Unit 5: Writing Classes

Question 1

What are the “big three”?

Answer 1

1. The instance variables
2. Accessor and Mutator methods
3. A toString method

Question 2

Instance Variables

Answer 2

Private variables that are used

Question 3

Accessor Methods

Answer 3

An instance method that gets the value of a property of an object

Question 4

Mutator Methods

Answer 4

An instance method that sets the value of a property of an object

Question 5

What differentiates private and public variables?

Answer 5

A private variable can only be directly accessed from within the class. A public variable can be accessed from outside the class.

Question 6

Static Variables

Answer 6

private static type name;
or,
private static type name = value;
– Example:
private static int theAnswer = 42;
static variable: Stored in the class instead of each object.
– A "shared" global field that all objects can access and modify.
– Like a class constant, except that its value can be changed.

Question 7

Final Static fields

Answer 7

public static final type name;
or,
public static final type name = value;
– Example:
public static final int NUMOFMONTHS = 12;
Final static variable:
– A class constant whose value cannot be changed. Usually public.
– ALL CAPS by convention.

Question 8

Instance Variables

Answer 8

private type name;
or,
private type name = value;
– Example:
private int id = 243342;
instance variable: Stored in an object instead of the class.
– each object has its own copy of the instance variable.

Question 9

Static vs Instance Call

Answer 9

A static method is called through the name of the class.
An instance method is called through the name of an object. 
public class Main {
public static void main(String[] args) {
BankAccount a = new BankAccount("Jim Smith");
//getID is instance
// uses object name + dot notation to call
System.out.println(a.getID());
//getNumAccounts is static
// uses class name + dot notation to call
System.out.println(BankAccount.getNumAccounts());
       }
}

Question 10

What is the difference between an interface and an abstract class?

(A) There is no difference.

(B) Abstract classes can have methods with bodies (code in them), but interfaces can not.

(C) Abstract classes can be instantiated, while interfaces can not.

(D) Abstract classes can be extended, but interfaces can not.

(E) Abstract classes can declare abstract methods, but interfaces can not.

Answer 10

(B) Abstract classes can have methods with bodies (code in them), but interfaces can not.

Question 11

Scope

Answer 11

• scope: The part of a program where a variable exists.
– From its declaration to the end of the { } braces
• A variable declared in a for loop exists only in that loop.
• A variable declared in a method exists only in that method.
public static void example() {
int x = 3;
for (int i = 1; i <= 10; i++) {
System.out.println(x);
}
// i no longer exists here
} // x ceases to exist here

Question 12

The “this.” keyword

Answer 12

“this.” : Within a non-static method or a constructor, the keyword
this is a reference to the current object—the object whose
method or constructor is being called.
– Refer to a field: this.field
– Call a method: this.method(parameters);
– One constructor this(parameters);
can call another:

Question 13

Variable shadowing

Answer 13

• shadowing: 2 variables with same name in same scope.
– Normally illegal, except when one variable is a field.
public class Point {
private int x;
private int y;
...
public Point(int x, int y) {
}
– In most of the class, x and y refer to the instance variables.
– In the constructor, x and y refer to the method's parameters.
The instance 
variables x and y 
are global 
variables.

The parameter
variables x and y
are local variables

Question 14

Fixing shadowing

Answer 14

"this" can be omitted if it is clear which variable is being referenced. The keyword "this" is helpful to fix the shadowing 
problem. 
public class Point {
private int x;
private int y;
public Point(int x, int y) {
this.x = x;
this.y = y;
}
}
Inside the constructor(or any method with shadowing):
– To refer to the data field x, say this.x
– To refer to the parameter x, say x

Question 15

Constructors

Answer 15

One constructor can call another using the this keyword. This 
helps us reuse another constructor's code.
public class Point {
private int x;
private int y;
public Point(int x, int y) {
this.x = x;
this.y = y;
}
public Point(){
this(0, 0);
     }
}

Question 16

Point Revisited

Answer 16

public class Point {
private int x;
private int y;
public Point(){
this(0, 0);
}
public Point(int x, int y){
this.x = x;
this.y = y;
}
public double distanceToPoint(Point other){
int dx = this.x – other.x;
int dy = this.y – other.y;
return Math.sqrt(dx * dx + dy * dy);
}
public double distanceToOrigin(){
Point origin = new Point();
return this.distanceToPoint(origin);
} }
}

Question 17

A “swap” method?

Answer 17

• Does the following swap method work? Why or why not?
public static void main(String[] args) {
int a = 7;
int b = 35;
// swap a with b?
swap(a, b);
System.out.println(a + " " + b);
// 7 35 (unchanged)
}
public static void swap(int a, int b) {
int temp = a;
a = b;
b = temp;
}

Question 18

1. Which statement about parameters is false?
   (A) The scope of parameters is the method in which they are defined.
   (B) Static methods have no implicit parameter this.
   (C) Two overloaded methods in the same class must have parameters with different names.
   (D) All parameters in Java are passed by value.
   (E) Two different constructors in a given class can have the same number of
   parameters.

Answer 18

(C) Two overloaded methods in the same class must have parameters with different names.

Question 19

2. Which of the following will cause an error message?
I Date d1 = new Date(8, 2, 1947);
Date d2 = d1;
II Date d1 = null;
Date d2 = d1;
III Date d = null;
int x = d.year();
(A) I only
(B) II only
(C) III only
(D) II and III only
(E) I, II, and III

Answer 19

(C) III only

Question 20

3. A client program creates a Date object as follows:
Date d = new Date(1, 13, 2002);
Which of the following subsequent code segments will cause an error?
(A) String s = d.toString();
(B) int x = d.day();
(C) Date e = d;
(D) Date e = new Date(1, 13, 2002);
(E) int y = d.year;

Answer 20

(E) int y = d.year;

Question 21

4. Which of the following correctly constructs a Date object in a client class?
   (A) Date d = new (2, 13, 1947);

(B) Date d = new Date(2, 13, 1947);

(C) Date d;
d = new (2, 13, 1947);

(D) Date d;
d = Date(2, 13, 1947);

(E) Date d = Date(2, 13, 1947);

Answer 21

(B) Date d = new Date(2, 13, 1947);