Develop Reference

Why can I chain setters with a class with builder class inside but cannot on a instance of a normal class?

I've got introduced to builder pattern while learning java and I can't understand how builder can chain the setter methods.
//this one compiles
new AlertDialog.Builder().setApplyButton("apply").setText("stop");
//this one doesn't compile
new NormalAlert().setApplyButton("apply").setText("stop");

Builder methods return this (the builder instance itself) so that more methods can be called on it. Generally, the method named build is defined to return the constructed object.
Example:
public class Person {
private String name;
private Integer age;
public static class Builder {
private String name;
private Integer age;
public Builder name(String name){
this.name = name;
return this;
}
public Builder age(Integer age){
this.age = age;
return this;
}
public Person build() {
return new Person(this);
}
}
private Person(Builder builder) {
this.name = builder.name;
this.age = builder.age;
}
}

Because in normal class setters usually return void you can't call another of your class setters on type void
Simple demonstration of builder pattern looks like this
public class Alert {
String prop1;
String prop2;
Alert(AlertBuilder builder){
this.prop1 = builder.prop1;
this.prop2 = builder.prop2;
}
}
class AlertBuilder{
String prop1;
String prop2;
public AlertBuilder setProp1(String prop1) {
this.prop1 = prop1;
return this;
}
public AlertBuilder setProp2(String prop2) {
this.prop2 = prop2;
return this;
}
public Alert build(){
return new Alert(this);
}
}
Where you have your main class and a builder class. Builder pattern keeps updating itself and when you are ready to build the "real" object you call build method and get all your fields at once.
I suggest you read more about Builder design patterns if you are interested.

how to replace an email in java through a method

first time I've tried this. I need to be able to replace an email for subclass Student and sublass Teacher after an email has been inputted, I have a parent class and superclass which is where I believe I need to add my changeEmail method. I may be a way off here but can I use stringBuilder or is there an easier way? Real noob when it comes to this.
SUBCLASS -
public class Teacher extends Member
{
private String qualifications;
public Teacher(String name, String email, String qualifications)
{
super(name, email);
this.qualifications = qualifications;
}
public String getQualifications()
{
return qualifications;
}
public String toString()
{
StringBuffer details = new StringBuffer();
details.append(super.getName());
details.append(' ');
if(qualifications != null && qualifications.trim().length() > 0) {
details.append("(" + qualifications + ")");
details.append(' ');
}
details.append(super.getEmail());
return details.toString();
}
}
SUBCLASS -
public class Student extends Member
{
private int attendance;
public Student(String name, String email)
{
super(name, email);
this.attendance = 0;
}
public int getAttendance()
{
return attendance;
}
public void markAttendance(int attendance)
{
this.attendance += attendance;
}
public void print()
{
System.out.println(super.getName() + " (" + attendance + ")");
}
}
SUPERCLASS -
public class Member
{
private String email;
private String name;
public Member(String name, String email)
{
this.name = name;
this.email = email;
}
public String getEmail()
{
return email;
}
public String getName()
{
return name;
}
public String changeEmail()
{
//..........
}
}

Since changeEmail is a public method in the superclass, the subclasses can access it too. Student (as well as Teacher) is a Member.
public String changeEmail(String newEmailAddress) {
String old = email;
this.email = newEmailAddress;
return old;
}
What I changed was adding a parameter (String newEmailAddress) and then set the new value to the email instance field.
(EDIT: I updated the answer to return the old email address. I don't know why a method like this would return anything but anyways..)

That is called inheritance, basically if you have some shared variables, you can use some parent class and with the keyword extends create some subclasses.
All subclasses, which inherits the parent class, can have their own class variables, but also are having the parent variables.
In your case you can image the diagram like that- obvious, doesnt?
So...
Parent class member is having these class variables:
- String : mail
- String : name
You have two subclasses- Student and Teacher:
Teacher class variables:
qualifications
mail, name (inherited from parent!)
Student class variables:
attendance
mail, name (inherited from parent!)
Notice- with the keyword super you are calling the constructor (or simply "class" other methods) from the parent, so in Teacher and Student class, you will call exactly following:
public Member(String name, String email) {
this.name = name;
this.email = email;
}
To be able change the email, you need following
1) implement methods in parent class
2) optional- add call to child classes, and for usage outside the class also add some external method (without this you can still use public parent class methods)
Eg.
in parent
public void changeEmail(String newEmail) {
this.email = newEmail;
}
public String changeEmailWithReturnOld(String newEmail) {
String oldMail = this.email;
changeEmail(newEmail); //calling above
return oldMail;
}
In childs
public String changeTheMailWithReturnOld(String newMail) {
return super.changeEmailWithReturnOld(newMail); //super means super class, parent
}
Clear? :)
Then you can call following:
Teacher teacher1 = new Teacher("foo", "foo#foo.foo", "whateverFoo");
teacher1.changeEmail("someNewFoo#foo.foo"); //parent method
teacher1.changeEmailWithReturnOld("someNewFoo#foo.foo"); //Child method

