Write short notes on any two:<br>a) Association<br>b) Multi-threading<br>c) Access modifiers

Association:

In object-oriented programming (OOP), association refers to the relationship between two or more classes or objects. It is a way of organizing and structuring data and behavior in a program. There are three main types of associations:

• One-to-One: In a one-to-one association, each object of one class is associated with exactly one object of another class. For example, a student object may be associated with a single transcript object.
• One-to-Many: In a one-to-many association, each object of one class can be associated with multiple objects of another class. For example, a customer object may be associated with multiple order objects.
• Many-to-Many: In a many-to-many association, each object of one class can be associated with multiple objects of another class, and vice versa. For example, a student object may be associated with multiple course objects, and each course object may be associated with multiple student objects.

Associations are represented in code using references or pointers. For example, in Java, a one-to-one association can be represented using the following code:

class Student {
    private Transcript transcript;

    public Student(Transcript transcript) {
        this.transcript = transcript;
    }

    public Transcript getTranscript() {
        return transcript;
    }
}

In this example, the Student class has a reference to a Transcript object. This allows the Student object to access the data and methods of the Transcript object.

Multi-threading:

Multi-threading is a technique that allows a computer program to execute multiple tasks concurrently. This can improve the performance of a program by allowing it to take advantage of multiple processors or cores.

There are two main types of multi-threading:

• Kernel-Level Multi-threading: In kernel-level multi-threading, the operating system manages the creation and scheduling of threads. This is the most common type of multi-threading.
• User-Level Multi-threading: In user-level multi-threading, the application itself manages the creation and scheduling of threads. This type of multi-threading is less common, but it can provide better performance in some cases.

Multi-threading can be implemented in a variety of ways. One common approach is to use a thread pool. A thread pool is a collection of threads that are available to execute tasks. When a task is submitted to the thread pool, it is assigned to a thread in the pool. The thread then executes the task and returns the result.

Multi-threading can be a powerful tool for improving the performance of a computer program. However, it can also be complex to implement and debug.

Access Modifiers:

Access modifiers are keywords that control the visibility of classes, methods, and variables. There are four access modifiers in Java:

• Public: Public members are accessible from anywhere in the program.
• Protected: Protected members are accessible from within the same package and from subclasses.
• Default (package-private): Default members are accessible from within the same package.
• Private: Private members are accessible only from within the same class.

Access modifiers are used to control the encapsulation of data and behavior in a program. Encapsulation is the process of bundling data and behavior together into a single unit. This makes it easier to maintain and reuse code.

For example, the following Java code uses access modifiers to control the visibility of the name variable:

class Person {
    private String name;

    public Person(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

In this example, the name variable is declared as private. This means that it is only accessible from within the Person class. This helps to protect the data in the name variable from being accidentally modified or accessed from outside the class.