Question

我是Android / Java中的一个非常新的程序员。以下是产生不清楚的结果的代码。另外，我提出了找到的“解决方案”，但并不满足我的好奇心。 `

//Here I define my class to hold data
public class ImageData {
    Date date;
    byte[] image;
}

// Here I initialize the array photos and put getter and setter
abstract class Variables {
    public static ImageData photos = new ImageData[4];
    static ImageData getPhotos(int n) {
        return photos[n];
    }
    static void setPhotos(ImageData photos, int n) {
        Variables.photos[n] = photos;
    }
}

// Here, at the very begining of app, initialize the array
        Variables.photos[0] = new ImageData();
        Variables.photos[1] = new ImageData();
        Variables.photos[2] = new ImageData();
        Variables.photos[3] = new ImageData();

// And now, I use those objects

    public void run() {// This DOES NOT WORK (all photos have the last image)
        // get 4 photos and save result in global
        int count;
        ImageData localPhoto = new ImageData();
        // clear global (at least the byte[]
        for (count = 0; count > 4; count++) {
            localPhoto.image = null;
            setPhotos(localPhoto, count);
        }
        // take 4 pictures
        for (count = 0; count < 4; count++) {
            localPhoto.image = getPhoto(ipCamera, port);
            localPhoto.date = Calendar.getInstance().getTime();
            setPhotos(localPhoto, count);
        }
        //...............................
    }

在我发表评论时，所有四张photos.image都是相同的（还有日期）。我使用以下方法工作：

   public void run() { // This WORKS
        // get 4 photos and save result in global
        int count;
        ImageData localPhoto; // declared, not initialized
        // clear global (at least the byte[] )
        for (count = 0; count > 4; count++) {
            localPhoto = new ImageData(); // <<<<<<<<<<<<<<<<< NEW
            localPhoto.image= null;
            setPhotos(localPhoto, count);
        }
        // take 4 pictures
        for (count = 0; count < 4; count++) {
            localPhoto = new ImageData(); // <<<<<<<<<<<<<<<<< NEW
            localPhoto.image = getPhoto(ipCamera, port);
            localPhoto.date = Calendar.getInstance().getTime();
            setPhoto(localPhoto, count);
        }
        //...............................
    }

但是我真的不明白为什么会这样。而且，我想以一种更优雅的方式初始化数组，但找不到。谢谢您的评论。谢谢！

Answer 1

public void run() {// This DOES NOT WORK (all photos have the last image)
    // get 4 photos and save result in global
    int count;
    ImageData localPhoto = new ImageData();

上面的行创建一个ImageData()对象，并为其命名为localPhoto。

    // clear global (at least the byte[]
    for (count = 0; count > 4; count++) {
        localPhoto.image = null;

这将设置先前创建的对象中的image字段。请注意，它在同一对象中将image设置为四次。

        setPhotos(localPhoto, count);

现在您调用setPhotos()并将引用传递给相同的对象四次。

    }
    // take 4 pictures
    for (count = 0; count < 4; count++) {
        localPhoto.image = getPhoto(ipCamera, port);
        localPhoto.date = Calendar.getInstance().getTime();
        setPhotos(localPhoto, count);

与上面的描述一样，每次重复此循环时，您都使用对同一对象的引用。因此，数组中的所有四个元素都指向同一个对象。

} }

现在看看您的更改

for (count = 0; count > 4; count++) { localPhoto = new ImageData(); // <<<<<<<<<<<<<<<<< NEW localPhoto.image= null; setPhotos(localPhoto, count); }

这可解决此问题，因为您在每次迭代中都创建了一个新对象。每次将引用localPhoto分配给一个不同的对象。

要更好地理解这一点，请阅读参考变量如何在Java中工作。

对象数组的概念问题

1 个答案: