C# VS JAVA 差异 (未完待续)

1. 静态构造函数

C#中有静态构造函数， Java中没有静态构造函数。其实Java中有一个类似静态构造函数的东东，称作静态初始化，或者静态代码块，可以通过这样的代码实现相同的功能：

但是Java中静态代码块和C#静态构造函数还是不一样的。C#中静态构造函数在其他静态成员初始化后再执行，而java中静态代码块和其他静态成员谁在先谁就先执行。

 class Parent{
     public static StaticVariable staticVariable = new StaticVariable("Parent - Static Variable1");
     public StaticVariable inStaticVariable = new StaticVariable("Parent - Instant Variable1");

     static
     {
         System.out.println("Parent - Static block/constructor");
         System.out.println(staticVariable == null);
         //System.out.println(staticVariable2 == null); not working because staticVariable2 is not defined
         staticVariable2 = new StaticVariable("Parent - Static Variable2 -  Static block");
     }

     {
         System.out.println("Parent - Initializer Block");
     }

     public static StaticVariable staticVariable2 = new StaticVariable("Parent - Static Variable2");
     public StaticVariable inStaticVariable2 = new StaticVariable("Parent - Instant Variable2");

     public Parent()
     {
         System.out.println("Parent - Instance Constructor");
     }
 }

View Java Code

     class StaticDemo
     {
         static int i = ;

         static StaticDemo()
         {
             Console.WriteLine(i);
             Console.WriteLine(j);
         }

         public static void Execute()
         {
         }

         static int j = ;
     }

View C# Code

2常量

Java中声明(实例/类)常量使用关键词(final/static final)。C#中声明(实例/类)常量使用关键词(readonly/const 或者 readonly static).

C#中必须使用类名去访问类层级的变量。Java中可以使用实例去访问类层级的变量，但是编译时会有警告。

Java Code and C# Code

 class ParentDef{
     public static final String STATICVALUE_STRING="Parent Static Variable";
     public String valueString="Parent Instant Variable";
 }

 class ChildRef extends ParentDef{
     public static final String STATICVALUE_STRING="Child Static Variable";
     public String valueString="Child Instant Variable";
 }

 public class BasicDemo {
     public static void main(String[] args) {
         //Child child = new Child();

         ParentDef pdf = new ParentDef();
         ParentDef pcdf = new ChildRef();
         ChildRef cdf = new ChildRef();
         System.out.println("V1");
         System.out.println(pdf.STATICVALUE_STRING);
         System.out.println(pdf.valueString);

         System.out.println("V2");
         System.out.println(pcdf.STATICVALUE_STRING);
         System.out.println(pcdf.valueString);

         System.out.println("V3");
         System.out.println(cdf.STATICVALUE_STRING);
         System.out.println(cdf.valueString);

     }

 }

     class InheritenceDemo
     {
         public static void Execute()
         {
             ParentDef pdf = new ParentDef();
             ParentDef pc = new ChildDef();
             ChildDef cdf = new ChildDef();

             Console.WriteLine("V1");
             Console.WriteLine(pdf.value);

             Console.WriteLine("V2");
             Console.WriteLine(pc.value);

             Console.WriteLine("V3");
             Console.WriteLine(cdf.value);
             Console.WriteLine(cdf.READONLYSTRING);

         }
     }

     class ParentDef
     {
         public const string Const_String = "Parent Const Varialbe";
         public static string STATICVALUE_STRING = "Parent Static Variable";
         public string value = "Parent Instant Variable";
     }

     class ChildDef:ParentDef
     {
         public readonly string READONLYSTRING="Child readonly variable";
         public readonly static string READONLYSTATICSTRING = "Child readonly  static variable";
         public static string STATICVALUE_STRING = "Child Static Variable";
         public string value = "Child Instant Variable";

         public ChildDef()
         {
             READONLYSTRING = "NEW Child readonly variable";
             //READONLYSTATICSTRING = "NEW Child readonly  static variable"; ERROR as satatic readonly variable can not be reassianged in instant constructor
         }
     }

3参数传递

C#中有ref关键词用来按引用传递参数。Java则没有，无法真正按引用传递参数。Java总是采用按值调用。方法得到的是所有参数值的一个拷贝，特别的，方法不能修改传递给它的任何参数变量的内容。

（1）：“在Java里面参数传递都是按值传递”这句话的意思是：按值传递是传递的值的拷贝，按引用传递其实传递的是引用的地址值，所以统称按值传递。
（2）：在Java里面只有基本类型和按照下面这种定义方式的String是按值传递，其它的都是按引用传递。就是直接使用双引号定义字符串方式：String str = “Java”;

C# code

　　class RefExample
　　{
　　static void Method(ref int i)
　　{
　　i = ;
　　}
　　static void Main()
　　{
　　int val = ;
　　Method(ref val); // val is now 44
　　}
　　}

4虚函数

C#中普通成员函数加上virtual关键字就成为虚函数.

