Introduction Flashcards
Java supports single an multilevel inheritance. What does that mean?
A class cannot extent from more than one class directly, but it can use a hierarchy
What is Polymorphism?
Polymorphism is the ability to process data differently depending on their types of inputs -> Same method name but with different parameters
What is dynamic Polymorphism?
Child class overrides the parent’s method
What is static Polymorphism?
Polymorphism which is resolved at compile time and thus does away with runtime virtual table lookups -> Multiple methods in one class have the same name
What are possible problems with Polymorphism?
Type identification during downcasting and fragile base class problem
When to use inheritance instead of composition?
Inheritance is used if classes fulfil the “is-a” condition and mainly provide additive functionality further down the class hierarchy
When to use composition instead of inheritance?
Composition allows to model objects that are made up of other objects -> the object(s) that compose or are contained by one object are destroyed too when that object is destroyed
What does the variable scope define?
The variable scope defines where variables can be seen
What method modifiers exist in java? And what is the default modifier?
public, protected, private, and package-private. Default is package-private
What is the problem with comparing two objects with the == operator?
Comparing two objects with the == operator returns false because it is not the values that get compared but the memory spaces -> Comparable and Comparator Interface
What are characteristics of abstract classes?
- Abstract modifier
- Abstract class can be subclassed, but it cannot be instantiated
- Abstract class has at least one abstract method but can declare both abstract and concrete methods
- A subclass derived from an abstract class must either implement ALL the abstract methods or be abstract itself
What are typical scenarios where we should prefer abstract classes over interfaces and concrete classes?
- Encapsulate some common functionality in one place that multiple subclasses will share
- Partially define an API that subclasses can easily extend and refine
- Subclasses need to inherit one or more common methods or fields with protected access
What is an interface?
An interface is an abstract type that contains a collection of method and constant variables
Why are interfaces used?
Interfaces are used to achieve abstraction, polymorphism and multiple inheritances
What are characteristics of an interface?
- We cannot instantiate interfaces directly
- Interfaces can be empty -> No variables or methods
- Interface methods cannot be protected or final
- Interface’s variables are public, static, and final by default and we are not allowed to change their visibility
What can we achieve by using interfaces?
- Behavioral Functionality
- Multiple Inheritances
- Polymorphism
What are the interface inheritance rules?
- Interface extending another interface -> All abstract methods are inherited
- Abstract class implementing an interface -> All abstract and default methods are inherited
What is the difference between a class and an interface?
- Concrete class is a blueprint for object creation
- Interface is a user-defined type which can have a collection of field constants and method signatures that will be overwritten by an implementing class
What is a functional interface?
A functional interface is an interface that contains only one abstract method
When to use an interface?
- Using multiple inheritances and if there are different class hierarchies
- When unrelated classes implement the interface
- When application functionalities have to be defined as a contract, but not concerned about who implements the behavior
When to use an abstract class?
- When trying to provide a base class with methods that the subclasses override
- If we have specified requirements and only partial implementation details
- While classes that extend abstract classes have several common fields or methods (that require non-public modifiers)
- If one wants to have non-final or non-static methods to modify the states of an object
For what can the super keyword be used?
- Access the parent class
- Super() method can be used to call the parent default constructor
- Accessing parent class variables
- Accessing methods from the parent class
For what can the this keyword be used?
- Reference to the current object whose method is being called
- Disambiguating field shadowing
- Referencing constructors of the same class
- Returning this to return the current class instance
- Access the outer class and it’s variables from within the inner class
What is field shadowing?
Example: If constructor parameters have the same name as instance fields
What is a generic constructor?
A generic constructor is a constructor that has at least one parameter of a generic type
What are anonymous classes?
Anonymous classes are inner classes with no name that you use to only instantiate one single object of that class ever -> You need to define that class and instantiate one object at the same time
Generally anonymous classes are just a particular case of nested classes
What are use cases for anonymous classes?
- Use inner classes in general use cases and anonymous classes in very specific ones in order to achieve a cleaner hierarchy
- Are usually used if the implementations of some methods of some classes needs to be modified on the fly
- Creating various event listeners in an application with graphical interface
What are nested classes?
A nested class is a class that is declared inside another class or interface
What is variable hiding?
- Local variables shadow instance variables with the same name
- When both the child and the parent classes have a variable with the same name, the child’s variable hides the one from the parent
What is method hiding?
When a child class defines a static method with the same signature as a static method in the parent class, then the child method hides the one in the parent class
What is an immutable object?
An immutable object is an object whose contents cannot be changed, so the internal state remains constant after it has been entirely created
What are the benefits of immutable objects?
- It can be safely shared among multiple threads
- Immutable objects are side-effects free because none of the objects referencing it will notice any difference
When are exceptions raised and what are possible reasons?
Exceptions are raised when the normal flow of the application is disrupted. Possible reasons are wrong input data, lose of a connection or an unexpected situation
When is the throw keyword used?
The throw keyword is used to explicitly throw an exception. This exception must be a subclass of Throwable but it cannot throw multiple exceptions with a single throw
When is the throws keyword used?
The throws keyword can be placed in the method declaration. It denotes which exceptions can be thrown from this method. These exceptions must be handles with a try-catch
What are checked exceptions?
Checked exceptions are checked at compile time
What are most common checked exceptions?
Most common checked exceptions are IOExceptions, FileNotFoundExceptions, ParseExceptions
What are unchecked exceptions?
Unchecked exceptions are thrown during runtime
What are most common unchecked exceptions?
Most common unchecked exceptions are ArrayIndexOutOfBoundsExceptions, IllegalArgumentException, NullPointerException
What is a JVM?
- Java Virtual Machine
- Is an implementation of a virtual machine which executes a java program
- It is an abstract computing machine with it’s own instruction set and manipulates various memory areas at runtime
How does a JVM work?
First interprets the bytecode, then stores class information in the memory area, and finally it executes the bytecode generated by the java compiler
What is a JRE?
- Java Runtime Environment
- Is a bundle of software components used to run java applications
What are components of a JRE?
- An implementation of a JVM
- Classes required to run the java programs
- Property files
What are components of a JVM?
- Class loaders
- Runtime data areas
- Execution engine
- Java native interface
- Native libraries
What is a JDK?
- Java Development Kit
- Provides environment and tools for developing, compiling, debugging, and executing a java program
What are components of a JDK?
- JRE
- Development tools
