This should do it:

float[] array;

// Initializing here... not part of the average computation
array = new float[1000];
for ( int i = 0; i < array.length; ++i )
{
 array[i] = random( -1, 1 );
}

// This is where we compute the average
float average = 0;
for ( int i = 0; i < array.length; ++i )
{
 average += array[i];
}
average /= (float)(array.length);

// The average should be close to zero
println( average );