Why is variable not always null? : <p>Within the selectMarkerIfHover method, the if statement allows for a case when "lastSelected" is not null. </p>

<p>For the life of me I cannot see a situation when this could ever happen, because it appears to always be either null to begin with, or set as null in the mouseMoved method. I need to understand how lastSelected could ever be null when the selectMarkerIfHover method is called. I have looked everywhere for help to solve this, but it is so specific, I could not find an answer.</p>

<pre><code>public class EarthquakeCityMap extends PApplet {

// You can ignore this.  It's to get rid of eclipse warnings
private static final long serialVersionUID = 1L;

// IF YOU ARE WORKING OFFILINE, change the value of this variable to true
private static final boolean offline = false;

/** This is where to find the local tiles, for working without an Internet connection */
public static String mbTilesString = "blankLight-1-3.mbtiles";

//feed with magnitude 2.5+ Earthquakes
private String earthquakesURL = "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/2.5_week.atom";

// The files containing city names and info and country names and info
private String cityFile = "city-data.json";
private String countryFile = "countries.geo.json";

// The map
private UnfoldingMap map;

// Markers for each city
private List&lt;Marker&gt; cityMarkers;
// Markers for each earthquake
private List&lt;Marker&gt; quakeMarkers;

// A List of country markers
private List&lt;Marker&gt; countryMarkers;

// NEW IN MODULE 5
private CommonMarker lastSelected;
private CommonMarker lastClicked;

public void setup() {       
    // (1) Initializing canvas and map tiles
    size(900, 700, OPENGL);
    if (offline) {
        map = new UnfoldingMap(this, 200, 50, 650, 600, new MBTilesMapProvider(mbTilesString));
        earthquakesURL = "2.5_week.atom";  // The same feed, but saved August 7, 2015
    }
    else {
        map = new UnfoldingMap(this, 200, 50, 650, 600, new Google.GoogleMapProvider());
        // IF YOU WANT TO TEST WITH A LOCAL FILE, uncomment the next line
        //earthquakesURL = "2.5_week.atom";
    }
    MapUtils.createDefaultEventDispatcher(this, map);


    // (2) Reading in earthquake data and geometric properties
    //     STEP 1: load country features and markers
    List&lt;Feature&gt; countries = GeoJSONReader.loadData(this, countryFile);
    countryMarkers = MapUtils.createSimpleMarkers(countries);

    //     STEP 2: read in city data
    List&lt;Feature&gt; cities = GeoJSONReader.loadData(this, cityFile);
    cityMarkers = new ArrayList&lt;Marker&gt;();
    for(Feature city : cities) {
      cityMarkers.add(new CityMarker(city));
    }

    //     STEP 3: read in earthquake RSS feed
    List&lt;PointFeature&gt; earthquakes = ParseFeed.parseEarthquake(this, earthquakesURL);
    quakeMarkers = new ArrayList&lt;Marker&gt;();

    for(PointFeature feature : earthquakes) {
      //check if LandQuake
      if(isLand(feature)) {
        quakeMarkers.add(new LandQuakeMarker(feature));
      }
      // OceanQuakes
      else {
        quakeMarkers.add(new OceanQuakeMarker(feature));
      }
    }

    // could be used for debugging
    printQuakes();

    // (3) Add markers to map
    //     NOTE: Country markers are not added to the map.  They are used
    //           for their geometric properties
    map.addMarkers(quakeMarkers);
    map.addMarkers(cityMarkers);

}  // End setup


public void draw() {
    background(0);
    map.draw();
    addKey();

}

/** Event handler that gets called automatically when the 
 * mouse moves.
 */
@Override
public void mouseMoved()
{
    // clear the last selection
    if (lastSelected != null) {
        lastSelected.setSelected(false);
        lastSelected = null;

