Check Your Understanding

Defining Classes

What is encapsulation? Why is it useful?
Instance variables are a part of the hidden implementation of a class, but they aren’t actually hidden from programmers who have the source code of the class. Explain to what extent the private reserved word provides information hiding.
Consider a class Grade that represents a letter grade, such as A+ or B. Give two choices of instance variables that can be used for implementing the Grade class.
Consider a class Time that represents a point in time, such as 9 a.m. or 3:30 p.m. Give two different sets of instance variables that can be used for implementing the Time class.
Show that the BankAccount(double initialBalance) constructor is not strictly necessary. That is, if we removed that constructor from the public interface, how could a programmer still obtain BankAccount objects with an arbitrary balance?
Conversely, could we keep only the BankAccount(double initialBalance) constructor and remove the BankAccount() constructor?
What is the this reference? Why would you use it?
Which of the methods in the CashRegister class are accessor methods? Which are mutator methods?
Write declarations for storing the following quantities. Choose between integers and floating-point numbers. Declare constants when appropriate.
1. The number of days per week
2. The number of days until the end of the semester
3. The number of centimeters in an inch
4. The height of the tallest person in your class, in centimeters

What is the value of mystery after this sequence of statements?

int mystery = 1;  
mystery = 1 - 2 * mystery;  
mystery = mystery + 1;

What is wrong with the following sequence of statements?

int mystery = 1;
mystery = mystery + 1; 
int mystery = 1 - 2 * mystery; 

Write the following Java expressions in mathematical notation.
1. dm = m * (Math.sqrt(1 + v / c) / Math.sqrt(1 - v / c) - 1);
2. volume = Math.PI * r * r * h;
3. volume = 4 * Math.PI * Math.pow(r, 3) / 3;
4. z = Math.sqrt(x * x + y * y);
What are the values of the following expressions? In each line, assume that
```
double x = 2.5;
double y = -1.5;  
int m = 18;
int n = 4; 
```
1. x + n * y - (x + n) * y
2. m / n + m % n
3. 5 * x - n / 5
4. 1 - (1 - (1 - (1 - (1 - n))))
5. Math.sqrt(Math.sqrt(n))
What are the values of the following expressions, assuming that n is 17 and m is 18?
1. n / 10 + n % 10
2. n % 2 + m % 2
3. (m + n) / 2
4. (m + n) / 2.0
5. (int) (0.5 * (m + n))
6. (int) Math.round(0.5 * (m + n))
What are the values of the following expressions? In each line, assume that
```
String s = "Hello";
String t = "World"; 
```
1. s.length() + t.length()
2. s.substring(1, 2)
3. s.substring(s.length() / 2, s.length())
4. s + t
5. t + s
Given a string s, write expressions for:
• The string consisting of the first letter
• The string consisting of the first and last letter
• The string consisting of all but the first and last letter
• The first half of the string (discarding the middle letter if the string has odd length) • The second half of the string (discarding the middle letter if the string has odd length) • The string consisting of the middle letter if the string has odd length, or the middle two letters otherwise

Find at least five compile-time errors in the following program.

public class HasErrors
{
     public static void main();
     {
        System.out.print(Please enter two numbers:)  
        x = in.readDouble;
        y = in.readDouble; 
        System.out.printline("The sum is " + x + y);
     }
}

Find three run-time errors in the following program.

public class HasErrors
{
     public static void main(String[] args) {
        Scanner in = new Scanner("System.in"); 
        System.out.print("Please enter an integer:"); 
        x = in.readInt();
        System.out.print("Please enter another integer: "); 
        x = in.readInt();
        System.out.println("The sum is " + x + y);
        int x = 0; int y = 0;
     }
}

Consider the following code:

CashRegister register = new CashRegister(); 
register.recordPurchase(19.93);
register.receivePayment(20, 0, 0, 0, 0); 
System.out.print("Change: ");
System.out.println(register.giveChange()); 

The code segment prints the total as 0.07000000000000028. Explain why. Give a recommendation to improve the code so that users will not be confused.

Explain the differences between 2, 2.0, ‘2’, “2”, and “2.0”.
Explain what each of the following program segments computes.
1. x = 2;
  y = x + x;
2. s = "2";
  t = s + s;