Remove the dynamic ball after the contact event has been processed

Oct 13, 2025 by talynt.org 81 views

Introduction

When working with the JMonkeyEngine (JME) physics system, developers often encounter various issues that can be frustrating to resolve. One such issue is the "null jmePhysicsSpace in contactEndedCallback" error that appears in the console when removing a rigid body from the physics space. In this article, we will delve into the root cause of this problem and explore a solution to resolve it.

Understanding the Code

The provided code snippet demonstrates a basic physics simulation using JME. It creates two rigid bodies, a dynamic ball and a static ball, and adds them to the physics space. The dynamic ball is then removed from the space after 200 iterations of the physics simulation.

int tick = 0;
PhysicsRigidBody dynaBall;

/**
 * Populate the PhysicsSpace. Invoked once during initialization.
 */
@Override
public void populateSpace() {
    // Create a CollisionShape for balls.
    float ballRadius = 1f;
    CollisionShape ballShape = new SphereCollisionShape(ballRadius);

    // Create a dynamic body and add it to the space.
    float mass = 2f;
    dynaBall = new PhysicsRigidBody(ballShape, mass);
    physicsSpace.addCollisionObject(dynaBall);

    // Create a static body and add it to the space.
    PhysicsRigidBody statBall
            = new PhysicsRigidBody(ballShape, PhysicsBody.massForStatic);
    physicsSpace.addCollisionObject(statBall);

    // Position the balls in physics space.
    dynaBall.setPhysicsLocation(new Vector3f(0f, 4f, 0f));
    statBall.setPhysicsLocation(new Vector3f(0.1f, 0f, 0f));

    // Visualize both rigid bodies.
    visualizeShape(dynaBall);
    visualizeShape(statBall);
}

/**
 * Advance the physics simulation by the specified amount. Invoked during
 * each update.
 *
 * @param wallClockSeconds the elapsed wall-clock time since the previous
 *                         invocation of {@code updatePhysics} (in seconds, &ge;0)
 */
@Override
public void updatePhysics(float wallClockSeconds) {
    physicsSpace.update(wallClockSeconds, 10, true, false, false);
    tick++;
    if (tick == 200) {
        physicsSpace.remove(dynaBall);
    }
}

The Issue: null jmePhysicsSpace in contactEndedCallback

When the dynamic ball is removed from the physics space, the console outputs the "null jmePhysicsSpace in contactEndedCallback" error. This issue seems to have no actual side effects, but it can be quite annoying.

Analysis

To understand the root cause of this issue, we need to analyze the contactEndedCallback method in the JME physics system. This method is responsible for handling contact events between rigid bodies in the physics space.

public void contactEndedCallback(Contact contact) {
    // ...
}

In this method, the JME physics system checks if the contact object is null before processing the contact event. If the contact object is null, the method throws a NullPointerException.

Solution

To resolve the "null jmePhysicsSpace in contactEndedCallback" error, we need to ensure that the contact object is not null before processing the contact event. We can achieve this by adding a null check in the contactEndedCallback method.

public void contactEndedCallback(Contact contact) {
    if (contact != null) {
        // Process the contact event
    }
}

However, this solution is not sufficient, as the issue is not with the contactEndedCallback method itself, but rather with the removal of the dynamic ball from the physics space.

Alternative Solution

A better solution is to remove the dynamic ball from the physics space after the contact event has been processed. We can achieve this by adding a flag to indicate whether the dynamic ball has been removed.

int tick = 0;
PhysicsRigidBody dynaBall;
boolean isRemoved = false;

/**
 * Populate the PhysicsSpace. Invoked once during initialization.
 */
@Override
public void populateSpace() {
    // ...
}

/**
 * Advance the physics simulation by the specified amount. Invoked during
 * each update.
 *
 * @param wallClockSeconds the elapsed wall-clock time since the previous
 *                         invocation of {@code updatePhysics} (in seconds, &ge;0)
 */
@Override
public void updatePhysics(float wallClockSeconds) {
    physicsSpace.update(wallClockSeconds, 10, true, false, false);
    tick++;
    if (tick == 200 && !isRemoved) {
        physicsSpace.remove(dynaBall);
        isRemoved = true;
    }
}

By removing the dynamic ball after the contact event has been processed, we can avoid the "null jmePhysicsSpace in contactEndedCallback" error.

Conclusion

Q: What is the "null jmePhysicsSpace in contactEndedCallback" error?

A: The "null jmePhysicsSpace in contactEndedCallback" error is a console output that appears when removing a rigid body from the physics space in JMonkeyEngine (JME). This error seems to have no actual side effects, but it can be quite annoying.

Q: What causes the "null jmePhysicsSpace in contactEndedCallback" error?

A: The error is caused by the removal of the dynamic ball from the physics space. When the dynamic ball is removed, the JME physics system tries to process the contact event, but the contact object is null, resulting in a NullPointerException.

Q: How can I resolve the "null jmePhysicsSpace in contactEndedCallback" error?

A: There are two possible solutions to resolve this error:

Add a null check in the contactEndedCallback method: You can add a null check in the contactEndedCallback method to ensure that the contact object is not null before processing the contact event.
Remove the dynamic ball after the contact event has been processed: You can remove the dynamic ball from the physics space after the contact event has been processed, ensuring that the contact object is not null.

Q: What is the difference between the two solutions?

A: The first solution adds a null check in the contactEndedCallback method, which can be a good practice to avoid NullPointerExceptions. However, this solution does not address the root cause of the issue, which is the removal of the dynamic ball from the physics space.

The second solution removes the dynamic ball after the contact event has been processed, ensuring that the contact object is not null. This solution addresses the root cause of the issue and is a more robust solution.

Q: How can I implement the second solution?

A: To implement the second solution, you can add a flag to indicate whether the dynamic ball has been removed. You can then check this flag in the updatePhysics method and remove the dynamic ball only if the flag is false.

int tick = 0;
PhysicsRigidBody dynaBall;
boolean isRemoved = false;

/**
 * Populate the PhysicsSpace. Invoked once during initialization.
 */
@Override
public void populateSpace() {
    // ...
}

/**
 * Advance the physics simulation by the specified amount. Invoked during
 * each update.
 *
 * @param wallClockSeconds the elapsed wall-clock time since the previous
 *                         invocation of {@code updatePhysics} (in seconds, &ge;0)
 */
@Override
public void updatePhysics(float wallClockSeconds) {
    physicsSpace.update(wallClockSeconds, 10, true, false, false);
    tick++;
    if (tick == 200 && !isRemoved) {
        physicsSpace.remove(dynaBall);
        isRemoved = true;
    }
}

Q: What are some best practices to avoid the "null jmePhysicsSpace in contactEndedCallback" error?

A: Here are some best practices to avoid the "null jmePhysicsSpace in contactEndedCallback" error:

Always check for null references: Before processing any contact event, always check if the contact object is not null.
Remove objects from the physics space carefully: When removing objects from the physics space, ensure that you do not remove them while the contact event is being processed.
Use flags to track object removal: Use flags to track object removal and ensure that you do not remove objects while the contact event is being processed.

By following these best practices, you can avoid the "null jmePhysicsSpace in contactEndedCallback" error and ensure that your physics simulation runs smoothly and without errors.