Exception Handling

The syntax for handling exception is:

try {}
catch (<MyException> e) {}
finally {}

If there is a try you need a catch or a finally or both. Every catch must precede the finally and they must be declared respecting the hierarchy from the more specialized to the less specialized (e.g. FileNotFoundException before IOException).

The code inside the finally is always executed, even if there is a return inside the try.

It’s possible to throw exceptions with the statement throw. You can throw every class descending from Throwable.

We have two type of exception: checked and unchecked exception. If an exception is checked this means that you have to handle it through a try...catch or you need to add to the method signature the clause throws, like public void readFromFile() throws IOException {}.

Error and Exception extend Throwable, RuntimeException extends Exception. Error, Throwable and RuntimeException are unchecked, Exception is checked.


You can use it to identify the truth about some statements when you are not in production. We have two types of assertions:

  • assert(<boolean expression>);
  • assert(<boolean expression>): <statement returning a value>;

The second one gives you some more information and converts the second statement in a string. If the boolean expression is false then we’ll have an AssertionError and you MUST NOT HANDLE IT, remember, it’s not in production!

Be aware that:

before Java 1.4 assert could be used as an identifier, so if we have a source code written before 1.4, we need to specify javac -source 1.3 com/Class.java so that assert will not be used as language keyword.

The option -source accept 1.3, 1.4, 1.5, 5, 1.6, 6

To enable or disable assertion at runtime:

  • java -ea or java -enableassertions to enable them;
  • java -da or java -disableassertions to disable them;
  • java -ea: com.foo.Bar to enable them for a specific class;
  • java -ea: com.foo... to enable them for a specific package and all subpackages;
  • java -ea -dsa enable all the assertions except the ones from system classes;
  • java -ea -da:com.foo... enable all the assertions except for a specific package;

How to use assertions

An AssertionError should not be handled.

An assertion must not be used when:

  • I want to validate arguments of a public method because when you disable them you are also disabling the validation;
  • I want to validate command-line arguments;
  • I risk to have side effects. Like when you are doing assert(booleanMethod()) it’s true that you are getting a boolean but are you sure that inside the method the state of the object is not changing?

An assertion can be used when:

  • I want to validate arguments of a private method because if it’s private you can assume the logic is correct (you already validated input coming from a public method using this private method);
  • I want to assert cases that, even in production, will never happen;