week03 Abstraction&relationaships&testing

Question 1

What is OOP?

Answer 1

OOP is a style of programming that models the real world. It uses objects and classes to create reusable code.

Question 2

True or false?
“OOP only uses Encapsulation and Inheritance as its main techniques”

Answer 2

False. OOP uses Modularity, Abstraction, Encapsulation, Inheritance, and Polymorphism.

polymorphism: ある1つのメソッドを呼び出しに対して、オブジェクト毎に異なる機能や動作を示すこと。多態性。

Question 3

What does modularity mean in OOP?

Answer 3

Modularity refers to dividing a program into separate modules that can be developed and tested independently.

Question 4

Why is encapsulation is important in OOP?

Answer 4

Encapsulation protects object integrity by hiding its internal state and requiring all interaction to be performed through its methods.

Question 5

What is inheritance in OOP?

Answer 5

Inheritance allows a new class to adopt the properties and methods an existing class.

Question 6

True or false?

“Polymorphism in OOP means that a single function can work in many different ways”

Answer 6

True. Polymorphism allows methods to do different things based on the object they are acting upon

Question 7

What are the benefits of using OOP?

Answer 7

OOP makes code more flexible, easier to maintain, and resusable, leading to efficient and effective programming practices.

Question 8

What does coupling in mean in Java?

Answer 8

Coupling is how closely different parts of a program work together. We want less couling to make changes easier.

Question 9

What is cohesion in Java?

Answer 9

Cohesion is when parts of a program that belong together are kept together. High cohesion makes a program more strong and organized.

Question 10

True or false?

“We want maximum cohesion in our Java programs”

Answer 10

True.

Question 11

Why maximum independence important in java?

Answer 11

Independence means each part can work on its own. This makes the progra more flexible and easier to fix.

Question 12

What is composition in OOP?

Answer 12

Composition is when a class includes objects from other classes as part of its structure. It’s ‘has-a’ relationship.

Question 13

True or false?
“A unidirectional association means both classes know about each other”

Answer 13

False. In a unidirectional association, only one class contains a reference to the other.

Question 14

True or false?
“In a bidirectional association, both objects contain references to one another.”

Answer 14

True.

Question 15

What is the Law of Demeter?

Answer 15

It is a rule for writing software. It says objects should only talk to close friends, not strangers. This means a class should only know limited things about other classes. It should talk only to its direct friends.

Question 16

What does making a parameter final do?

Answer 16

It does not let you change the parameter’s value inside the method.

Question 17

How to write symbolic constants in Java?

Answer 17

Use final keyword. Write class-level constants in ALL_CAPS and local constatnts in camelCase.

Question 18

True or false?
“Assign a value to a symbolic constant when it’s declared.”

Answer 18

True. A symbolic constant should get its value immediately when declared.

Question 19

When to assign value to a final instance variable?

Answer 19

Assign during object construction if it’s meant to only set once.

Question 20

True or false?
“Class-level constants should be static and final for memory efficiency”

Answer 20

True. Static means it’s class-wide, final means it’s unchangeable. It’s memory efficient.

Question 21

Where does the equals method come from?

Answer 21

The equals method is inherited from the Object class, which is the parent class for all Java classes. Every class you create automatically has this method. (, and it’s a good practice to override it for proper equality comparison based on the class’s fields.)

Question 22

What are the three methods from Object that are commonly overridden?

Answer 22

1. toString(): gives a string representation of the object.
2. equals(Object obj): checks if this object is equal to another object.
   3.hashCode(): provides the hash code of the object, which should be consistent with equals.

Question 23

True or false?
“Without overriding, equals checks if two object are the exact same object.”

Answer 23

True. The default implementation of equals in the Object class uses == to determine if two references point to the same object in memory, not if they are logically equal.

Question 24

True or false?
“A student object can access another Student’s private fields.”

Answer 24

True. Within the equals method, you can access the private fields of another object of the same class directly. This is because the scope of private access control is within the same class, not just the same object.

Question 25

What does hashCode() do in Java?

Answer 25

hashCode() is a method that gives a number for an object. This number comes from a hash function. This function takes data of any size and gives back a fixed size number. The idea is to make a unique number for different object states. If two objects are the same, they should have the same hash number.

Question 26

What is hashing?

Answer 26

Hashing is when you use a function to find the right index for an item in a list or table. It is much faster than seraching every place if you know the index. To use hashing in Java, hashCode() must work well with equals().

Question 27

Why are bad hash fucntions a problem?