    }
    selectMarkerIfHover(quakeMarkers);
    selectMarkerIfHover(cityMarkers);
}

// If there is a marker under the cursor, and lastSelected is null 
// set the lastSelected to be the first marker found under the cursor
// Make sure you do not select two markers.
// 
private void selectMarkerIfHover(List&lt;Marker&gt; markers)
{
    // TODO: Implement this method
    // Abort if there's already a marker selected
    if (lastSelected != null) {
        return;
    }

    for(Marker m : markers) {
        CommonMarker marker = (CommonMarker)m;
        if(marker.isInside(map,mouseX,mouseY)) {
            lastSelected = marker;
            marker.setSelected(true);
            return;
        }
    }
}

/** The event handler for mouse clicks
 * It will display an earthquake and its threat circle of cities
 * Or if a city is clicked, it will display all the earthquakes 
 * where the city is in the threat circle
 */
@Override
public void mouseClicked()
{
    // TODO: Implement this method
    // Hint: You probably want a helper method or two to keep this code
    // from getting too long/disorganized
    if(lastClicked != null) {
        unhideMarkers();
        lastClicked = null;
    }
    else if(lastClicked == null) {
        checkEarthquakes();
        if(lastClicked == null) {
            checkCities();
        }
    }

}

// Helper method that will check if an earthquake marker was clicked on
// and respond appropriately
private void checkEarthquakes() {
    if (lastClicked != null) return;
    // Loop over the earthquake markers to see if one of them is selected
    for (Marker m : quakeMarkers) {
        EarthquakeMarker marker = (EarthquakeMarker)m;
        if (!marker.isHidden() &amp;&amp; marker.isInside(map, mouseX, mouseY)) {
            lastClicked = marker;
            // Hide all the other earthquakes and hide
            for (Marker mhide : quakeMarkers) {
                if (mhide != lastClicked) {
                    mhide.setHidden(true);
                }
            }
            for (Marker mhide : cityMarkers) {
                if (mhide.getDistanceTo(marker.getLocation()) 
                        &gt; marker.threatCircle()) {
                    mhide.setHidden(true);
                }
            }
            return;
        }
    }
}

// Helper method that will check if a city marker was clicked on
// and respond appropriately
private void checkCities() {
    if (lastClicked != null) return;
    // Loop over the earthquake markers to see if one of them is selected
    for (Marker marker : cityMarkers) {
        if (!marker.isHidden() &amp;&amp; marker.isInside(map, mouseX, mouseY)) {
            lastClicked = (CommonMarker)marker;
            // Hide all the other earthquakes and hide
            for (Marker mhide : cityMarkers) {
                if (mhide != lastClicked) {
                    mhide.setHidden(true);
                }
            }
            for (Marker mhide : quakeMarkers) {
                EarthquakeMarker quakeMarker = (EarthquakeMarker)mhide;
                if (quakeMarker.getDistanceTo(marker.getLocation()) 
                        &gt; quakeMarker.threatCircle()) {
                    quakeMarker.setHidden(true);
                }
            }
            return;
        }
    }       
}


// loop over and unhide all markers
private void unhideMarkers() {
    for(Marker marker : quakeMarkers) {
        marker.setHidden(false);
    }

    for(Marker marker : cityMarkers) {
        marker.setHidden(false);
    }
}

// helper method to draw key in GUI
private void addKey() { 
    // Remember you can use Processing's graphics methods here
    fill(255, 250, 240);

    int xbase = 25;
    int ybase = 50;

    rect(xbase, ybase, 150, 250);

    fill(0);
    textAlign(LEFT, CENTER);
    textSize(12);
    text("Earthquake Key", xbase+25, ybase+25);

    fill(150, 30, 30);
    int tri_xbase = xbase + 35;
    int tri_ybase = ybase + 50;
    triangle(tri_xbase, tri_ybase-CityMarker.TRI_SIZE, tri_xbase-CityMarker.TRI_SIZE, 
            tri_ybase+CityMarker.TRI_SIZE, tri_xbase+CityMarker.TRI_SIZE, 
            tri_ybase+CityMarker.TRI_SIZE);

