一个关于构造方法的问题

Question

   初学java，做一个扑克牌的作业。创建了三个类：Bridge/Cardmain/Piece。代码分别如下：   
   public class Bridge {
private Piece[] pieces;
private String[] numbers;
private String[] images;public Bridge(){    Piece[] pieces = new Piece[52];
    String[] numbers={"King","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen"};
    String[] images={"Diamonds","Clubs","Hearts","Spades"};
  
}
public void setorder(){
for(int j=0;j<images.length;j++){
  for(int i=0;i<numbers.length;i++){
      pieces[12*j+i].setimage(images[j]);
      pieces[12*j+i].setnumber(numbers[i]);
  }
}
}
public void getpieces(int j,int k){
    System.out.println(pieces[12*j+k].getnumber()+" Of "+pieces[12*j+k].getimage());
}}
public class Piece{
private String number;
private String image;
private boolean t;public Piece(){
number="Ace";
image="Diamonds";
t=false;
}public String getnumber(){
return number;
}
public String getimage(){
return image;
}
public void setnumber(String x){
number=x;
}
public void setimage(String y){
image=y;
}
}
public class Cardmain {
    public static void main(String args[]){
        Scanner s = new Scanner(System.in);
        Bridge nbridge =new Bridge();
        nbridge.setorder();        System.out.println("Please input your to-distribute cardnumber: ");
        int num = s.nextInt();
        System.out.println("Please input your to-distribute cardimage: ");
        System.out.println("***************Option******************");
        System.out.println("0:Diamond  1:Clubs  2:Hearts  3:Spades ");
        System.out.println("***************************************");
        int im = s.nextInt();
        nbridge.getpieces(im, num);
        
   }
}但在执行到这一步时有问题： Bridge nbridge =new Bridge();
        nbridge.setorder();
新建的nbridge对象在构造时numbers数组和images数据都有赋值，调用setorder方法时调试是就发现nbridge中数组的引用都为空了，这究竟是怎么回事呢？

Answer 1

private Piece[] pieces;
private String[] numbers;
private String[] images;public Bridge(){    Piece[] pieces = new Piece[52];
    String[] numbers={"King","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen"};
    String[] images={"Diamonds","Clubs","Hearts","Spades"};
  
}这个是2个对象  所以等于没有初始化

Answer 2

public Bridge(){    Piece[] pieces = new Piece[52];
    String[] numbers={"King","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen"};
    String[] images={"Diamonds","Clubs","Hearts","Spades"};
  
}＝＝〉
public Bridge(){    pieces = new Piece[52];
    numbers={"King","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen"};
    images={"Diamonds","Clubs","Hearts","Spades"};
  
}

Answer 3

public Bridge(){
    // 你原来的写法，是局部变量
    pieces = new Piece[52];
    for (int i = 0; i < pieces.length; i++) {
      // 初始化了数组，还要初始化每个元素
      pieces[i] = new Piece();
    }
    numbers={"King","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen"};
    images={"Diamonds","Clubs","Hearts","Spades"};
  
}

Answer 4

Piece[] pieces = new Piece[52];
只是声明了一个对象数组，但并未初始化

Answer 5

感谢2楼回复！但我删去String[]后netbeans反而报错，说缺少";"，不是语句，这又是怎么回事呢？

Answer 6

{"Diamonds","Clubs","Hearts","Spades"}的方式只能在变量命名初始化时候用
如果在构造方法里初始化，用new String[] {"Diamonds","Clubs","Hearts","Spades"}

Answer 7

也就是说，你可以
private String[] images = {"Diamonds","Clubs","Hearts","Spades"};直接初始化
或者在构造方法里
images = new String[] {"Diamonds","Clubs","Hearts","Spades"};

Answer 8

private Piece[] pieces;
private String[] numbers;
private String[] images;public Bridge(){    Piece[] pieces = new Piece[52];
    String[] numbers={"King","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen"};
    String[] images={"Diamonds","Clubs","Hearts","Spades"};
  
}之前有朋友提出了这个错误，你在constructor中使用的String[] numbers={"King","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen"}; 
String[] images={"Diamonds","Clubs","Hearts","Spades"};
和你在class里的private String [] number；
private String[] images;并不是相同的对象。等于是你在constructor中申明了两个同你的private members无关的对象，所以你的constructor并没有初始化其中两个private members。正确的应该是
private Piece[] pieces;
private String[] numbers;
private String[] images;public Bridge(){    Piece[] pieces = new Piece[52];//array可以在用的时候再初始化，不过用时小心NullPointerException
    this.numbers={"King","Ace","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen"};
    this.images={"Diamonds","Clubs","Hearts","Spades"};
  
