Solution for GrayImage free response question on 2012 AP Computer Science exam - https://compsci.rocks

countWhitePixels-for.java

public int countWhitePixels() {
  int cnt = 0;
  for (int r=0; r<pixelValues.length; r++) {
    for (int c=0; c<pixelValues[r].length; c++) {
      if (pixelValues[r][c] == WHITE)
        cnt++; 
    }
  }
  return cnt; 
}

countWhitePixels-foreach.java

public int countWhitePixels() {
  int cnt = 0;
  for (int[] row: pixelValues) {
    for (int val: row) {
      if (val == WHITE)
        cnt++;
    }
  }
  return cnt; 
}

GrayImage.java

public class GrayImage {
  public static final int BLACK = 0;
  public static final int WHITE = 255;
  private int[][] pixelValues;
  public int countWhitePixels() {
    // Part A
  }
  public void processImage() {
    // Part B
  }

processImage.java

public void processImage() {
  for (int r=0; r<pixelValues.length-2; r++) {
    for (int c=0; c<pixelValues[r].length-2; c++) {
      pixelValues[r][c] -= pixelValues[r+2][c+2];
      if (pixelValues[r][c] < BLACK)
        pixelValues[r][c] = BLACK;
    }
  }
}