CODEWITHSUNDEEP
javalinked-listindexoutofboundsexception
hacke
hacke

Reputation: 299

LinkedList out of bounds exception

I'm trying to make an implementation of Tom Nichols' low pass filter. My approach was like this, but I get an out of bounds exception all the time even though I've guarded against it:

private float  filterValue(LinkedList<Float> listOfItems)
    {
        final float ALPHA = 0.2f;
        float filtered;

        int marker = listOfItems.size();

        if (listOfItems.size()>2)
             marker = listOfItems.size() - 1;

        float current = listOfItems.pollLast();
        float previous = listOfItems.get(marker); <<-- line 282

        filtered = (float) previous + ALPHA*(current - previous);

        return filtered;


    }

error log:

12-16 15:54:51.148: E/AndroidRuntime(2055): FATAL EXCEPTION: main
12-16 15:54:51.148: E/AndroidRuntime(2055): java.lang.IndexOutOfBoundsException
12-16 15:54:51.148: E/AndroidRuntime(2055):     at      java.util.LinkedList.get(LinkedList.java:519)
 12-16 15:54:51.148: E/AndroidRuntime(2055):    at se.macke.velocitysensor.SensorActivity$MySensorEventListener.filterValue(SensorActivity.java:282)
 12-16 15:54:51.148: E/AndroidRuntime(2055):    at se.macke.velocitysensor.SensorActivity$MySensorEventListener.getAccelerometer(SensorActivity.java:249)
12-16 15:54:51.148: E/AndroidRuntime(2055):     at se.macke.velocitysensor.SensorActivity$MySensorEventListener.onSensorChanged(SensorActivity.java:231)
12-16 15:54:51.148: E/AndroidRuntime(2055):     at android.hardware.SystemSensorManager$ListenerDelegate$1.handleMessage(SystemSensorManager.java:204)
12-16 15:54:51.148: E/AndroidRuntime(2055):     at android.os.Handler.dispatchMessage(Handler.java:99)

Suggestions, please.

/M

Upvotes: 0

Views: 5659

Answers (2)

Kevin Bowersox
Kevin Bowersox

Reputation: 94499

Size returns the actual size of the list, it is not zero indexed. The get method retrieves an item from the list which uses a zero based index. The guard condition does not prevent this issue.

int marker = listOfItems.size();  //assume size = 3,
if (listOfItems.size()<2)   //condition is not met.
    marker = listOfItems.size() - 1;

float previous = listOfItems.get(marker); //attempts to access non-existent index of 3

Fix

private float  filterValue(LinkedList<Float> listOfItems)
{
    final float ALPHA = 0.2f;
    float filtered;

    int marker = listOfItems.size();

    if (listOfItems.size() > 0){

        marker = listOfItems.size() - 1;

        float current = listOfItems.pollLast();
        float previous = listOfItems.get(marker); <<-- line 282

        filtered = (float) previous + ALPHA*(current - previous);
    }
    return filtered;
}

Upvotes: 2

Subhrajyoti Majumder
Subhrajyoti Majumder

Reputation: 41230

if (listOfItems.size()<2)
    marker = listOfItems.size() - 1;

What if listOfItems.size() is zero then var marker would be -1 which cause IndexOutOfBoundsException.

you should put lower bound > 0. 

if (listOfItems.size()>0 && listOfItems.size()<2)
    marker = listOfItems.size() - 1;

Upvotes: 3

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