How can I throw a general exception in Java?

You could create your own Exception class:

public class InvalidSpeedException extends Exception {

  public InvalidSpeedException(String message){
     super(message);
  }

}

In your code:

throw new InvalidSpeedException("TOO HIGH");

You could use IllegalArgumentException:

public void speedDown(int decrement)
{
    if(speed - decrement < 0){
        throw new IllegalArgumentException("Final speed can not be less than zero");
    }else{
        speed -= decrement;
    }
}

Well, there are lots of exceptions to throw, but here is how you throw an exception:

throw new IllegalArgumentException("INVALID");

Also, yes, you can create your own custom exceptions.

A note about exceptions. When you throw an exception (like above) and you catch the exception: the String that you supply in the exception can be accessed throw the getMessage() method.

try{
    methodThatThrowsException();
}catch(IllegalArgumentException e)
{
  e.getMessage();
}

It really depends on what you want to do with that exception after you catch it. If you need to differentiate your exception then you have to create your custom Exception. Otherwise you could just throw new Exception("message goes here");

The simplest way to do it would be something like:

throw new java.lang.Exception();

However, the following lines would be unreachable in your code. So, we have two ways:

1. Throw a generic exception at the bottom of the method.
2. Throw a custom exception in case you don't want to do 1.