What concept does the following Java code illustrate?

class Book {
    private String title;
    private double price;
    public Book(String title, double price) {
        this.title = title;
        this.price = price;
    }
    public String getTitle() {
        return this.title;
    }
    public void setPrice(double price) {
        if(price > 0) {
            this.price = price;
        }
    }
}