Java中其实没有虚函数的概念，它的普通函数就相当于C#的虚函数，动态绑定是Java的默认行为。如果Java中不希望某个函数具有虚函数特性，可以加上final关键字变成非虚函数.

5空接口

在Java和C#中空接口都是合法的。都可以定义空接口。空接口还是很奇怪的存在，个人的理解是空接口仅做标记使用，无其他含义。如果你只需要在运行时区分这些类型，一个更佳的解决方式是使用自定义属性(attribute)。如果你希望在编译时区分这些类型，就只好使用空接口了。 而JDK中定义的很多空接口很多都是前者，个人认为这是一种不良的设计，反观CLR,我们则很难找到一个空接口。 所以.Net的设计在这点上来看跟合理点。
JDK中定义的空接口
java.io.Serializable;
java.lang.Cloneable
java.lang.annotation.Annotation;
java.rmi.Remote;
java.util.RandomAccess;

6Delegate 和 Event

Java欠缺C#中的事件功能，java中没有delegate这种用法。在java中实现事件功能其实就是通过实现EventListener接口实现观察者模式。C#则是通过EventHandler的实例来实现事件的功能。

class EventDemo
    {
        public static void RunDemo()
        {
            SMS sms = new SMS();
            SmsReceiver r = new SmsReceiver(sms);
            sms.SendSms("","Hello world");
        }
    }

    public class SMS
    {
        public EventHandler<SmsEventArgs> SmsEvent = (o, e) => { };
        public EventHandler SmsEvent2;

        protected virtual void OnSmsEvent(SmsEventArgs e)
        {
            //EventHandler<SmsEventArgs> handler = this.SmsEvent;
            //if (handler != null)
            //{
            //    handler(this, e);
            //}

            this.SmsEvent(this, e);

        }

        protected virtual void OnSmsEvent2()
        {
            EventHandler handler = this.SmsEvent2;

            if (handler != null)
            {
                handler(this,null);
            }
        }

        public void SendSms(string phone, string message)
        {
            SmsEventArgs e = new SmsEventArgs();
            e.Message = message;
            e.ToPhone = phone;
            OnSmsEvent(e);
        }
    }

    public class SmsEventArgs:EventArgs
    {
        public string ToPhone { get; set; }
        public string Message { get; set; }
    }

    public class SmsReceiver
    {
        public SmsReceiver(SMS sms)
        {
            sms.SmsEvent += new EventHandler<SmsEventArgs>(sms_SmsEvent);
        }

        void sms_SmsEvent(object sender, SmsEventArgs e)
        {
            Console.WriteLine(e.ToPhone + ":" + e.Message);
        }

    }

View C# Code

public class AskEvent extends EventObject {

    private static final long serialVersionUID = 1L;
    private Object Evnetsource;
    private String name; 

    public Object getEvnetsource() {
        return Evnetsource;
    }

    public String getName() {
        return name;
    }

    public AskEvent(Object source,String name) {
        super(source);
        Evnetsource = source;
        this.name = name;
    }

}

public interface Listener extends EventListener {
    public void listen(AskEvent ae); 

}

public class Ask {

        private Listener l ;
        private List<String> names = new ArrayList<String>();

        public void addListener(Listener l){
            this.l = l;
        }

        public void addName(String name){
            names.add(name);
        }

        public void setFlag(boolean flag){
            if(flag){
                if(names.size()==) System.out.println("Input Name！！！");
                for(int i = ;i<names.size();i++){
                    l.listen(new AskEvent(this,names.get(i)));
                }
                names.clear();
            }
        }

}

public class EventTest {

    public static void main(String[] args) {
        System.out.println("START");
        Scanner scan = new Scanner(System.in);
        Ask ask = new Ask();
         ask.addListener(new Listener(){
             public void listen(AskEvent ae) {
                 if(ae.getName().equals("a")) System.out.println(ae.getName() + "good man");
                 else System.out.println(ae.getName() + "bad man");
             }
         });

         while(true){
                System.out.print("input name:");
                final String name = scan.nextLine();
                if(name.equals("exit")) break;
                if(name.equals("print")) {
                    ask.setFlag(true);
                    continue;
                }
                ask.addName(name);
            }
            System.out.println("OVER");
        }
    }

View Java Code

7Exception

Java中Checked Exception是必须要被try_catch捕获，编译器会强制检查的。在C#中是否捕获Exception并不是由编译器强制的。