leading zeroes) |
20 |
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B |
Full month name |
February |
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b or h |
Abbreviated month name |
Feb |
|
m |
Two-digit month (with leading zeroes) |
02 |
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d |
Two-digit day (with leading zeroes) |
09 |
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e |
Two-digit day (without leading zeroes) |
9 |
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A |
Full weekday name |
Monday |
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a |
Abbreviated weekday name |
Mon |
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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 |
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L |
Three-digit milliseconds (with leading zeroes) |
047 |
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N |
Nine-digit nanoseconds (with leading zeroes) |
047000000 |
|
P |
Uppercase morning or afternoon marker |
PM |
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p |
Lowercase morning or afternoon marker |
pm |
|
z |
RFC 822 numeric offset from GMT |
-0800 |
|
Z |
Time zone |
PST |
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s |
Seconds since 1970-01-01 00:00:00 GMT |
1078884319 |
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E |
Milliseconds since 1970-01-01 00:00:00 GMT |
1078884319047 |
Table 3-6. Flags for printf
|
Flag |
Purpose |
Example |
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+ |
Prints sign for positive and negative numbers |
+3333.33 |
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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 |
|
< |
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