#010 Create Your Own Java Classes – 여행하는 개발자

2013년 8월 23일2016년 2월 4일 by nobaksan

Lab Exercises

Exercise 1: Create your own class

So far, you have dealt with a single class that contains main(..) method. In this exercise, you are going to create multiple classes among which one class invokes another class – more precisely, a method in a class invokes a method of another class.

Build and run an application that uses newly created StudentRecord class

(1.1) Build and run an application that uses newly created StudentRecord class

0. Start the NetBeans IDE if you have not done so.
1. Create a NetBeans project

Select File from top-level menu and select New Project.
Observe that the New Project dialog box appears.
Select General under Categories section and Java Application under Projects section.
Click Next.

Under Name and Location pane, for the Project Name field, enter MyStudentRecordExampleProject.
For the Create Main Class field, enter StudentRecordExample. (Figure-1.10 below)
Click Finish.

Figure-1.10: Create a new project

Observe that the MyStudentRecordExampleProject project node is created under Projects pane of the NetBeans IDE and IDE generated StudentRecordExample.java is displayed in the editor window of the IDE.

2. Write StudentRecord.java.

Right click MyStudentRecordExampleProject and select New->Java Class.

Observe that the Name and Location pane of the New Java Class dialog box appears.
For the Class Name field, enter StudentRecord. (Figure-1.11 below)
Click Finish.

Figure-1.11: Create StudentRecord.java

Observe that the IDE generated StudentRecord.java is displayed in the editor window.
Modify the IDE generated StudentRecord.java as shown in Code-1.12 below. Study the code by paying special attention to the bold-fonted comments.

public class StudentRecord {

/** Creates a new instance of StudentRecord */
public StudentRecord() {
}

// Declare instance variables.
private String name;
private double mathGrade;
private double englishGrade;
private double scienceGrade;

// Declare static variables.
private static int studentCount = 0;

/**
* Returns the name of the student
*/
public String getName(){
return name;
}

/**
* Changes the name of the student
*/
public void setName(String temp ){
name =temp;
}

/**
* Computes the average of the english, math and science
* grades
*/
public double getAverage(){
double result =0;
result =(getMathGrade()+getEnglishGrade()+getScienceGrade() )/3;
return result;
}

/**
* Returns the number of instances of StudentRecords
*/
public static int getStudentCount(){
return studentCount;
}

/**
* Increases the number of instances of StudentRecords.
* This is a static method.
*/
public static void increaseStudentCount(){
studentCount++;
}

// Instance methods
public double getMathGrade() {
return mathGrade;
}

public void setMathGrade(double mathGrade) {
this.mathGrade = mathGrade;
}

public double getEnglishGrade() {
return englishGrade;
}

public void setEnglishGrade(double englishGrade) {
this.englishGrade = englishGrade;
}

public double getScienceGrade() {
return scienceGrade;
}

public void setScienceGrade(double scienceGrade) {
this.scienceGrade = scienceGrade;
}
}

Code-1.12: StudentRecord.java

3. Modify the IDE generated StudentRecordExample.java as shown in Code-1.13.

Study the code while paying special attention to the bold-fonted comments.
Observe that StudentRecordExample class uses StudentRecord class.

public class StudentRecordExample {

public static void main(String[] args) {

// Create an object instance of StudentRecord class.
StudentRecord annaRecord =new StudentRecord();

// Increament the studentCount by invoking a static method.
StudentRecord.increaseStudentCount();

// Create another object instance of StudentRecord class.
StudentRecord beahRecord =new StudentRecord();

// Increament the studentCount by invoking a static method.
StudentRecord.increaseStudentCount();

// Create the 3rd object instance of StudentRecord class.
StudentRecord crisRecord =new StudentRecord();

// Increament the studentCount by invoking a static method.
StudentRecord.increaseStudentCount();

// Set the names of the students.
annaRecord.setName(“Anna”);
beahRecord.setName(“Beah”);
crisRecord.setName(“Cris”);

