You can use getter and setter methods to read and update private fields. A getter returns a field’s value, while a setter changes it.

These methods let a class control how its data is accessed and updated.

In this example, you will create a Book class with a private title field. A setter will prevent the title from being null or blank.

Source code

public class GetterSetterExample {
    void main() {
        Book book = new Book("Learning Java");

        IO.println("Original title: " + book.getTitle());

        book.setTitle("Exploring Java");

        IO.println("Updated title: " + book.getTitle());
    }
}

class Book {
    private String title;

    Book(String title) {
        this.title = validateTitle(title);
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = validateTitle(title);
    }

    private static String validateTitle(String title) {
        if (title == null || title.isBlank()) {
            throw new IllegalArgumentException(
                    "Title must not be null or blank.");
        }

        return title;
    }
}

Explanation

The title field is private:

private String title;

Code in GetterSetterExample cannot access this field directly. For example, the following statement would cause a compilation error:

book.title = "Exploring Java";

Instead, the class provides methods for reading and updating the value.

Reading a value with a getter

The getTitle() method returns the current title:

public String getTitle() {
    return title;
}

The calling code uses it like this:

IO.println("Original title: " + book.getTitle());

A getter commonly uses the prefix get followed by the field name, such as getTitle().

Updating a value with a setter

The setTitle() method accepts a new title:

public void setTitle(String title) {
    this.title = validateTitle(title);
}

It validates the input before assigning it to the field. If validation fails, the assignment does not happen, and the existing title stays unchanged.

The calling code updates the title using:

book.setTitle("Exploring Java");

Validating the title

Both the constructor and setter call the same validation method:

private static String validateTitle(String title) {
    if (title == null || title.isBlank()) {
        throw new IllegalArgumentException(
                "Title must not be null or blank.");
    }

    return title;
}

The method rejects null, empty strings, and strings containing only whitespace.

The || operator evaluates the second condition only when the first condition is false. If title is null, Java skips title.isBlank(), avoiding a NullPointerException.

The validation method is private because it is an implementation detail. It is static because it does not use any object’s fields.

Run the example

Compile and run the file:

javac GetterSetterExample.java
java GetterSetterExample

Output

Original title: Learning Java
Updated title: Exploring Java

Private fields with controlled access are an example of encapsulation. Add setters only for values that callers should be allowed to change. A field that should remain fixed can be initialized through a constructor and exposed through a getter.