Java ,import not resolved,inheritance,inner class

Begging java programming recently, run into an error. please help
Have two classes , PersonTest.java:
public class PersonTest {
public static void main(String[] args) {
Person person1=new Person("dummy","sdymmt","20","male","washington");
System.out.println("Name: "+person1.getName());
System.out.println("Surname: "+person1.getSurname());
System.out.println("Age: "+person1.getAge());
System.out.println("Gender:" +person1.getGender());
System.out.println("Birthplace: "+person1.getBirthplace());
Person person2= new Person(400);
System.out.println("Income:"+person2.getX()+" mije leke");
System.out.println("Tax:"+person2.Taksat()+" mije leke");
Student student1= new Student("adsd","zedsdsadza");
System.out.println("emri"+student1.getEmer());
}
}
and also Person.java :
public class Person {
private String Name;
private String Surname;
private String Age;
private String Gender;
private String Birthplace;
private double x;
public Person()
{
}
public Person(String Name, String Surname, String Age, String Gender, String Birthplace) {
this.Name = Name;
this.Surname = Surname;
this.Age = Age;
this.Gender = Gender;
this.Birthplace = Birthplace;
}
public String getName() {
return Name;
}
public String getSurname() {
return Surname;
}
public String getAge() {
return Age;
}
public String getGender() {
return Gender;
}
public String getBirthplace() {
return Birthplace;
}
public Person(double x) {
this.x = x;
}
public double getX() {
return x;
}
double Taksat() {
return (0.1 * x);
}
public class Student extends Person {
private String University;
private String Faculty;
public Student(String Universiteti, String Fakulteti) {
super(Name, Surname, Age, Gender, Birthplace);
this.Faculty = Fakulteti;
this.University = Universiteti;
}
public String getFaculty() {
return Faculty;
}
public String getUniversity() {
return University;
}
}
}
Two classes are in the same default package. How to fix the fact that the test class doesn't recognize the inner class student as a class.

Nested non static class are called Inner Classes those classes cannot live without the Outer class (which wrapped them).
Java docs
An instance of InnerClass can exist only within an instance of
OuterClass and has direct access to the methods and fields of its
enclosing instance.
To instantiate an inner class, you must first instantiate the outer
class. Then, create the inner object within the outer object with this
syntax:
OuterClass.InnerClass innerObject = outerObject.new InnerClass();
Try using:
Person.Student student = person1.new Student(PARAMETERS);
Important Mark:
Of course, you should highly consider that this is not a good design, because you may want this classes to be visible outside of the Person class but also because Person.Student inherits from Person, which it's already contains the Student class, which usually looks like a loop or a circle relationship, which usually not a good idea for the first place.

Because there is no Student class. Since it nested, it's Person.Student

new to java constructor help needed

I have an past exam question that says:
"Create a class Element that records the name of the element as a String and has a public method, toString that returns the String name. Define a constructor for the class (that should receive a String to initialise the name)."
I gave it a go and don't where to go from here...
main class is:
public class builder {
public static void main(String[] args) {
element builderObject = new element(elementName);
}
}
and constructor is:
import java.util.*;
class element {
public int getInt(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.println("Enter the first number");
String elementName = scan.nextLine();
System.out.println("%s");
}
public String toString() {
return elementName;
}
}

Don't get frustrated. Please read java tutorials first and understand the concepts. your exam question is very clear on what you need to do. Atleast for this question, you need to know what is constructor, the purpose of having toString() in a class.
May be the below can help you.
public class Element {
private String elementName;
public Element(String elementName) {
this.elementName = elementName;
}
#Override
public String toString() {
return elementName;
}
}

I can't think of a way to explain this without actually giving the answer, so....
public class Element { /// Create class Element
private final String name; // Record the 'name'
public Element(String name) { // constructor receives and sets the name
this.name = name;
}
public String toString() { // public method toString() returns the name
return name;
}
}

You are missing the constructor itself. The point of constructors is to initialize the object, usually by saving the given parameters to data members.
E.g.:
class Element {
/** A data member to save the Element's name */
private String elementName;
/** A constructor from an Element's name*/
public Element(String elementName) {
this.elementName = elementName;
}
#Override
public String toString() {
return elementName;
}
}

class Element {
private String name = "";
/**
/* Constructor
/**/
public void Element(final String name){
this.name = name;
}
#Override
public String toString(){
return name;
}
}

