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