Avoiding multiple if conditions in a Car class
If you don't want to return early (because you want to go through all checks to warn about all problems), then you can use a flag to keep track of any problems:

for (Car car : carList) {
    boolean goodToGo = true; 
    if (!car.engineOn()) {
        car.warn("How are you planning to participate in a race with an off engine??");
        goodToGo = false;
    }
    if (!car.driverBeltOn()) {
        car.warn("Please wear a damn belt!");
        goodToGo = false;
    }
    if (goodToGo) {
        // Vroooomm....
    }
}


Btw, I guess you forgot a return statement here:


public boolean driverBeltOn() {
    this.driverBeltOn;
}



And I recommend to rename your methods to isDriverBeltOn and isEngineReady, as it's a fairly common practice to prefix boolean getters with "is...".