Fields address and phone are public. Why are public fields disliked in Java? What really changes when switching to private + getter/setter?

In Java, public fields are not recommended because they violate the principle of encapsulation — one of the fundamentals of object-oriented programming. When using public fields:

• There is no control over how and when data is changed.
• It is impossible to add logic when reading or writing (e.g., validation).
• It is difficult to change the internal implementation of a class without affecting external code.

Switching to private fields with getter/setter allows:

• Controlling access to fields (e.g., making a field read-only).
• Adding additional logic when getting or setting a value (validation, transformation).
• Maintaining compatibility when changing the internal structure of the class.

Example:

public class User {
    private String phone;

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        if (phone != null && phone.matches("\\+\\d{11}")) {
            this.phone = phone;
        } else {
            throw new IllegalArgumentException("Invalid phone number");
        }
    }
}

This approach makes the code more reliable and maintainable.