Parsing GPX files (Android compatible).

GpxParser.java

package pl.kamituel.gpx;

import java.io.IOException;
import java.io.InputStream;

public class GpxParser {
    private InputStream mIs = null;
    private StringBuilder mStringBuilder = new StringBuilder();

    public GpxParser (InputStream is) {
        mIs = is;
    }

    public TrkPt nextTrkPt () throws IOException {
        mStringBuilder.delete(0, mStringBuilder.length());

        int c;
        while ( (c = mIs.read()) != -1 ) {
            mStringBuilder.append((char)c);

            TrkPt trkpt = new TrkPt();
            if (trkpt.parse(mStringBuilder)) {
                return trkpt;
            }
        }
        return null;
    }

    public TrkPtInputStream getTrkPtStream () {
        return new TrkPtInputStream(this);
    }
}

GpxSmooth.java

package pl.kamituel.gpx;

import java.io.IOException;
import java.util.Vector;

/**
 * Created by kls on 6/18/13.
 */
public class GpxSmooth {
    public static Iterable<TrkPt> compressGpx (GpxParser gpx, Double threshold) throws IOException {
        Vector<TrkPt> res = new Vector<TrkPt>();

        TrkPt pN = null;
        TrkPt pN_1 = null;
        TrkPt pN_2 = null;

        while ((pN = gpx.nextTrkPt()) != null) {
            if (threshold != null && pN_1 != null && pN_2 != null) {
                if (distanceFromTheLine(pN, pN_2, pN_1) > threshold) {
                    res.add(pN);
                }
            } else {
                res.add(pN);
            }

            pN_2 = pN_1;
            pN_1 = pN;
        }

        return res;
    }

    public static double distanceFromTheLine(TrkPt p1, TrkPt p2, TrkPt x) {
        double A = p1.getLon() - p2.getLon();
        double B = p2.getLat() - p1.getLat();
        double C = p1.getLat() * (p2.getLon() - p1.getLon()) - p1.getLon() * (p2.getLat() - p1.getLat());

        return (A * x.getLat() + B * x.getLon() + C) / Math.sqrt(A*A + B*B);
    }
}

TrkPt.java

package pl.kamituel.gpx;

import java.security.InvalidParameterException;

public class TrkPt {
    private final static String START_TAG = "<trkpt ";
    private final static String START_ANY_TAG = "<";
    private final static String END_TAG_EMPTY = "/>";
    private final static String END_TAG_FULL = "</trkpt>";

    private final static String ATTR_LAT = "lat=\"";
    private final static String ATTR_LON = "lon=\"";
    private final static String ELEM_ELE = "<ele>";

    private double mLat = 0.0;
    private double mLon = 0.0;
    private double mEle = Double.MIN_VALUE;

    public TrkPt () {
    }

    private boolean parse (String s) {
        int lat = s.indexOf(ATTR_LAT);
        int lon = s.indexOf(ATTR_LON);
        int ele = s.indexOf(ELEM_ELE);

        if (lat < 0 || lon < 0) {
            //throw new InvalidParameterException("trkpt without lat or lon attribute");
            return false;
        }

        int endLat = s.indexOf("\"", lat + ATTR_LAT.length());
        int endLon = s.indexOf("\"", lon + ATTR_LON.length());

        mLat = Double.parseDouble(s.substring(lat+ATTR_LAT.length(), endLat));
        mLon = Double.parseDouble(s.substring(lon+ATTR_LON.length(), endLon));

        if (ele > 0) {
          mEle = Double.parseDouble(s.substring(ele + ELEM_ELE.length(), s.indexOf("<", ele + ELEM_ELE.length())));
        }

        //System.out.println("lat " + (lat+ATTR_LAT.length()) + " endLat " + endLat);

        return true;
    }

    public boolean parse (StringBuilder s) {
        int startTag = s.indexOf(START_TAG);
        if (startTag < 0) return false;

        int nextTag = s.indexOf(START_ANY_TAG, startTag);
        int endTagEmpty = s.indexOf(END_TAG_EMPTY, startTag);
        int endTagFull = s.indexOf(END_TAG_FULL, startTag);

        if (endTagEmpty + END_TAG_EMPTY.length() == s.length()) {
            // It's like <trkpt .../>EOF
            //System.out.println("matches 0 " + s.substring(startTag, endTagEmpty + END_TAG_EMPTY.length()));
            return parse(s.substring(startTag, endTagEmpty + END_TAG_EMPTY.length()));
        } else if (nextTag > 0 && endTagEmpty > 0 && nextTag < endTagEmpty) {
            // It's like <trkpt ...><...
            if (nextTag == endTagFull) {
                // It's like <trkpt ...></trkpt>
                return parse(s.substring(startTag, endTagFull + END_TAG_FULL.length()));
            } else if (endTagFull > 0) {
                // It's like <trkpt ...>...</trkpt>
                //System.out.println("matches 2 " + s.substring(startTag, endTagFull + END_TAG_FULL.length()));
                return parse(s.substring(startTag, endTagFull + END_TAG_FULL.length()));
            } else {
                return false;
            }
        } else if (nextTag > 0 && endTagFull > 0) {
            //System.out.println("matches 3 " + s.substring(startTag, endTagFull + END_TAG_FULL.length()));
            return parse(s.substring(startTag, endTagFull + END_TAG_FULL.length()));
        } else {
            //System.out.println("no match " + s.toString());
            return false;
        }

    }

    public double getLat() {
        return mLat;
    }

    public double getLon() {
        return mLon;
    }

    public double getEle () {
         return mEle;
    }
}

TrkPtInputStream.java

package pl.kamituel.gpx;

import java.io.IOException;
import java.io.InputStream;

/**
 * Created by kls on 6/19/13.
 */
public class TrkPtInputStream extends InputStream {
    private GpxParser mParser;
    private StringBuilder mBuffer = new StringBuilder();

    public TrkPtInputStream(GpxParser parser) {
        mParser = parser;
    }

    @Override
    public int read() throws IOException {
        if (mBuffer.length() == 0) {
            TrkPt point = mParser.nextTrkPt();
            if (point == null) {
                return -1;
            }
            mBuffer.append(point.getLat() + "," + point.getLon() + "\n");
        }

        int res = mBuffer.charAt(0);
        mBuffer.deleteCharAt(0);
        return res;
    }
}

Should it be useful to anybody, I made a library for this: https://github.com/ticofab/android-gpx-parser .