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从codes学java tiger之varargs

BlueO2 — Mon, 26 Jun 2006 12:25:00 GMT

public class Artist {

    String[] others;
     /** Creates a new instance of Artist */
     public Artist() {
    }
     public Artist(String name,String  others) {
         for (String other : others) {
            System.out.println(other);
        }
         this .others = others;
    }
     public void wildestMethod(Object  objs) {
     // this method will accept any arguments
    }
     public static void main(String[] args) {
        Artist artist = new Artist( " You " , " 1 " , " 2 " );
        artist.wildestMethod( 1 , 3.5 , " AA " , new Artist());
    }

}

BlueO2 2006-06-26 20:25 发表评论

从code学习(f��n)java tiger之自动装��?拆箱

BlueO2 — Thu, 22 Jun 2006 10:34:00 GMT

public class AutoBoxing {

     /** Creates a new instance of AutoBoxing */
     public AutoBoxing() {
    }
     public void boxingDemo() {
         // auto boxing
        Integer i = 0 ;
         float f = 1.66f ;
        Float F = f;
         // auto unboxing
        Integer I = new Integer( 1 );
         int i2 = I;
         // null value test, it will case NullPointerException
        Integer I2 = null ;
         int i3 = I2;
    }
     public void testOperator() {
        Integer i = 1 ;
         while ( true ) {
            i ++ ;
            System.out.println( " Counter: " + i);
             if (i > 5 ) break ;
        }
    }
     public void testCompare() {
         // it's equal because -127~127 are immutable objects
        Integer i = 1 ;
        Integer i2 = 1 ;
         if (i == i2) System.out.println( " 1:Equal " );
         else System.out.println( " 1:Not Equal " );
         // it's not equal because j and j2 are different objects
        Integer j = 200 ;
        Integer j2 = 200 ;
         if (j == j2) System.out.println( " 200:Equal " );
         else System.out.println( " 200:Not Equal " );
    }
     public void testControl() {
        Boolean flag = true ;
        Integer i = 20 ;
        Integer j = 30 ;
         if (flag) {
            System.out.println( " Boolean affects " );
        }
         if (i < j)
            System.out.println( " Integer affects " );
    }
     public void testMethod( double arg) {
        System.out.println( " public void testMethod(double arg) is invoked " );
    }
     public void testMethod(Integer arg) {
        System.out.println( " public void testMethod2(Integer arg) is invoked " );
    }
     public static void main(String args[]) {
        AutoBoxing auto = new AutoBoxing();
        auto.testCompare();
        auto.testOperator();
        auto.testControl();
         int i = 1 ;
         // public void testMethod(Integer arg) wouldn't be invoked
         // because  public void testMethod(double arg) will be invoked in JDK1.4
         // Java tiger consider the backward capability
        auto.testMethod(i);
        auto.boxingDemo();
    }

}

BlueO2 2006-06-22 18:34 发表评论

从code学习(f��n)java tiger �?枚�D

BlueO2 — Thu, 22 Jun 2006 10:22:00 GMT

public enum User {
Admin,User,Guest,Unknown
}

public class Login {

     private User user;
    EnumMap < User,String > userName = new EnumMap < User, String > (User. class );
     /** Creates a new instance of Login */
     public Login() {
        userName.put(User.Admin, " Administrator " );
        userName.put(User.User, " David " );
        userName.put(User.Guest, " Steve " );
    }
     public boolean isAdmin(User user) {
         if (user.equals(User.Admin)) {
             return true ;
        }
         return false ;
    }
     public void printUserRole() {
        User[] users = user.values();
         for (User u : user.values()) {
            System.out.println(u.toString());
        }
    }
     public void isRole(User user) {
         switch (user) {
             case Admin:
                System.out.println( " admin " );
                 break ;
             case User:
                System.out.println( " User " );
                 break ;
             case Guest:
                System.out.println( " Guest " );
                 break ;
             default :
                System.out.println( " unknow " );
        }
    }
     public static void main(String[] args) {
        Login login = new Login();
        System.out.println(login.isAdmin(User.Admin));
        login.printUserRole();
        login.isRole(User.User);
    }

}

BlueO2 2006-06-22 18:22 发表评论

BlueO2 — Tue, 20 Jun 2006 10:18:00 GMT

摘要: public class Generic { /** */ /** Creates a new instance of Generic */ ... 阅读全文

BlueO2 2006-06-20 18:18 发表评论

斗胆�l�Thinking in JAVA挑错

BlueO2 — Sat, 08 Oct 2005 04:05:00 GMT

今天晚上被讨厌的vba郁闷�Q�以后谁再跟我说vb��单易用我跟谁急。由于计划上每天都要�l�箋讨论java基础语言�Ҏ(gu��)��，所以最�q�也�q�璞归真�Q�重读thinking in java。发��C��一个问题如下：(x��)