// Print anna’s name.
System.out.println(“Name = ” + annaRecord.getName());

// Print number of students.
System.out.println(“Student Count = “+StudentRecord.getStudentCount());
}

}

Code-1.13: StudentRecordExample.java

4. Build and run the program

Right click MyStudentRecordExampleProject and select Run.
Observe the result in the Output window of the NetBeans IDE. (Figure-1.12 below)

Name = Anna
Student Count = 3

Figure-1.12: Result of running the application

Solution: The solution to this exercise is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javacreateclass/samples/MyStudentRecordExampleProject. You can just open and run it.

5. (For your own exercise) Do make changes as following and build and run the application.

Modify StudentRecordExample.java as following:
- Create another StudentRecord object using new keyword, call it myOwnRecord
- Call setName() method of the myOwnRecord object passing “myOwn” as the value to set
- Display the name of the myOwnRecord object
- Set Math grade of myOwnRecord object
- Set English grade of myOwnRecord object
- Set Science grade of myOwnRecord object
- Display the average grade of myOwnRecord object

Summary

In this exercise, you have learned how to create and use your own class called StudentRecord class using new keyword.

Exercise 2: Build and run Java applications that use static (class) variables and static methods

When a number of objects are created from the same class blueprint, they each have their own distinct copies of instance variables. Class variables are referenced by the class name itself, <Class-name>.<static-variable>. For more information, please read Understanding Instance and Class Members section of the Java tutorial.

(2.1) Build and run an application that uses static variables and instance variables

1. Create a NetBeans project

Select File from top-level menu and select New Project.
Observe that the New Project dialog box appears.
Select General under Categories section and Java Application under Projects section.
Click Next.
Under Name and Location pane, for the Project Name field, enter MyStaticVariablesExampleProject.
For the Create Main Class field, enter StaticVariablesExample. (Figure-2.11 below)
Click Finish.

Figure-2.11:Create MyStaticVariablesProject project

Observe that the MyStaticVariablesExampleProject project node is created under Projects pane of the NetBeans IDE and IDE generated StaticVariablesExample.java is displayed in the editor window of the IDE.

2. Write Variables.java.

Right click MyStaticVariablesExampleProject and select New->Java Class.
Observe that the Name and Location pane of the New Java Class dialog box appears.
For the Class Name field, enter Variables. (Figure-2.12 below)
Click Finish.

Figure-2.12: Create Variables.java

Observe that the IDE generated Variables.java is displayed in the editor window.
Modify the IDE generated Variables.java as shown in Code-2.13 below. The modification is to create a couple of static variables and a couple of instance variables.

public class Variables {

// Static variables
static int staticintA = 10;
static String staticStringB =”I am static string”;

// Instance variables
int instanceintA = 20;
String instanceStringB = “I am instance string”;

}

Code-2.13: Variables.java

3. Modify StaticVariablesExamples.java to access static and instance variables of Variables class as shown in Code-2.14 below.

public class StaticVariablesExample {

public static void main(String[] args) {

// Access static variables of Variables class.
// Note that you don’t have to create an object instance
// of Variables class.
System.out.println(“Variables.staticintA = ” + Variables.staticintA);
System.out.println(“Variables.staticStringB = ” + Variables.staticStringB);
Variables.staticStringB = “Life is good!”;
System.out.println(“Variables.staticStringB = ” + Variables.staticStringB);

// Access instance variables of Variables class.
// Note that you have to create an object instance
// of Variables class before you access them.
Variables objectInstance1 = new Variables();
Variables objectInstance2 = new Variables();
objectInstance1.instanceintA = 1;
System.out.println(“objectInstance1.instanceintA = ” + objectInstance1.instanceintA);
objectInstance2.instanceintA = 3;
System.out.println(“objectInstance2.instanceintA = ” + objectInstance2.instanceintA);

