如何访问java中对象数组中对象的变量

Question

I am working on game development in android

I have a class called droid in my model package and i have a constructor called update() in my main game panel class

I would like to make an array of droids and access them in my main game panel and from constructors within the main game panel class. I am able to do this from the main game panel constructor but not from the update constructor. ie Whenever I try to access the x position of one of the droids in the update constructor of the main game panel class I get this error: "The type of the expression must be an array type but it resolved to Droid"

package net.test.droid.model;

public class Droid {

private Bitmap bitmap;  // the actual bitmap
private int x;          // the X coordinate
private int y;  
private boolean touched;    // if droid is touched/picked up

public Droid(Bitmap bitmap, int x, int y) {
    this.bitmap = bitmap;
    this.x = x;
    this.y = y;
}



public Bitmap getBitmap() {
    return bitmap;
}
public void setBitmap(Bitmap bitmap) {
    this.bitmap = bitmap;
}
public int getX() {
    return x;
}
public void setX(int x) {
    this.x = x;
}
public int getY() {
    return y;
}
public void setY(int y) {
    this.y = y;
}



public void draw(Canvas canvas) {
        canvas.drawBitmap(bitmap, x, y, null);
}
}

in main game

public class MainGamePanel extends SurfaceView implements
    SurfaceHolder.Callback {

  public Droid droid_array;
  public MainGamePanel(Context context) {
    super(context);
    // adding the callback (this) to the surface holder to intercept events
    getHolder().addCallback(this);
    Droid[] droid_array = new Droid[5];
    for (int i = 0; i < 5; i++) {
        droid_array[i] = new Droid(BitmapFactory.decodeResource(
                getResources(), R.drawable.ic_launcher),                              droid_x_pos + i*10, droid_y_pos);
    }
droid_array[1].setX(666);
}

the last line works fine however whenI try to use it in update() I get the error

public void update() {
test=droid_array[1].getX();
}

the above line returns error "The type of the expression must be an array type but it resolved to Droid"

Answer 1

Here is your problem:

public Droid droid_array;

Has type Droid . This is your class level property. Inside the MainGamePanel constructor you hide the class level property with this variable:

Droid[] droid_array

Once you leave the MainGamePanel constructor the Droid[] droid_array variable goes out of scope.

The Update method references the public Droid droid_array class property, which is not an array.

Answer 2

Basically, there are two variables with the name droid_array .

1. public Droid droid_array; //this is of type Droid
2. Droid[] droid_array = new Droid[5]; //this is an array of Droid type

At the last line of the MainGamePanel(Context contect) constructor, droid_array is accessible as an array because droid_array is of the method scope.

But in the update() method, there is no such array as droid_array . This makes the Droid droid_array available there which is not an array but an object of Droid type.

You will need to do something like this.

public class MainGamePanel extends SurfaceView implements
    SurfaceHolder.Callback {

  public Droid[] droid_array;
  public MainGamePanel(Context context) {
    super(context);
    // adding the callback (this) to the surface holder to intercept events
    getHolder().addCallback(this);

    droid_array = new Droid[5];
    for (int i = 0; i < 5; i++) {
        droid_array[i] = new Droid(BitmapFactory.decodeResource(
                getResources(), R.drawable.ic_launcher),                               droid_x_pos + i*10, droid_y_pos);
    }
    droid_array[1].setX(666);
  }

  public void update() {
      test=droid_array[1].getX();
  }
}