Java 如何在box2d中实现传感器之间的交互？

我不知道如何使传感器平台的交互变得活跃。大概是这样的：

BeginContact { 
     if (sensorA && sensorB){ 
         platform.getBody () 
         setActive (true);
     }
}

但我不知道怎么做。请帮帮我

public class WorldUtils {

public static World createWorld() {
    return new World(Constants.WORLD_GRAVITY, true);
}

public static Body createPlatform(World world) {

    SpawnType spawnType = RandomUtils.getRandomSpawnType();

    BodyDef bodyDef = new BodyDef();
    bodyDef.type = BodyDef.BodyType.StaticBody;
    bodyDef.position.set(new Vector2(GameStage.pointToCreate, spawnType.getY()));

    PolygonShape shape = new PolygonShape();
    FixtureDef fixtureDef = new FixtureDef();
    fixtureDef.density = spawnType.getDensity();
    fixtureDef.shape = shape;

    shape.setAsBox(spawnType.getWidth() / 2, spawnType.getHeight() / 2);
    PolygonShape polygonShapeSensor = new PolygonShape();
    polygonShapeSensor.setAsBox(Constants.PLATFORM_WIDTH/2+0.5f, Constants.PLATFORM_HEIGHT/2-1.5f,
            new Vector2(Constants.PLATFORM_WIDTH/2-0.5f, Constants.PLATFORM_HEIGHT/2-2f), bodyDef.angle);

    FixtureDef myPolygonFixtureDef = new FixtureDef();
    myPolygonFixtureDef.shape = polygonShapeSensor;
    myPolygonFixtureDef.isSensor = true;

    Body body = world.createBody(bodyDef);
    body.createFixture(fixtureDef);
    body.createFixture(myPolygonFixtureDef);
    body.resetMassData();
    PlatformUserData userData = new PlatformUserData(spawnType.getWidth(), spawnType.getHeight());
    body.setUserData(userData);
    shape.dispose();
    return body;
}


public static Body createCharacter(World world) {

    BodyDef bodyDef = new BodyDef();
    bodyDef.type = BodyDef.BodyType.DynamicBody;
    bodyDef.position.set(new Vector2(Constants.CHARACTER_X, Constants.CHARACTER_Y));

    PolygonShape shape = new PolygonShape();
    shape.setAsBox(Constants.CHARACTER_WIDTH / 2, Constants.CHARACTER_HEIGHT / 2);
    PolygonShape polygonShapeSensor = new PolygonShape();
    polygonShapeSensor.setAsBox(Constants.CHARACTER_WIDTH/2+0.5f, Constants.CHARACTER_HEIGHT/2-1.5f,
            new Vector2(Constants.CHARACTER_WIDTH/2-0.5f, Constants.CHARACTER_HEIGHT/2-2f), bodyDef.angle);

    FixtureDef myPolygonFixtureDef = new FixtureDef();
    myPolygonFixtureDef.shape = polygonShapeSensor;
    myPolygonFixtureDef.isSensor = true;

    Body body = world.createBody(bodyDef);
    body.setGravityScale(Constants.CHARACTER_GRAVITY_SCALE);
    body.createFixture(shape, Constants.CHARACTER_DENSITY);
    body.createFixture(myPolygonFixtureDef);
    body.resetMassData();
    body.setUserData(new CharacterUserData(Constants.CHARACTER_WIDTH, Constants.CHARACTER_HEIGHT));
    shape.dispose();
    body.setBullet(true);
    body.setSleepingAllowed(false);
    return body;
}
}




public class GameStage extends Stage implements ContactListener {

...............................................................

    @Override
public void beginContact(Contact contact) {
    Body a = contact.getFixtureA().getBody();
    Body b = contact.getFixtureB().getBody();


    if ((BodyUtils.bodyIsRunner(a) && BodyUtils.bodyIsGround(b)) ||
            (BodyUtils.bodyIsGround(a) && BodyUtils.bodyIsRunner(b))) {


    }

}
}



public class BodyUtils {
public static boolean bodyIsRunner(Body body) {
    UserData userData = (UserData) body.getUserData();
    return userData!=null && userData.getUserDataType() == UserDataType.CHARACTER;
}

public static boolean bodyIsGround (Body body) {
    UserData userData = (UserData) body.getUserData();
    return userData != null && userData.getUserDataType() == UserDataType.PLATFORM;
}

这是一个很难回答的问题？