Description

Lab 7
[Objective]
• Learn how to define a Java class and create its object
• Learn how to define and use instance variables
• Learn how to define and use instance methods
• Learn how to use get and set methods
• Learn how to use ArrayList and make the object as its element.

[Before Exercises]

Step1: How to define a circle on a 2D plane?
A circle has three attributes including the radius, the x coordinate and the y coordinate.
We can define a class named Circle, in which there are three private attributes.

public class Circle { private double radius; private double x; private double y;
}
Step2: Define the methods of a circle.
Define three public methods for computing the area, perimeter and print position of the circle.

public class Circle { private double radius; private double x; private double y;

public double area() { return radius*radius*Math.PI;
}
public double perimeter () {
return 2*Math.PI*radius;
}
public void position() {
System.out.printf(“Position of the cricle is
(%.1f,%.1f) “,x,y);
}
}
Step3: How to use the class Circle?
Create another class named CircleTest in the same package, in which there is a main method to be used.
In the main method, we can create an object of Circle by using the statement as follows:
Circle c1=new Circle( );
After that, we want to know the perimeter, area and position about the c1, so we need to invoke the method of c1.
public class CircleTest {
public static void main(String[] args) {
Circle c1=new Circle();
System.out.printf(“The area of c1 is %.2f “, c1.area());
System.out.printf(“The perimeter of c1
is %.2f “, c1.perimeter());
c1.position();
}
}
When we run the program, the result would as follows:

Step4: Set and get the values of the attributes

If we set or get the radius of a circle object in main method directly, it would lead to an error because of its private privilege.

We can define several public methods in class Circle for getting or setting the class variables, and we can check the validity of input value in the set method.

public class Circle { private double radius; private double x; private double y;

public double area() { return radius*radius*Math.PI;
}
public double perimeter () {
return 2*Math.PI*radius;
}
public void position() {
System.out.printf(“Position of the cricle is
(%.1f,%.1f) “,x,y);
}
public double getRadius() { return radius;
}
public void setRadius(double radius) { if (radius > 0) {
this.radius = radius;
}
}
public double getX() {
return x;
}
public void setX(double x) {
this.x = x;
}
public double getY() {
return y;
}
public void setY(double y) { this.y = y;
}
}

Sample output:

Step5: How to manage multiple circle objects ?
We can use an array or an ArrayList to manage them.
In the main method, create an arrayList with a Circle type, to store many objects of Circle. Add the following code at the end of main method.

ArrayList<Circle> circleList=new ArrayList<Circle>();
circleList.add(c1);
System.out.printf(“Radius of %d circle is %.2f: “,1,circleList.get(0).getRadius());
Sample output:

Step5: Add more circles in the ArrayList.
Add the following code at the end of main method.

for(int i=1;i<5;i++) {
circleList circleList circleList circleList
.add(new Circle());
.get(i).setRadius(i);
.get(i).setX(Math.random()*5);
.get(i).setY(Math.random()*5);
}

System.out.println(“—Begin to print the
circle list—“);
for(int i=0;i<5;i++) {
System.out.printf(“The area of %d circle is %.2f “,
i+1, circleList.get(i).area()); System.out.printf(“The perimeter
is %.2f “,
circleList.get(i).perimeter());
}

[Exercises]

1. Declare a class named User. The class contains:
a. Private data fields:
i. String account;
ii. String password; iii. double money;
b. Implement a public method named introduce() to print the user name and his account balance.
c. Implement a public method expense(double value,Scanner in). It withdraws the money from the user account if the password is correct.
d. Implement a public method income(double value). It deposits the money to the user account.
e. Implement the getter and setter methods for each private field of the class User.
In the same package, we create a class named UserTest, which has a main method.

Statements in main method:
User user =new User();
Scanner in = new Scanner(System.in);
user.setUser(“Lucy”); user.setPassword(“123456”); user.setMoney(1000); user.introduce(); user.expense(2000,in); user.expense(500,in); user.income(1000); user.introduce(); in.close();

Sample output:

2. Design a class named Food. The class contains:
a. Private data fields:
i. int id；
ii. String name; iii. String type; iv. int size;
v. double price;
b. Implement a public method named getMenu() to
print all the information of this food object.
c. Implement the getter and setter method for each
private field of Food.

In FoodTest class, create four objects of Food as follows:

Object Name id name type size price
pizza1 1 pizza Seafood 11 12
pizza2 2 pizza Beef 9 10
Fried rice 3 fried
rice Seafood 5 12
Noodles 4 noodles Beef 6 14

Create an ArrayList<Food> to add those four Food objects, and then show the information of them as follows by iterating the ArrayList<Food> we created.

Sample output:

3. Design a class named softOpening. The class contains no data fields but:
a. Implement a public static method named
generateMenu() to generate 4 object of Food and add them to the ArrayList<Food>.
b. Implement a public static method named
getMenu(ArrayList<Food>) to print the items in the ArrayList<Food> as designed.
c. Implement a public static method named to
generateUser(Scanner in) to generated a user whose account and money is get by using the Scanner object ‘in’.
d. Implement a public static method named UserConsume(ArrayList<Food>,User user,Scanner in) to invoke the getMenu, ask user to select the foods in the
Menu, count the cost and invoke the expense of the user.
e. Invoke the method introduce() of the User
object to show his/hers balance.
Statements in main method:
Scanner in = new Scanner(System.in);
ArrayList<Food> foodList = genarateMenu(); //generate a Menu User user = genarateUser(in); //generate a user
user.introduce(); //show the account of the user userConsume(foodList,user,in); //user consume
user.introduce(); //show the account of the user in.close();

Sample output: