Abstraction, Program Design and Its Impact：练习题

Consider the following class header for a class named Book. Which of the following is a valid constructor for this class if the class has private instance variables title (String) and pages (int)?

Which of the following best describes the purpose of a constructor in a Java class?

A class Student has an instance variable gpa. The constructor is defined as follows:

public Student(double gpa) {
gpa = gpa;
}

What happens when new Student(3.5) is called, assuming the instance variable is also named gpa?

A class Square is being designed. It needs to store the length of a side. Which of the following is the most appropriate way to declare the instance variable for side length to ensure encapsulation?

Consider the following class definition:

public class Runner {
private double distance;
public Runner(double d) {
distance = d;
}
public void addDistance(double d) {
distance += d;
}
}

If we execute the following code:
Runner r1 = new Runner(5.0);
r1.addDistance(2.5);
What is the value of distance in the object r1?

Part a: A software developer needs to create a Student class. Define a constructor that initializes a student's name (String) and their current GPA (double).
Part b: Write a method isEligible that returns true if the GPA is greater than or equal to 2.0, and false otherwise.
Part c: Explain why the instance variables should be declared as private and how the constructor helps in maintaining encapsulation.