Questions and Answers
What types of inheritance does Java support?
Java supports single inheritance.
How do you extend a class in Java?
By using the 'extends' keyword.
How are classes accessed if they are not marked as public?
Only subclasses and classes from the same package can access them.
What do all Java objects implicitly extend from?
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What must you ensure when using constructors and inheritance?
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How do you access inherited class members in Java?
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What happens if you override a method in Java?
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What are the rules for method overriding in Java?
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What is the difference between overriding and method hiding?
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What happens if you create a final method in a parent class?
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What can you say about variables in Java inheritance?
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What is an abstract class in Java?
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What must the first concrete class extending an abstract class do?
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What is an interface in Java?
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What are the conditions for implementing multiple interfaces?
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What is the default interface method?
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What should happen if two interfaces with default methods conflict?
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What must be provided to use a static method in an interface?
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What does polymorphism allow in Java?
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What is required when casting an object from a superclass to a subclass?
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What can you do with polymorphic parameters?
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Study Notes
Inheritance in Java
- Java supports single inheritance, allowing a class to inherit from only one parent class, which enhances code clarity.
- A class can implement multiple interfaces, allowing for greater flexibility in design.
Extending Classes
- The
extends
keyword is used to indicate a child class deriving from a parent class, granting access to public and protected members. - Constructors in a child class can call parent constructors using
super()
, with ordering requirements for explicit calls.
Access Modifiers
- Public access modifier on a class allows access from any other class; absence of the modifier restricts access to subclasses and classes in the same package.
Object Hierarchy
- All Java objects implicitly extend
java.lang.Object
if they do not extend another class.
Constructor Behavior
- The child's constructor must match the parent's constructor signatures if the parent requires arguments.
- If not explicitly called, a no-argument constructor is automatically provided by the compiler.
Accessing Members
- Public and protected members can be accessed in child classes as if they were defined there, using
this.
for local context andsuper.
for parent context.
Method Overriding
- Non-private parent methods can be overridden in child classes using the same signature and return type; private methods cannot be overridden but can be "shadowed".
Overriding Rules
- Overridden method in child must match signature, be equally or more accessible, and conform to exception handling rules of the parent method.
Method vs Variable Behavior
- Variables cannot be overridden; they can only be hidden by declaring another variable with the same name in the child class.
- For variable reference,
super.
is used to access parent class variables from child classes.
Abstract Classes and Methods
- Abstract classes cannot be instantiated and may contain both abstract and concrete methods, but must implement all abstract methods in the first concrete subclass.
Interfaces Characteristics
- Interfaces define methods that implementing classes must provide, allowing classes to implement multiple interfaces for greater functionality.
Rules for Interfaces
- Interfaces cannot be instantiated, must have public access, and may not be final. All methods are considered public and abstract unless defined as static.
Default Methods in Interfaces
- Introduced in Java 8, default methods allow interfaces to include method bodies, providing backward compatibility while allowing overriding.
Static Methods in Interfaces
- Static methods can be defined in interfaces, accessible using
InterfaceName.methodName();
, and do not conflict across multiple interface implementations.
Polymorphism in Java
- Allows objects to be referenced by their own type, a superclass, or an interface type, affecting accessible properties and methods based on the reference type.
Casting in Java
- Implicit casting occurs when moving from subclass to superclass, while explicit casting is needed for superclass to subclass.
- Runtime exceptions may occur if an object is not an instance of the target type being cast.
Polymorphic Parameters
- Methods can use superclass or interface types as parameters, enabling flexibility to accept any subclass instances.
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Description
These flashcards cover essential concepts in Java class design, including types of inheritance and extending classes. Understanding these principles is crucial for effective Java programming and software design. Test your knowledge on how Java handles inheritance and class relationships to enhance your coding skills.