JDK5.0的11个主要新特征(二)

时间:2021-04-03 15:14:27

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());
    }
}