Estou tentando escrever um objeto que criei em um arquivo usando o fluxo de saída do objeto e sempre que executo o código, ele gera uma NotSerializableException. Por favor, diga-me se você vê o que fiz de errado.

Guardar método:

public static void saveEntity(PhysicsBody b, File f) throws IOException {
    if (!f.exists())
        f.createNewFile();
    ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(f));
    oos.writeObject(b);
    oos.close();
}

Saída de erro:

java.io.NotSerializableException: PhysicsBody
at java.io.ObjectOutputStream.writeObject0(Unknown Source)
at java.io.ObjectOutputStream.writeObject(Unknown Source)
at PhysicsUtil.saveEntity(PhysicsUtil.java:15)
at applet.run(applet.java:51)
at java.lang.Thread.run(Unknown Source)

PhysicsBody classe:

import java.awt.geom.Point2D;
import java.util.ArrayList;

public class PhysicsBody {

protected float centerX;
protected float centerY;
protected float minX, minY, maxX, maxY;
protected float mass;
protected ArrayList<Vertex> vertices = new ArrayList<Vertex>();
protected ArrayList<Edge> edges = new ArrayList<Edge>();

public PhysicsBody(float mass) {
    this.mass = mass;
}

public void addVertex(Vertex v) {
    if (v != null)
        vertices.add(v);
}

public void addEdge(Edge e) {
    if (e != null)
        edges.add(e);
}

public MinMax projectToAxis(Point2D.Float axis) {
    float dotP = axis.x * vertices.get(0).x + axis.y * vertices.get(0).y;
    MinMax data = new MinMax(dotP, dotP);

    for (int i = 0; i < vertices.size(); i++) {
        dotP = axis.x * vertices.get(i).x + axis.y * vertices.get(i).y;
        data.min = Math.min(data.min, dotP);
        data.max = Math.max(data.max, dotP);
    }
    return data;
}

public void calculateCenter() {
    centerX = centerY = 0;

    minX = 10000.0f;
    minY = 10000.0f;
    maxX = -10000.0f;
    maxY = -10000.0f;

    for (int i = 0; i < vertices.size(); i++) {
        centerX += vertices.get(i).x;
        centerY += vertices.get(i).y;

        minX = Math.min(minX, vertices.get(i).x);
        minY = Math.min(minY, vertices.get(i).y);
        maxX = Math.max(maxX, vertices.get(i).x);
        maxY = Math.max(maxY, vertices.get(i).y);
    }

    centerX /= vertices.size();
    centerY /= vertices.size();
}

}