// The static variable can be accessed from an object instance.
System.out.println(“objectInstance1.staticintA = ” + objectInstance1.staticintA);
objectInstance1.staticintA = 220;
System.out.println(“objectInstance1.staticintA = ” + objectInstance1.staticintA);
System.out.println(“Variables.staticintA = ” + Variables.staticintA);

// The static variable can be accessed from multiple object instances.
objectInstance2.staticintA = 550;
System.out.println(“objectInstance1.staticintA = ” + objectInstance1.staticintA);
System.out.println(“objectInstance2.staticintA = ” + objectInstance2.staticintA);
System.out.println(“Variables.staticintA = ” + Variables.staticintA);
}

}

Code-2.14: StaticVariablesExample.java

4. Build and run the program

Right click MyStaticVariablesExampleProject and select Run.
Observe the result in the Output window of the NetBeans IDE. (Figure-2.15 below)

Variables.staticintA = 10
Variables.staticStringB = I am static string
Variables.staticStringB = Life is good!
objectInstance1.instanceintA = 1
objectInstance2.instanceintA = 3
objectInstance1.staticintA = 10
objectInstance1.staticintA = 220
Variables.staticintA = 220
objectInstance1.staticintA = 550
objectInstance2.staticintA = 550
Variables.staticintA = 550

Figure-2.15: Result

5. Observe compile error when you try to access instance variable as if it is a static variable.

Add the code fragment of Code-2.16 below to the StaticVariablesExample.java.
Observe the compile error. (Figure-2.17 below)
Remove the code fragment you just added to remove the compile errors.

Variables.instanceintA = 3;

Code-2.16: Compile error

Figure-2.17: Compile error

Solution: The solutions to this exercise are provided as ready-to-open-and-run NetBeans projects as part of hands-on lab zip file. You can find them as <LAB_UNZIPPED_DIRECTORY>/javacreateclass/samples/MyStaticVariablesExampleProject and <LAB_UNZIPPED_DIRECTORY>/javacreateclass/samples/MyStaticMethodsExampleProject. You can just open and run them.

6. (For your own exercise) Modify StaticVariablesExample.java to display the addition of staticintA static variable and instanceintA instance variable.

(2.2) Build and run an application that uses static methods and non-static methods

0. Start the NetBeans IDE if you have not done so.
1. Create a NetBeans project

Select File from top-level menu and select New Project.
Observe that the New Project dialog box appears.
Select General under Categories section and Java Application under Projects section.
Click Next.
Under Name and Location pane, for the Project Name field, enter MyStaticMethodsExampleProject.
For the Create Main Class field, enter StaticMethodsExample.
Click Finish.

Observe that the MyStaticMethodsExampleProject project node is created under Projects pane of the NetBeans IDE and IDE generated StaticMethodsExample.java is displayed in the editor window of the IDE.

2. Write Methods.java.

Right click MyStaticMethodsExampleProject and select New->Java Class.
Observe that the Name and Location pane of the New Java Class dialog box appears.
For the Class Name field, enter Methods.
Click Finish.
Observe that the IDE generated Methods.java is displayed in the editor window.
Modify the IDE generated Methods.java as shown in Code-2.21 below. The modification is to create a couple of static and non-static methods.

public class Methods {

// Static variable
static int a = 0;

// Static method
static void staticMethod(int i) {
System.out.println(“staticMethod(“+ i +”) entered”);
}

// Anonymous static method. The things inside the anonymous
// static method get executed when the class is loaded.
static { //static block
System.out.println(“Anonymous static method entered, a = ” + a);
a += 1; // same thing as a = a + 1
System.out.println(“Anonymous static method exiting, a = ” + a);
}

// Non-static method
void myNonStaticMethods(int i){
System.out.println(“myNonStaticMethod(“+ i +”) entered”);
}
}

Code-2.21: Methods.java

3. Modify StaticMethodsExamples.java to invoke static and non-static methods as shown in Code-2.22 below.

public class StaticMethodsExample {

public static void main(String[] args) {

// Access a static variable of Demo class. Note that you don’t have to
// create an object instance of Demo class.
System.out.println(“Methods.a = ” + Methods.a);

