How do subclasses set private properties by inheriting the setName method of parent class?

Mechanism for subclasses to inherit parent class methods to access private attributes

The inheritance mechanism in Java, especially how subclasses access the private properties of the parent class through the parent class method, is often confusing. This article will explore this issue in depth and illustrate it with a specific example.

Scene description

Suppose we have a parent class Employee and a subclass Manager :

Parent Employee :

 public abstract class Employee {
    private String name;

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public abstract void showMsg();
}

Subclass Manager :

 public class Manager extends Employee {
    private Clerk clerk;

    public Manager() {}
    public Manager(Clerk clerk) { this.clerk = clerk; }

    @Override
    public void showMsg() { System.out.println(this.getName()); }
}

In the test code, we can observe that even if name attribute is declared private in Employee class, Manager instance can still set and print the value of the attribute through the setName method.

answer

The key lies in Java's access control and object polymorphism.

• Encapsulation of private attributes: The private keyword ensures that name attribute can only be accessed by methods inside Employee class. Subclass Manager cannot access name directly.

• Method inheritance: Manager inherits the setName method of Employee . This does not mean that Manager has gained direct access to name attribute.

• Polymorphism: When we create a Manager object and call the setName method, although the method on Manager object is called, the setName method defined in Employee class is actually executed. Because the setName method is defined in the Employee class, it has permission to access private members of Employee class (including name ).

Therefore, Manager does not directly access name , but indirectly operates name attribute through the inherited setName method. This reflects the encapsulation and polymorphism of Java: encapsulation protects the integrity of the data, and polymorphism allows subclasses to indirectly operate private members of the parent class through the parent class's methods. This does not break the encapsulation, but takes advantage of the features of object-oriented programming.

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