In Java classes, how can we set the value of a private data member safely?

Introduction / Context:
Encapsulation is a key concept in object oriented programming, and Java enforces it through access modifiers such as private. When a field is declared private, it cannot be accessed directly from outside the class. Instead, classes typically expose controlled ways to read and write these fields. This question focuses on the correct technique to safely set the value of a private data member from outside the class without breaking encapsulation or access rules.

Given Data / Assumptions:
- We have a Java class with one or more fields declared with the private modifier.
- Other classes in the same or different package need to change the value of that private field.
- We want to preserve encapsulation, validation and control over how values are assigned.
- We are not using reflection or advanced frameworks, only standard Java language features.

Concept / Approach:
Private fields are visible only inside the class that declares them. To allow controlled modification from outside, we typically provide a setter method, often public or sometimes protected, that takes a parameter, performs validation if needed and then assigns the value to the private field. This approach hides the internal representation and enables future changes to validation logic without affecting callers. Direct access from other classes, global variables or static tricks all violate the idea of encapsulation and are not supported by the language for private members.

Step-by-Step Solution:
Step 1: Recall that the private modifier restricts visibility of a field to the class that defines it. Step 2: Recognize that code in other classes cannot legally reference that private field by name. Step 3: Understand that the idiomatic Java solution is to define a setter method, for example setAge(int age), which is declared public or protected. Step 4: Inside the setter, we assign the incoming parameter value to the private field, possibly after validation or transformation. Step 5: Compare this with the options and select the one that accurately describes using a setter method to assign the value inside the class.

Verification / Alternative check:
To verify, imagine a class Person with private int age. If another class wants to change age, the only legal and clean approach is something like person.setAge(25). Any attempt such as person.age = 25 from outside the Person class would cause a compilation error because age is private. Making the field static does not change its access level. Therefore, the only option that matches Java syntax and encapsulation principles is the one that mentions a setter method that assigns the value inside the class body.

Why Other Options Are Wrong:
Option B suggests accessing the private field directly from any other class, which is not allowed by Java and will not compile.
Option C claims that declaring the field static allows modification from anywhere, but static changes scope across instances, not visibility; access is still restricted by private.
Option D mentions a global variable pointing at the private field, which is not a supported or safe mechanism in Java and breaks the idea of encapsulation.

Common Pitfalls:
A common pitfall is to relax encapsulation by making fields public instead of defining proper getters and setters. Another mistake is to assume that being in the same package allows direct access to private fields, which is not true. Some beginners also confuse static with public access. Following the getter setter pattern keeps the class flexible, allows validation and logging and prevents unexpected changes to the internal state from outside code.

Final Answer:
The correct way is by providing a public or protected setter method that assigns the value to the private field inside the class.