// Invoke a static method of Methods class. Note that you don’t have to
// create an object instance of Methods class.
Methods.staticMethod(5);

// The static variable can be accessed from an object instance.
Methods d = new Methods();
System.out.println(“d.a = ” + d.a);

// The static method can be invoked from an object instance.
d.staticMethod(0);

// The same static variable can be accessed from multiple instances.
Methods e = new Methods();
System.out.println(“e.a = ” + e.a);
d.a += 3;
System.out.println(“Methods.a = ” + Methods.a);
System.out.println(“d.a = ” + d.a);
System.out.println(“e.a = ” + e.a);

// Compile error
// Methods.myNonStaticMethod(3);
}

}

Code-2.22: StaticMethodsExamples.java

4. Build and run the program

Right click MyStaticMethodsExampleProject and select Run.
Observe the result in the Output window of the NetBeans IDE. (Figure-2.23 below)

Anonymous static method entered, a = 0
Anonymous static method exiting, a = 1
Methods.a = 1
staticMethod(5) entered
d.a = 1
staticMethod(0) entered
e.a = 1
Methods.a = 4
d.a = 4
e.a = 4

Figure-2.23: Result

5. Observe compile error when you try to invoke non-static method as if it is a static method.

Add the code fragment of Code-2.24 below to the StaticMethodsExample.java.

Methods.myNonStaticMethod(3);

Code-2.24: Compile error expected

Observe the compile error. (Figure-2.25 below)
Remove the code fragment you just added to remove the compile errors.

Figure-2.25: Compile error

Summary

In this exercise, you have built and run Java applications that use static variables and static methods.

Exercise 3: Overloading

In this exercise, you are going to exercise the concept of overloading. Please note that overloading and overriding are two different concepts.

Add two overloaded methods to the StudentRecord class

(3.1) Use two overloaded methods to the StudentRecord class

1. Add two overloaded myprint() methods to the StudentRecored.java as shown in Code-3.11 below. The code fragments that need to be added are highlighted in bold and blue-colored font.

public class StudentRecord {

// Declare instance variables.
private String name;
private double mathGrade;
private double englishGrade;
private double scienceGrade;
private double average;

// Declare static variables.
private static int studentCount = 0;

/**
* Returns the name of the student
*/
public String getName(){
return name;
}

/**
* Changes the name of the student
*/
public void setName(String temp ){
name =temp;
}

/**
* Computes the average of the english,math and science
* grades
*/
public double getAverage(){
double result =0;
result =(getMathGrade()+getEnglishGrade()+getScienceGrade() )/3;
return result;
}

/**
* Returns the number of instances of StudentRecords
*/
public static int getStudentCount(){
return studentCount;
}

/**
* Increases the number of instances of StudentRecords
*/
public static void increaseStudentCount(){
studentCount++;
}

public double getMathGrade() {
return mathGrade;
}

public void setMathGrade(double mathGrade) {
this.mathGrade = mathGrade;
}

public double getEnglishGrade() {
return englishGrade;
}

public void setEnglishGrade(double englishGrade) {
this.englishGrade = englishGrade;
}

public double getScienceGrade() {
return scienceGrade;
}

public void setScienceGrade(double scienceGrade) {
this.scienceGrade = scienceGrade;
}

// Overloaded myprint(..) methods
public void myprint(){
System.out.println(“First overloaded method: Nothing is passed on”);
}

public void myprint(String name ){
System.out.println(“Second overloaded method: Name:”+name);
}

public void myprint(String name, double averageGrade){
System.out.print(“Third overloaded method: Name:”+name+” “);
System.out.println(“Average Grade:”+averageGrade);
}

}

Code-3.11: StaticAndInstanceMethods.java

3. Modify StudentRecoredExample.java as shown in Code-3.12 below. The code fragments that need to be added are highlighted in bold and blue-colored font.

public class StaticVariblesMethods {