# Lab 11
## Exercise 1
You are given the Java classes `Circle`, `Rectangle` and `Test` below.
```java
public class Circle {
private double radius;
public Circle(double r) {
radius = r;
}
public double area() {
return radius * radius * Math.PI;
}
}
public class Rectangle {
private double length;
private double width;
public Rectangle(double l, double w) {
length = l;
width = w;
}
public double area() {
return length * width;
}
}
public class Test {
public static void main(String[] args) {
Rectangle r = new Rectangle(30.1, 10.2);
Circle c = new Circle(5.3);
System.out.println("r=" + r);
System.out.println("c=" + c);
}
}
```
Execute `Test` and write down the output.
```
r=Rectangle@19e1023e
c=Circle@3a4afd8d
# RAM address
```
## Exercise 2
You are given the Java classes `Test2` below. Execute the program together with the `Rectangle` class in Exercise 1.
```java
public class Test2 {
public static void main(String[] args) {
Rectangle r = new Rectangle(1, 2);
System.out.println("r=" + r);
System.out.println("area=" + r.area());
r = new Rectangle(3, 4);
System.out.println("r=" + r);
System.out.println("area=" + r.area());
r = null;
System.out.println("r=" + r);
System.out.println("area=" + r.area());
}
}
```
Execute `Test2` and write down the output. Can you explain the output?
```
r=Rectangle@27ddd392
area=2.0
r=Rectangle@2a098129
area=12.0
r=null
Exception in thread "main" java.lang.NullPointerException
```
## Exercise 3
You are given the Java classes `Triangle` below.
```java
public class Triangle {
private int base;
private int height;
public Triangle() {
base = 0;
height = 0;
}
public Triangle(int base, int height) {
this.base = base;
this.height = height;
}
public void printThis() {
System.out.println(this);
}
public static void main(String[] args) {
Triangle t = new Triangle(1, 2);
System.out.println(t);
t.printThis();
}
}
```
Execute `Triangle` and write down the output. Can you explain the output? Discuss what `this` is in an object instance.
```
Triangle@6504e3b2
Triangle@6504e3b2
# RAM Address
```
## Exercise 4
You are given the Java classes Date and DateUser below.
```java
public class Date {
private int day;
private int month;
private int year;
public Date() {
this(1, 1, 1970);
}
public Date(int day, int month, int year) {
this.day = day;
this.month = month;
this.year = year;
}
public String toString() {
return "[" + day + "/" + month + "/" + year + "]";
}
}
public class DateUser {
public static void main(String[] args) {
Date d1 = new Date();
System.out.println(d1);
}
}
```
(a) Execute `DateUser` and observer the output. Can you explain the purpose of the program statement “`this(1, 1, 1970)`” in the no-argument constructor of `Date`?
(b) Remove the `toString()` method from the `Date` class. Compile and execute the program again. What is the difference?
```
# (a)
[1/1/1970]
# (b)
Date@28a418fc
# RAM Address
```
## Exercise 5
Consider the following class.
```java
public class QuestionOne {
public final int A = 345;
public int b;
private float c;
private void methodOne(int a) {
b = a;
}
public float methodTwo() {
return 23;
}
}
```
Identify the invalid statements in the program below. For each invalid statement, state why it is invalid.
```java
public class Q1Main {
public static void main(String[] args) {
QuestionOne q1 = new QuestionOne(); //line1
q1.A = 12; //line2
q1.b = 12; //line3
q1.c = 12; //line4
q1.methodOne(12); //line5
q1.methodOne(); //line6
System.out.println(q1.methodTwo()); //line7
q1.b = q1.methodTwo(); //line8
}
}
```
```
Q1Main.java:17: cannot assign a value to final variable A
q1.A = 12;
^
Q1Main.java:19: c has private access in QuestionOne
q1.c = 12;
^
Q1Main.java:21: methodOne(int) has private access in QuestionOne
q1.methodOne(12);
^
Q1Main.java:22: methodOne(int) in QuestionOne cannot be applied to ()
q1.methodOne();
^
Q1Main.java:24: possible loss of precision
found : float
required: int
q1.b = q1.methodTwo();
^
5 errors
Tool completed with exit code 1
```
## Exercise 6
Suppose that the class `Foo` is defined as below.
```java
public class Foo {
public int i;
public static String s;
public void imethod() { }
public static void smethod() { }
}
```
Let f be an instance of `Foo`. Determine if each of the following Java statements is valid or invalid.
| Code | valid | invalid |
| ---- | ----- | ------- |
| System.out.println(f.s); | [x] | [] |
| f.smethod(); | [x] | [] |
| System.out.println(Foo.i); | [] | [x] |
| System.out.println(Foo.s); | [x] | [] |
| Foo.imethod(); | [] | [x] |
| Foo.smethod(); | [x] | [] |
## Exercise 7
A `Point` object instance represents a Cartesian coordinate and thus has two member variables, x and y. The Java program `Distance` below instantiates two `Point` object instances to represent the coordinates (4, 5) and (11, 4). The program then displays the distance between the two coordinates by invoking the `distance()` method in the `distance() `method.
Expression to get the distance of two Points (x1, y1) (x2, y2) is `distance = Math.sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2));`
```
C:\> java Ex7
Distance = 7.0710678118654755
```
```java
public class Ex7 {
public static void main(String[] args) {
Point p1 = new Point(4, 5);
Point p2 = new Point(11, 4);
System.out.println("Distance = " + p1.distance(p2));
}
}
```
Complete the `Point` class.
```java
public class Point {
private int x;
private int y;
public Point() {
this(0, 0);
}
public Point(int x, int y) {
setPoint(x, y);
}
public void setPoint(int x, int y) {
if (x >= 0 && y >= 0) {
this.x = x;
this.y = y;
}
}
public int getX() {
return x;
}
public int getY() {
return y;
}
public double distance(Point p) {
// COMPLETE THIS METHOD
}
}
```
## Exercise 8
Study the validity of the programs below. Correct those invalid statements.
```java
public class Student {
public static int numberOfStudent = 0; //line 1
public String name; //line 2
public void Student(String name) { //line 3
numberOfStudent++; //line 4
setName(name); //line 5
}
public int getNumberOfStudent() { //line 6
return; //line 7
}
public String setName(String name) { //line 8
name = name; //line 9
}
}
public class Ex8 {
public static void main(String args[]) {
Student stu1 = new Student("John Chan"); //line 10
System.out.println(numberOfStudent); //line 11
System.out.println(name); //line 12
}
}
```
```
C:\> java Ex8
1
John Chan
```