Inheritance. SOTE notebook. November 06, n Unidirectional association. Inheritance ("extends") Use relationship

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Julius Arnold
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1 Inheritance 1..n Unidirectional association Inheritance ("extends") Use relationship Implementation ("implements") What is inherited public, protected methods public, proteced attributes What is not inherited private methods, attributes constructor If the super class has a non trivial constructor, the inheriting class (sub class) alos has to have a constructor that is calling the constructor of the super class. 1

2 Driver * Car Book 2

This means that entire super class must be abstract.

3 If a method of a super class may not be overwritten it has to be declared as final. If a method of a super class has to be overwritten, it has to be declared as abstract. This means that entire super class must be abstract. Casting It is not possible to implicitly downcast from one class to another: It is also not possible to explicitly downcast (wé get a class cast exception): 3

4 An explicit and implicit upcast is possible: Attention : Even if there is an explicit upcast, still the method of the sub class is going to be invoked! 4

5 Comparion of abstract classes and interfaces Communalities: both have at least one abstract method abstracts methods of both, interfaces and abstract classes, have to be implemented from inheriting respectively implementing class both can not be instantiated Differences: Interfaces only have abstract methods (exception: default methods in Java 8), whereas abstract class may have non abstract methods. Interface may not have attributes (exception: static final), abstract class can have attributes key word: interfaces <=> implements, abstractes <=> extends 5

6 Error handling Java enforces the error handling compiler complaints 1. Alternative to deal with error: Throw error to calling method: 2. Alternative: Catch error and deal with it 6

7 Hiearchy of class Throwable thrown by JVM (typically not handled) Error Exception IOException RunTimeException OutOfMemoryError StackOverflowError AssertionError FileNotFoundException NullPointerException ClassCastException IndexOutOfBoundsException 7

8 List of all the runtime exceptions 8

9 Important rules for dealing with exceptions don't catch errors, typically there is not much you can do anyhow catch errors as close to the point where there problem is introduced as possible. Don't throw the exception up to the main methods never write an empty catch block (you won't find the error) catch the exception as specific as possible (e.g. catch FileNotFoundException instead of an Exception). Also catch blocks can contain errors, that have to be handled (try catch or throw) You can writer your own exception, but make sure that this exception does not already exist 9

10 You can write / implement your own exceptions: and use (catch or throw) it: 10

Combination try catch try catch finally try finally (exception is not treated) try catch catch try catch catch finally finally blocks are always

11 Combination try catch try catch finally try finally (exception is not treated) try catch catch try catch catch finally finally blocks are always exectuted. Typical use case: Working with ressources such as databases, filesystems, network; and the ressources have to be released in any case. 11

12 The sequence of catch-blocks matter! The SOTE1Exception-block is unreachable code, sind all Exceptions will be caught by the first catch-block. 12

More on Exception Handling

Chapter 18 More on Exception Handling Lecture slides for: Java Actually: A Comprehensive Primer in Programming Khalid Azim Mughal, Torill Hamre, Rolf W. Rasmussen Cengage Learning, 2008. ISBN: 978-1-844480-933-2