week04 inheritande&exceptions

Question 1

What is an array in Java?

Answer 1

An array is a fixed-size collection that can store multiple items of the same type. The size is set when the array is created and cannot change. Arrays are zero-indexed, which means the first element is at index 0.

Question 2

True or false?
“You can change the size of an array after it is created.”

Answer 2

False. Once created, the size of a Java array cannot change. You must create a new array if you need a different size.

Question 3

How do you find the number of elements in an array?

Answer 3

Use the length property of the array. if array is an array, array.length gives you the number of elements.

Question 4

What should you be careful about with arrays of objects?

Answer 4

An array of objects is initialized with null for each element. You must assign an object to each position before using it. Accessing null elements will cause a NullPointerException.

Question 5

True or false:
“System.arraycopy() creates a deep copy of an array.”

Answer 5

False. It creates a shallow copy, meaning it copies the references, not the actual objects.

Question 6

What is a multi-dimensional array in Java?

Answer 6

It’s an array of arrays. You can imagine it is like a table with rows and columns.

Question 7

What is an ArrayList in Java?

Answer 7

An ArrayList is a list that backed by an array. It can grow and shrink. It’s part of Java’s collections Framework and provides more functions than a simple array.

Question 8

True or false:
“An ArrayList has a fixed size”

Answer 8

False. Unlike arrays, an ArrayList can change its size dynamically.

Question 9

What are key differences between an ArrayList and an array?

Answer 9

ArrayList can change size, only stores objects, and has many methods. Arrays have fixed size, can store primitives, have no methods, and can be multi-dimensional.

Question 10

When is it better to use an ArrayList?

Answer 10

Use ArrayList when you need a dynamic collection that chagnes size, when you’re storing objects, and when you want useful methods like add, remove, and contains.

Question 11

When should you use an array instead of an ArrayList?

Answer 11

Use an array when you know the collection size won’t change, need to store primitives, or don’t need the extra features of ArrayList.

Question 12

How do you find the size of an array in Java?

Answer 12

Use the length property. Example: arrayName.length returns the size as an int.

Question 13

What can an array contain in Java?

Answer 13

A single kind of ‘thing’ - primitives, object of a class, but all must be the same type.

Question 14

What is composition in OOP?

Answer 14

A ‘has-a’ relationship where a class includes object references as instance variables, indicating ownership

Question 15

What is aggregation in OOP?

Answer 15

A specific ‘has-a’ relationship where a class contains a collection of objects, which can exist independently.

Question 16

What is the difference between composition and aggregation in OOP?

Answer 16

Composition implies ownership and a strong relationship; without the containing object, the contained objects do not make sense. Aggregation allows contained objects to exist independently.

Question 17

What does the “is-a” relationship signify in Java, and how is it implemented?

Answer 17

It signifies a class inheriting from another class, indicating a parent-child relationship. Implemented with the extends keyword, where the subclass “is a” specialized version of the superclass.

Question 18

What are the pillars of Object-Oriented Programming (OOP) in Java?

Answer 18

Encapsulation, information hiding, inheritance, polymorphism, and abstraction.

Question 19

How does Java’s inheritance relate to the Object class?

Answer 19

Every class in Java inherits from the Object class, gaining access to methods like toString(), equals(), and hashCode().

Question 20

What are the rules of inheritance in Java regarding superclass and subclass?

Answer 20

Java supports only single inheritance; a subclass can have only one superclass, ensuring a clear “is-a” relationship.

Question 21

Can a superclass have multiple subclasses? What must each subclass represent?

Answer 21

Yes, a superclass can have any number of subclasses. Each subclass must be a specialized version of the superclass.

Question 22

How should common attributes be managed in an inheritance hierarchy?

Answer 22

Common attributes should be declared in the superclass. Subclass constructors must pass these attributes to the superclass constructor using super().

Question 23

Should instance variables in the superclass be duplicated in the subclass?

Answer 23

No, instance variables should not be duplicated. Subclasses inherit them, including private ones, but access private variables through public or protected methods.

Question 24

How does inheritance support encapsulation in Java?

Answer 24

By grouping related instance variables and methods in a superclass, subclasses inherit and can use these, promoting independence and reducing coupling.

Question 25

What happens if the `super()` constructor call is not explicitly made in a subclass constructor?

Answer 25

Java attempts to call the superclass's default (no-argument) constructor automatically. If not available, the subclass must explicitly call a different superclass constructor.

Question 26

How can a subclass access non-private and private superclass members?

Answer 26

Non-private members can be accessed directly. Private members are accessed indirectly through public or protected methods of the superclass.

Question 27

What is method overriding and the role of the `protected` visibility modifier?

Answer 27

Overriding allows subclass methods to implement superclass method signatures. `Protected` makes variables or methods accessible to subclasses and package classes.

Question 28

What is the difference between overriding and hiding methods in Java?

Answer 28

Overriding changes a superclass method's behavior in the subclass. Hiding occurs when a subclass defines a static method with the same signature as a static superclass method.

Question 29

What are final classes and methods in Java?

Answer 29

Final classes cannot be extended, and final methods cannot be overridden. This is useful for locking down behavior to prevent further modification.

Question 30

What is the base class of all exceptions in Java?

Answer 30

`Throwable`. Exceptions in Java extend Throwable, meaning they can be thrown and caught.

Question 31

How does Java handle exceptions compared to Python?

Answer 31

Java uses `try` and `catch` blocks, similar to Python, but it adopts a Look Before You Leap (LBYL) approach, using exceptions only for exceptional situations, not for control flow.

Question 32

What are the components of Java's exception handling structure?

Answer 32

Wrap risky code in a `try` block, `catch` exceptions with catch blocks, and optionally use a `finally` block for code that runs regardless of exceptions.

Question 33

How do you document a method that throws an exception in Java?

Answer 33

Add `throws ExceptionName` to the method signature and use an `@throws` tag in the Javadoc.

Question 34

What are the two types of exceptions in Java?

Answer 34

Checked exceptions (must be caught or declared to be thrown) and unchecked exceptions (runtime errors, often due to programming mistakes).

Question 35

How should you handle unchecked exceptions in Java?

Answer 35

Unchecked exceptions should not be caught routinely; they indicate programming errors that need to be fixed.

Question 36

What must you do with checked exceptions in Java?

Answer 36

Must be handled or declared, as they represent recoverable conditions. The compiler enforces this rule.

Question 37

What is exception chaining in Java?

Answer 37

When an exception causes another exception, both can be seen in the stack trace. Be cautious with exceptions thrown from `finally` blocks, as they can obscure the original exception.

Question 38

How do you create custom exceptions in Java?

Answer 38

Extend `Exception` for checked exceptions, or extend `RuntimeException` for unchecked exceptions.

Question 39

What happens if `super()` is not called explicitly in a subclass constructor?

Answer 39

Java tries to call the superclass's no-argument constructor automatically. If unavailable, you must call a different superclass constructor explicitly.