Tutorial 27: Runtime Exceptions

Now we have talked a lot about exceptions in Java. We have seen how to handle exceptions using the try-catch block and throwing exceptions. We have also seen how to throw multiple exceptions and the order in which exceptions should be thrown if they have a parent child relationship because of polymorphism. 

There is one last thing that I have to teach you about exceptions. There are two types of exceptions in Java. These are the checked exceptions and unchecked exceptions.

So far, we have been dealing with checked exceptions, which force us to handle them before the program can compile. However, there is the second group of exceptions that we haven’t yet dealt with, known as the unchecked exceptions.

With unchecked exceptions, the program will compile, but will then exit with an error. Unchecked exceptions are also called runtime exceptions because they only manifest when the program is compiled.

However, please note that when your program has runtime exceptions, it is indicative of very serious and fundamental problems in the code. In the following example, we are going to look at three common examples of runtime errors. Note that in principle, these errors should not exist.

Divide by Zero Runtime Errors

If you try to divide any number by zero, Java will not complain, but when you run the program, it will not compile. Instead, it will throw an ArithmeticException. You will either have to fix the error in the code, or handle the exception by throwing it or using a try-catch block.

int y = 0;

int x = 5;

int z = x/y

Figure 1: Divide by zero ArithmeticException

Trying to Access an Array Index beyond its Bounds

If you try to access an array index beyond the highest index, Java will at runtime throw an ArrayOutOfBoundsException. Remember that array indices always start from zero. So if the array has 3 items, this means that the third item has the index 2. So if you try to access index 3, which doesn’t exist, you will get a runtime error.

String[] names = {“Brian”, “Veronicah”, “Anthony”};

System.out.println(names[3]);

Figure 2: ArrayIndexOutOfBoundsException

 

Null Pointer

You may create a string reference but point it to nothing, like this: String name = null;

If you try to use a method on an uninitialized object i.e. on the reference variable, Java will throw a NullPointerException at runtime.

Figure 3: NullPointerException

While you can handle runtime exceptions when you know they exist, the ideal scenario is to never have them in the first place. Some runtime exceptions such as the NullPointerException are usually common among people learning Java for the first time, because they might not have learnt how to initialize objects.

In the next tutorial, we are going to look at abstract classes in Java.

In case you have any question on Exceptions or any other part of this course, please let me know by dropping them in the comments section below.

Until next time, take care.