}

Answer 9

Piece[] pieces = new Piece[52];
没有new对象

Answer 10

这问题是局部变量和this混淆不清的引起的，建议你看看成员变量和局部变量的区别，然后再看看构造器初始化的过程。另外你的硬编码问题很严重。
下边也是一个扑克牌的例子，你参考一下。
-----------------PuKe.java-------------------
import java.util.*;/**
 * 扑克
 * http://www.juheit.com
 */
public class PuKe{

/**
 * 花色
 */
private int pukeColor;

/**
 * 大小
 */  
private int pukeNum;

/**
 * 构造器
 */
public PuKe(int pukeColor, int pukeNum){
this.pukeColor = pukeColor;
this.pukeNum = pukeNum;
}

/**
 * Setter Getter
 */
public int getPukeColorInt(){
return this.pukeColor;
}

/**
 * 构建一幅扑克牌
 */
public List createPukeList(){

List<PuKe> pukeList = new ArrayList<PuKe>();

//四种花色，每种花色13张牌
for(int i = IPuKeConstant.PUKE_COLOR_B; i <= IPuKeConstant.PUKE_COLOR_P; i ++){
for(int j = IPuKeConstant.PUKE_NUM_A; j <= IPuKeConstant.PUKE_NUM_K; j ++){
pukeList.add(new PuKe(i, j));
}
}
//添加大小王
pukeList.add(new PuKe(IPuKeConstant.PUKE_COLOR_K, IPuKeConstant.PUKE_NUM_L));
pukeList.add(new PuKe(IPuKeConstant.PUKE_COLOR_K, IPuKeConstant.PUKE_NUM_G));

return pukeList;
}

/**
 * 分堆
 */
public List createPukeGroupList(List pukeList){

//扑克堆集合
List<PuKeGroup> pukeGroupList = new Vector<PuKeGroup>();//同时取放，防止并发异常

////////进行分组
//迭代每张扑克牌
Iterator pukeIterator = pukeList.iterator();

while(pukeIterator.hasNext()){
PuKe currentPuke = (PuKe)pukeIterator.next();
//取得扑克花色
int pukeColor = currentPuke.getPukeColorInt();

//定义查找标识
boolean findFlag = false;

//对堆进行迭代
Iterator pukeGroupIterator = pukeGroupList.iterator();

while(pukeGroupIterator.hasNext()){
//取得当前的堆
PuKeGroup currentPkeGroup = (PuKeGroup)pukeGroupIterator.next();
//取得堆的扑克花色
int pukeGroupColor = currentPkeGroup.getPukeColor();

if(pukeColor == pukeGroupColor){
currentPkeGroup.getPukeSubList().add(currentPuke);
findFlag = true;//找到，反转标识
break;
}
}

//如果没有找到
if(!findFlag){
List<PuKe> pukeSubList = new ArrayList<PuKe>();//床架扑克子集合
pukeSubList.add(currentPuke);//将扑克放到子集合中

pukeGroupList.add(new PuKeGroup(pukeColor, pukeSubList));
}
}
return pukeGroupList;
}
}/**
 * 扑克堆
 * http://www.juheit.com
 */
class PuKeGroup{ /**
 * 花色
 */
private int pukeColor;

/**
 * 扑克集合
 */
private List<PuKe> pukeSubList;

/**
 * 构造器
 */
public PuKeGroup(int pukeColor, List pukeSubList){
this.pukeColor = pukeColor;
this.pukeSubList = pukeSubList;
}

/**
 * 取得扑克花色
 */
public int getPukeColor(){
return this.pukeColor;
}

/**
 * 取得扑克分组的扑克集合
 */
public List getPukeSubList(){
return this.pukeSubList;
}
} /**
 * 扑克常量接口
 * http://www.juheit.com
 */
interface IPuKeConstant{

/**
 * 扑克A
 */
public static final int PUKE_NUM_A = 1; /**
 * 扑克K
 */
public static final int PUKE_NUM_K = 13; /**
 * 扑克小王
 */
public static final int PUKE_NUM_L = 14;

/**
 * 扑克大王
 */
public static final int PUKE_NUM_G = 15;

/**
 * 扑克花色黑桃
 */
public static final int PUKE_COLOR_B = 1;

/**
 * 扑克花色红桃
 */
public static final int PUKE_COLOR_R = 2;

/**
 * 扑克花色草花
 */
public static final int PUKE_COLOR_F = 3;

/**
 * 扑克花色方片
 */
public static final int PUKE_COLOR_P = 4; /**
 * 扑克花色王
 */
public static final int PUKE_COLOR_K = 5;

}

Answer 11

  多谢11楼和13楼的指点~真的是初学java~还有很多搞不清楚，以后定多调试多看程序！