在第9�?持有你的对象arrays的比较章节，有如此代码片断：(x��)
String[] s1 = new String[5];
Arrays.fill(s1,"hi");
String[] s2 = {"hi","hi","hi","hi","hi"};
System.out.println(Arrays.equals(s1,s2));
输出�l�果true
Bruce�l�出的解释是s1的所有元素指向同一个对象，s2却拥�?个不同对象，但是Arrays.equals比较的是内容�Q�还�Ҏ(gu��)��通过Object.equals()�Q�所以结果�ؓ(f��)true.
首先�Q�我们看看Arryas.fill(s1,"hi")如何工作�Q?BR>SRC:
public static void fill(Object[] a, Object val) {
        fill(a, 0, a.length, val);
    }
public static void fill(Object[] a, int fromIndex, int toIndex,Object val){
        rangeCheck(a.length, fromIndex, toIndex);
        for (int i=fromIndex; i            a[i] = val;
    }

昄��所有的s1中原素指向了相同的一个对�?hi"�q�里Bruce没错�Q�那么后面的s2是否拥有5个不同的对象呢？我们做以下代码测�?其中捎带着String�c�d��的测�?
import java.util.Arrays;
public class profit{
public static void main(String args[]){
String s1 = "Hi";
String s2 = "Hi";
String s3 = new String("Hi");
String s4 = new String("Hi");
if(s1==s2) System.out.println("s1 s2 equal");
if(s3==s4) System.out.println("s3 s4 equal");
//System.out.println(s3.hashCode());
//System.out.println(s4.hashCode());
String[] strArray = new String[2];
Arrays.fill(strArray,"hello");
if(strArray[0]==strArray[1]) System.out.println("strArray equal");
String[] strArray2 = {"hello","hello"};
if(strArray2[0]==strArray2[1]) System.out.println("strArray2 equal");
String[] strArray3 = {new String("hello"),new String("hello")};
if(strArray3[0]==strArray[1]) System.out.println("strArray3 equal");

输出�l�果�Q?BR>---------- Run ----------
s1 s2 equal
strArray equal
strArray2 equal

Output completed (1 sec consumed) - Normal Termination
首先�Q�各位应该明白最基础的东�?==实现的比较是��比较，��是非reference指向的具体对象内容，而是是否指向了同一个对象。第二，String�c�d��被设计�ؓ(f��)非可变类�Q�就是说 String s1 = "hi";与String s2 = "hi";其实指向了同一个对象，只有用new构造函数的时候才�?x��)��生新的对象，而不��内�Ҏ(gu��)��否已�l�在内存中存在。如下文字阐�q�C��一些：(x��)
s = new String("Initial Value");
每次都会(x��)调用构造器�Q�生成新对象�Q�性能低下且内存开销大，�q�且没有意义�Q�因为String对象不可改变�Q�所以对于内容相同的字符�Ԍ��只要一个String对象来表�C�就可以了。也��p��Q�多�ơ调用上面的构造器创徏多个对象�Q�他们的String�c�d��属性s都指向同一个对象�?
上面的结��Z��q�样一个事实：(x��)对于字符串常量，如果内容相同�Q�Java认�ؓ(f��)它们代表同一个String对象。而用关键字new调用构造器�Q��L��?x��)创��Z��个新的对象，无论内容是否相同
ok�Q�回�?/FONT>我们的话题上来String[] s2 = {"hi","hi","hi","hi","hi"};是否产生�?个不同的对象呢？我写的简单的��试代码

String[] strArray2 = {"hello","hello"};
if(strArray2[0]==strArray2[1]) System.out.println("strArray2 equal");
的结果大家应该得��Z��l�论了吧�?BR>那么�Q�bruce�l�出的代码的�l�果之所以�ؓ(f��)true��是因�ؓ(f��)�Q�这么多个数�l�中的成�?hi"其实都是同一个对象。那么我们��l�看Arrays.equals是否比较的内容呢�Q?BR>SRC�Q?BR>public static boolean equals(Object[] a, Object[] a2) {
        if (a==a2)
            return true;   //如果指向同一对象�Q�当然是相同的，�q�回true
        if (a==null || a2==null)
            return false; //如果为null��没有相�{�的一��_(d��)��false

        int length = a.length;
        if (a2.length != length) //长度不同
            return false;

        for (int i=0; i            Object o1 = a[i];
            Object o2 = a2[i];
            if (!(o1==null ? o2==null : o1.equals(o2))) //此处为关键，object的equals比较�Q�Arrays.equals�q�没有专门�ؓ(f��)String�c�d��重蝲此方法，全部都依赖Object的方法比�?BR>                return false;
        }

return true;
}

那么我们再看看Object�q�个�?w��i)根的equals�Ҏ(gu��)��吧：(x��)
public boolean equals(Object obj) {
return (this == obj);
}

短短几行代码�Q�但是，是浅比较毫无疑问�Q�所以，bruce说的�Q�依赖的是Object.equals�Q�比较的是内容，更是错误的�?BR>但是必须��的是�Q�当String数组传递进来的时候，�׃��java的RTTI执行期型别��L识，Arryas.equals调用的其实是String.equals�Ҏ(gu��)��Q�而String.equals�Ҏ(gu��)��是比较内容的�Q�所以，此处我想是bruce没有说明白，而又偏偏列�D了String的例子。如果是自己写的�c�L��没有复写equals�Ҏ(gu��)��Q�此时就调用Object.equals了，那么�Ҏ(gu��)��不可能比较内容，所以Arrays.equals大家一定要知道怎么回事再用�?/P>

BlueO2 2005-10-08 12:05 发表评论