HashMap dont stay stable in android

Question

Hi I am trying to save a object in HashMap if it is not exist than if it is exsit I want to control its value with new data. If data change than I want to do sth. else. But whenever I tried to compare new data and hash value I saw they are same on every time . How can I handle with this issue. There is code:

BluetoothLeDevice deviceLe;
private Map<String, byte[]> mMacMap;
byte [] integer0 =new byte[4];
byte[] tempInteger0=new byte[4];

public void addSensor(String macId, BluetoothLeDevice deviceLe) {
        byte [] addSensorrecord=deviceLe.getScanRecord();
        int j=0;
        for(int i=15;i<19;i++)
        {
            integer0 [j]=addSensorrecord[i];
            j++;
        }
        if (mMacMap.containsKey(macId)) {
            tempInteger0 = mMacMap.get(macId);

            if(!integer0 .equals(tempInteger0))
            {
                mMacMap.remove(macId);
                mMacMap.put(macId, integer0 );
                new SendBLEData().execute(deviceLe);
            }

        } else {
            final byte [] LocalInteger0=new byte[4];
            int t=0;
            for(int i=15;i<19;i++)
            {
                LocalInteger0[t]=addSensorrecord[i];
                t++;
            }
            mMacMap.put(macId, LocalInteger0);
            new SendBLEData().execute(deviceLe);
        }
    }

Answer 1

I am guessing, that your problem is here:

!Integer0.equals(tempInteger0))

I think you want to compare two arrays; and you are surprised to find them to be different ... all the time.

Your problem is: equals() on arrays doesn't do a comparison of the array content. In other words: this call to equals() only gives "true" if the arrays you are comparing ... are one and the same , like in:

  int a[] = { 1 };
  int b[] = a;
  int c[] = { 1 };

Here:

 a.equals(b) --> true

but

 a.equals(c) --> false

When comparing array content matters, then you should use ArrayList instead. Because two ArrayList objects are equal when they contain exactly the same equal elements.

And you see: you are using that equals on arrays to make a decision in your code. So, you either change to ArrayLists; or use Arrays.equals() as user hamsty suggested.

Answer 2

Just a few additions to the already posted answers.

The remove below is not necessary, a simple put will replace the old value

mMacMap.remove(macId);
mMacMap.put(macId, integer0 );

From the javadoc

If the map previously contained a mapping for the key, the old value is replaced by the specified value. (A map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true.)

Have you considered making bytes 15-19 into a string and adding them onto the maps key? This would eliminate the array compare and make the lookups much faster.

Answer 3

 !Integer0.equals(tempInteger0)) 

is your problem.

Use this to compare the content of arrays:

Arrays.equals(Integer0, tempInteger0)

Answer 4

The problem is the following sequence of events:

1. macId not in mMacMap , insert new byte[4]; into the map
2. macId in mMacMap , the array created in the previous step never matches integer0 due to the Array comparison problem mentioned by the other answer, replace macId in the map with a reference to integer0
3. macId in mMacMap , since the array is a reference to integer0 , it will always compare positively and the contents will no longer be updated.
4. Repeat 3.

Basically caused by these two issues:

1. Array#equals does not behave intuitively, use the static method Arrays.equals
2. Java is heavily reference-based, so if you insert something into a map it will not be copied, but simply a new reference is created; this may bite you if you change a shared object afterwards (like the array).