    fill(0, 0, 0);
    textAlign(LEFT, CENTER);
    text("City Marker", tri_xbase + 15, tri_ybase);

    text("Land Quake", xbase+50, ybase+70);
    text("Ocean Quake", xbase+50, ybase+90);
    text("Size ~ Magnitude", xbase+25, ybase+110);

    fill(255, 255, 255);
    ellipse(xbase+35, 
            ybase+70, 
            10, 
            10);
    rect(xbase+35-5, ybase+90-5, 10, 10);

    fill(color(255, 255, 0));
    ellipse(xbase+35, ybase+140, 12, 12);
    fill(color(0, 0, 255));
    ellipse(xbase+35, ybase+160, 12, 12);
    fill(color(255, 0, 0));
    ellipse(xbase+35, ybase+180, 12, 12);

    textAlign(LEFT, CENTER);
    fill(0, 0, 0);
    text("Shallow", xbase+50, ybase+140);
    text("Intermediate", xbase+50, ybase+160);
    text("Deep", xbase+50, ybase+180);

    text("Past hour", xbase+50, ybase+200);

    fill(255, 255, 255);
    int centerx = xbase+35;
    int centery = ybase+200;
    ellipse(centerx, centery, 12, 12);

    strokeWeight(2);
    line(centerx-8, centery-8, centerx+8, centery+8);
    line(centerx-8, centery+8, centerx+8, centery-8);

}



// Checks whether this quake occurred on land.  If it did, it sets the 
// "country" property of its PointFeature to the country where it occurred
// and returns true.  Notice that the helper method isInCountry will
// set this "country" property already.  Otherwise it returns false.    
private boolean isLand(PointFeature earthquake) {

    // IMPLEMENT THIS: loop over all countries to check if location is in any of them
    // If it is, add 1 to the entry in countryQuakes corresponding to this country.
    for (Marker country : countryMarkers) {
        if (isInCountry(earthquake, country)) {
            return true;
        }
    }

    // not inside any country
    return false;
}

// prints countries with number of earthquakes
private void printQuakes() {
    int totalWaterQuakes = quakeMarkers.size();
    for (Marker country : countryMarkers) {
        String countryName = country.getStringProperty("name");
        int numQuakes = 0;
        for (Marker marker : quakeMarkers)
        {
            EarthquakeMarker eqMarker = (EarthquakeMarker)marker;
            if (eqMarker.isOnLand()) {
                if (countryName.equals(eqMarker.getStringProperty("country"))) {
                    numQuakes++;
                }
            }
        }
        if (numQuakes &gt; 0) {
            totalWaterQuakes -= numQuakes;
            System.out.println(countryName + ": " + numQuakes);
        }
    }
    System.out.println("OCEAN QUAKES: " + totalWaterQuakes);
}



// helper method to test whether a given earthquake is in a given country
// This will also add the country property to the properties of the earthquake feature if 
// it's in one of the countries.
// You should not have to modify this code
private boolean isInCountry(PointFeature earthquake, Marker country) {
    // getting location of feature
    Location checkLoc = earthquake.getLocation();

    // some countries represented it as MultiMarker
    // looping over SimplePolygonMarkers which make them up to use isInsideByLoc
    if(country.getClass() == MultiMarker.class) {

        // looping over markers making up MultiMarker
        for(Marker marker : ((MultiMarker)country).getMarkers()) {

            // checking if inside
            if(((AbstractShapeMarker)marker).isInsideByLocation(checkLoc)) {
                earthquake.addProperty("country", country.getProperty("name"));

                // return if is inside one
                return true;
            }
        }
    }

    // check if inside country represented by SimplePolygonMarker
    else if(((AbstractShapeMarker)country).isInsideByLocation(checkLoc)) {
        earthquake.addProperty("country", country.getProperty("name"));

        return true;
    }
    return false;
}

}
</code></pre>
