尝试存储字节数组时，为什么会出现java.lang.NullPointerException？

Question

Why do I get a java.lang.NullPointerException when I try to store a Array of Bytes? Can you maybe explain to me why it doesn't work this way? How can I do it better without NullPointerException?

Exception in thread "Thread-3" java.lang.NullPointerException
    at test.Screenblocks_Data.set_Image(Screenblocks_Data.java:13)
    at desktop_share_client.ScreenBlocks.run(ScreenBlocks.java:120)

---

public class Screenblocks_Data implements java.io.Serializable {
    public int Screenblocks_Counter = 0;
    public int[][] positions = new int[200][2];
    public Jpeg[] sub_images = new Jpeg[200];

    public Screenblocks_Data() {

    }

    public void set_Image(byte[] temp_image) {
        sub_images[Screenblocks_Counter].set_sub_image(temp_image);
    }

    public byte[] get_Image(int position) {
        return sub_images[Screenblocks_Counter].sub_image;
    }
}

---

public class Jpeg {

    public byte[] sub_image = null;

    public void set_sub_image(byte[] temp_image) {

        sub_image = new byte[temp_image.length];

        sub_image = temp_image;
    }
}

Answer 1

You just bought a bag to carry 200 apples. But without filling the bag, you are trying to eat an apple :).

You never initialized zeroth (or Screenblocks_Counter th element) element. First you have to add element at zero position and then access it.

 sub_images[Screenblocks_Counter] = new Jpeg();
 sub_images[Screenblocks_Counter].set_sub_image(temp_image);