Answer 27

A bad hash function gives the same number for different objects. This is called a collision. Collisions make things slow because they need extra work to find the right object.

Question 28

True or false?
“You can use this in a static method”

Answer 28

False. Static methods belong to a class, so it doesn’t know what instance is invoked, which means it doesn’t understand what this is.

Question 29

True or false?
“Hashing always gives a unique result for each object”

Answer 29

False. Hasing tries to give unique results but sometimes gives the same number for different objects. This is why we have to deal with collisions.

Question 30

What is an enum in Java?

Answer 30

An enum is a special type in Java used to define collections of constants. It is typesafe, meaning you can use it to declare variables that can only take one of the predefined constants. Enums are a clean way to group together related constants and namage them.

Question 31

What are key properties of enums in Java?

Answer 31

Enums have named constants that are public, static, and final. Java does not allow you to create new enum instances because constructors are private. Enums are always static, so we access enum constants with the enum type name.

Question 32

When should you use enums?

Answer 32

Use enums when you know all possible values of a variable at compile time, like days of the week. For example, if you have an enum Day with values MONDAY, TUESDAY, etc., it’s impossible to add a new day ike FUNDAY. This makes the code safer and more predictable because it limits the values to a known, fixed set, preventing errors like assigning an invalid value to a variable of the enum type.

Question 33

What are static variables in Java?

Answer 33

Static variables belong to the class, not any instatnce. All instance share the same static variable. They are set up when the class is first used and stay the same for every instance. Use the static keyword to create them.

Question 34

True or false?
“Static methods can use non-static variables”

Answer 34

False. Static methods cannot use non-static variables. They don’t have this, so they can’t access instance variables.

Question 35

True or false?
“Static variables can be constants.”

Answer 35

True. Static variables can be constants if they are final. They can be pulic or private. A public static final variable is a constant that can be used anywhere, like Math.PI.

Question 36

When do you use static methods?

Answer 36

Use static methods when the method doesn’t need to access instance variables. They are good for actions that don’t change the state of an object, like Math.max();

Question 37

What are static factory methods?

Answer 37

Static factory methods are methods that create and return instances of a class. They can have descriptive names, reuse instances, return subtypes, and vary return types based on input, offering more flexibility than constructors.

Question 38

What are some downsides of static factory methods?

Answer 38

They can be hard to tell apart from other static methods. Also, classes without public constructors can’t be subclassed, which can limit how you use them.

Question 39

True or false?
“You can always use == for comparing double values if they look the same.”

Answer 39

False. Even if two double values look the same, their underlying binary representation might not match exactly due to floating-point precision.Use compare method in Double class.

Question 40

What does the compare method in the Double class do?

Answer 40

The compare method in the Double class evaluates two double values. It returns a negative value if the first double is smaller than the second, zero if both doubles are equal. and a positive value if the first double is larger than the second.

Question 41

What does type-safe mean for enumerations in Java?

Answer 41

It means the set of constants in an enum is fixed and know at compile time. New values cannot be added at runtime, ensuring only predefined values are used.

Question 42

Why are enums considered type-safe in Java?

Answer 42

Enums are type-safe because they limit the values to a predefined set know at compile time, preventing the use of invalid or unexpected values.

Question 43

What is a static initialization block in Java?

Answer 43

A static initialization block is a code block that executes once when the class is loaded into the JVM, used for initializing static variables or performing class-specific setup operations.

Question 44

Why is a static initialization block used in Java?

Answer 44

It’s used for complex initializations of static variables or to execute setup actions that need to happen before the class is first used.

Question 45

What is unidirectional association?

Answer 45

A one-way relationship where one class know about or interacts with another class, but not vice versa.

Question 46

What is bidirectional association?

Answer 46

Both classes are aware of each other and can interact mutually. This means that each class has a reference to the other, allowing both to access each other’s properties or methods.

Question 47

What does dependency mean in Java?

Answer 47

In Java, a dependency occurs when a class uses another class’s methods, variables, or data types, implying reliance on that class for functionality.

Question 48

What is delegation in Java?

Answer 48

Delegation is a design pattern where an object hands over its responsibilities to another object to perform specific tasks or functionalities.

Question 49

Why is delegation used in Java?

Answer 49

Delegation is used for decoupling objects, promoting code reusability and flexibility, and achieving separation of concerns.

Question 50

How does delegation differ from inheritance in Java?

Answer 50

Unlike inheritance where a subclass inherits behavior from a parent class, in delegation, an object relies on another object to perform certain tasks, without inheriting from it.