Java Basics 108 – Class and Object
STEPS
1) Create New Project.
Project Name: exams
Main Class: exams.ExamDetails
New Project created.
Make a copy of Startup Codes
package exams; public class ExamDetails { public static void main(String[] args) { // TODO code application logic here } } |
2) CREATE JAVA CLASS
2-1) Delete all original codes and paste the Startup Codes above.
package exams; public class ExamDetails { public static void main (String[] args){ // TODO code application logic here } } |
2-2) Add second class StudentResults
package exams; public class ExamDetails { public static void main(String[] args) { // TODO code application logic here } } class StudentResults { } |
2-3) Add properties to the StudentResults class.
package exams; public class ExamDetails { public static void main(String[] args) { // TODO code application logic here } } class StudentResults { String Full_Name; String Exam_Name; String Exam_Score; String Exam_Grade; } |
2-4) Declare a StudentResults object called aStudent
package exams; public class ExamDetails { public static void main(String[] args) { StudentResults aStudent = new StudentResults(); } } class StudentResults { String Full_Name; String Exam_Name; String Exam_Score; String Exam_Grade; } |
2-5) Assign property value to aStudent
Try printing the value.
package exams; public class ExamDetails { public static void main(String[] args) { StudentResults aStudent = new StudentResults(); aStudent.Exam_Name = "VB.NET"; String exam = aStudent.Exam_Name; System.out.println(exam); } } class StudentResults { String Full_Name; String Exam_Name; String Exam_Score; String Exam_Grade; } |
2-6) Change access to class property in StudentResults.
Try running the code.
package exams; public class ExamDetails { public static void main(String[] args) { StudentResults aStudent= new StudentResults(); aStudent.Exam_Name="VB.NET"; String exam=aStudent.Exam_Name; System.out.println(exam); } } class StudentResults { private String Full_Name; private String Exam_Name; private String Exam_Score; private String Exam_Grade; } |
At this point, the compiler will output error message:
Comment (//) those lines that have got errors.
2-7) Add an initiating method (Constructor ) to StudentResults.
a) The name of the method is similar to the name of the class.
b) The parameters of the method are assigned to the property of the class.
package exams; public class ExamDetails { public static void main(String[] args) { StudentResults aStudent = new StudentResults(); //aStudent.Exam_Name = "VB.NET"; //String exam = aStudent.Exam_Name; //System.out.println(exam); } } class StudentResults { private String Full_Name; private String Exam_Name; private String Exam_Score; private String Exam_Grade; StudentResults(String name, String grade) { Full_Name = name; Exam_Grade = grade; } } |
At this point, if you try to compile, you may get errors because the new object declaration parameter doesn’t match with the constructor parameter declaration.
2-8) Change the constructor declaration.
a) Omitting parameter
b) Assigning default values to all class properties.
package exams; public class ExamDetails { public static void main(String[] args) { StudentResults aStudent = new StudentResults(); } } class StudentResults { private String Full_Name; private String Exam_Name; private String Exam_Score; private String Exam_Grade; StudentResults() { Full_Name="No name given"; Exam_Name="Unknown"; Exam_Score="No Score"; Exam_Grade="Unknown"; } } |
Note: delete the statement marked with “//”
2-9) Create method to change class property.
package exams; public class ExamDetails { public static void main(String[] args) { StudentResults aStudent = new StudentResults(); } } class StudentResults { private String Full_Name; private String Exam_Name; private String Exam_Score; private String Exam_Grade; StudentResults() { Full_Name = "No name given"; Exam_Name = "Unknown"; Exam_Score = "No Score"; Exam_Grade = "Unknown"; } String fullName(String aName) { Full_Name = aName; return Full_Name; } } |
2-10) Edit the main method.
a) Add a call to class method.
b) Add an output statement to print the class property value.
package exams; public class ExamDetails { public static void main(String[] args) { StudentResults aStudent = new StudentResults(); String sName = aStudent.fullName("Bill Gates"); System.out.println(sName); } } class StudentResults { private String Full_Name; private String Exam_Name; private String Exam_Score; private String Exam_Grade; StudentResults() { Full_Name = "No name given"; Exam_Name = "Unknown"; Exam_Score = "No Score"; Exam_Grade = "Unknown"; } String fullName(String aName) { Full_Name = aName; return Full_Name; } } |
2-11) Add another method to class StudentResults
package exams; public class ExamDetails { public static void main(String[] args) { StudentResults aStudent = new StudentResults(); String sName = aStudent.fullName("Bill Gates"); System.out.println(sName); } } class StudentResults { private String Full_Name; private String Exam_Name; private String Exam_Score; private String Exam_Grade; StudentResults() { Full_Name = "No name given"; Exam_Name = "Unknown"; Exam_Score = "No Score"; Exam_Grade = "Unknown"; } String fullName(String aName) { Full_Name = aName; return Full_Name; } String examName(String examCode){ if (examCode.equals("VB")){ Exam_Name="Visual Basic.NET"; } else if (examCode.equals("JV")){ Exam_Name="Java"; } else if (examCode.equals("C#")){ Exam_Name="C# .NET"; } else if (examCode.equals("PH")){ Exam_Name="PHP"; } else { Exam_Name="No Exam Selected"; } return Exam_Name; } } |
2-12) Edit main method
a) Add a call to the new class method.
b) Add an output statement to print the returning value.
package exams; public class ExamDetails { public static void main(String[] args) { StudentResults aStudent = new StudentResults(); String sName = aStudent.fullName("Bill Gates"); String exam = aStudent.examName("VB"); System.out.println(sName); System.out.println(exam); } } class StudentResults { private String Full_Name; private String Exam_Name; private String Exam_Score; private String Exam_Grade; StudentResults() { Full_Name = "No name given"; Exam_Name = "Unknown"; Exam_Score = "No Score"; Exam_Grade = "Unknown"; } String fullName(String aName) { Full_Name = aName; return Full_Name; } String examName(String examCode) { if (examCode.equals("VB")) { Exam_Name = "Visual Basic.NET"; } else if (examCode.equals("JV")) { Exam_Name = "Java"; } else if (examCode.equals("C#")) { Exam_Name = "C# .NET"; } else if (examCode.equals("PH")) { Exam_Name = "PHP"; } else { Exam_Name = "No Exam Selected"; } return Exam_Name; } } |
3) More Class Methods
Continue at http://www.homeandlearn.co.uk/java/more_class_methods.html
REFERENCE
http://www.homeandlearn.co.uk/java/write_your_own_java_classes.html
http://www.homeandlearn.co.uk/java/field_variables.html
http://www.homeandlearn.co.uk/java/class_constructor.html
http://www.homeandlearn.co.uk/java/accessing_class_variables.html