进入Hashmap返回null

Question

I have a basic Hashmap code that wont work for some reason.

ConcurrentHashMap<Bitmap, byte[]> pixels = new ConcurrentHashMap<Bitmap, byte[]>();

it key is bitmap, and the value are its bytes that I get using this code:

public byte[] getPixels(Bitmap bmp) {
    int bytes = bmp.getRowBytes() * bmp.getHeight();
 buffer = ByteBuffer.allocateDirect(bytes);
    bmp.copyPixelsToBuffer(buffer);
    buffer.clear();
    return buffer.array();
}

In the hashmap for all the bitmaps I have I put:

    pixels.put(bitmap1, getPixels(b)); 

And when I want to get the value (bytes) back I do:

    byte[] pixelData = pixels.get(bitmap1);

and for some odd reason its always null!, why? I tried on different bitmaps, they all return null, and it is the same bitmap..

Answer 1

It must be the same instance of Bitmap. If you create another instance of same Bitmap, it'll not be the same object.

For instance:

Bitmap b1 = BitmapFactory.decodeFile("mybitmap.png");
ConcurrentHashMap<Bitmap, byte[]> pixels = new ConcurrentHashMap<Bitmap, byte[]>();
pixels.add(b1);
[...]
Bitmap b2 = BitmapFactory.decodeFile("mybitmap.png");
byte[] barray1 = pixels.get(b1);
byte[] barray2 = pixels.get(b2);

barray1 is not null

barray2 is null

Answer 2

First things: the javadoc of Android's Bitmap makes no guarantee that it supports equals() and hashCode() if they have the same content, which means that if they don't, you're screwed.

Second: the javadoc of ByteBuffer.allocateDirect() states that:

Whether or not it has a backing array is unspecified.

Therefore --> don't use that, use ByteBuffer.allocate() instead which is guaranteed to have a backing array.

But I guess the problem is with the first point.

Answer 3

Whenever you use a HashMap and you treat a certain type of object as a key , you need to consider whether that object's equals() method is designed properly for custom equality checks. In most cases it is not, so you need to re-implement the equals() method.

The prevailing logic in Java is that when you override equals() , you must also override the hashCode() method. (See this post )

In other words, you need to derive a class CustomBitmap from Bitmap and add new implementations of equals() and hashCode() to the new class. Then it can be used successfully as a key in a HashMap .