You don't have a constructor in there. A constructor typically looks something like this:
public class MyClass {
private String name;
private int age;
//This here is the constructor:
public MyClass(String name, int age) {
this.name = name;
this.age = age;
}
//here's a toString method just for demonstration
#Override
public String toString() {
return "Hello, my name is " + name + " and I am " + age + " years old!";
}
}
You should be able to use that as a guideline for making your own constructor.

class Element
{
private String name = "UNSET";
public String getName() { return name; }
public Element(String name) {
this.name = name;
}
public String toString() {
return getName();
}
}

You are missing a constructor you might be looking for something like this
public class Element{
private String name;
public Element(String name){ //Constructor is a method, having same name as class
this.name = name;
}
public String toString(){
return name;
}
}
A note
I take you are starting with java, In java class names usually start with capital letter, thus element should be Element. Its important that one picks up good habits early..

How to use a variable of one class, in another in Java?

I'm just working through a few things as practice for an exam I have coming up, but one thing I cannot get my head round, is using a variable that belongs to one class, in a different class.
I have a Course class and a Student class. Class course stores all the different courses and what I simply want to be able to do is use the name of the course, in class Student.
Here is my Course class:
public class Course extends Student
{
// instance variables - replace the example below with your own
private Award courseAward;
private String courseCode;
public String courseTitle;
private String courseLeader;
private int courseDuration;
private boolean courseSandwich;
/**
* Constructor for objects of class Course
*/
public Course(String code, String title, Award award, String leader, int duration, boolean sandwich)
{
courseCode = code;
courseTitle = title;
courseAward = award;
courseLeader = leader;
courseDuration = duration;
courseSandwich = sandwich;
}
}
And here is Student:
public class Student
{
// instance variables - replace the example below with your own
private int studentNumber;
private String studentName;
private int studentPhone;
private String studentCourse;
/**
* Constructor for objects of class Student
*/
public Student(int number, String name, int phone)
{
studentNumber = number;
studentName = name;
studentPhone = phone;
studentCourse = courseTitle;
}
}
Am I correct in using 'extends' within Course? Or is this unnecessary?
In my constructor for Student, I am trying to assign 'courseTitle' from class Course, to the variable 'studentCourse'. But I simply cannot figure how to do this!
Thank you in advance for your help, I look forward to hearing from you!
Thanks!

Am I correct in using 'extends' within Course? Or is this unnecessary?
Unfortunately not, if you want to know whether your inheritance is correct or not, replace extends with is-a. A course is a student? The answer is no. Which means your Course should not extend Student
A student can attend a Course, hence the Student class can have a member variable of type Course. You can define a list of courses if your model specifies that (a student can attend several courses).
Here is a sample code:
public class Student{
//....
private Course course;
//...
public void attendCourse(Course course){
this.course = course;
}
public Course getCourse(){
return course;
}
}
Now, you can have the following:
Student bob = new Student(...);
Course course = new Course(...);
bob.attendCourse(course);

I assume a Course is not a Student, so inheritance between those classes is probably a bad idea.

You have to declare them public.
A better way is the keep them private, and code a public getter for that variable. for example:
public Award getCourseAward(){
return this.courseAward;
}

Course should not extend Student. If you want to access the courseTitle field of Course, you need to pass a reference to a Course object to the Student and then do course.CourseTitle.

You cannot access private attributes of a class from another, this is one of the main principles of OOP: encapsulation. You have to provide access method to those attribute, you want to publish outside the class. The common approach is setter/getters - getters only, if you want to have your class immutable. Look here: http://en.wikipedia.org/wiki/Mutator_method#Java_example

It does not make sense to arbitrarily extend classes. Student is not a Course or vice versa, so you cannot extend them like that.
What you need to do is:
create a Course first:
Course aCourse = new Course(..);
create a Student:
Student aStudent = new Student(..);
assign the Course to the Student:
aStudent.setCourse(aCourse.title);

Extending Student with Couse because they are not of the same kind. Extending one class with another happens when specializing a more general (in a sense) one.
The solution would be to pass courseTitle as an argument of the Student constructor

There should be 3 separate objects here, a Course, a Student, and an Enrollment. An enrollment connects a Student to a Course, a Course has many Students, and a Student can enroll in many courses. None of them should extend each other.

First,
You are extending Student class in Course class, which means, student class gets all the coruse class properties. So, the student class does not have the courseTitle property.
Second, yes, it is unnesessary - you need to do the following:
public class Course
{
private Award courseAward;
private String courseCode;
public String courseTitle;
private String courseLeader;
private int courseDuration;
private boolean courseSandwich;
public Course(String code, String title, Award award, String leader, int duration, boolean sandwich)
{
courseCode = code;
courseTitle = title;
courseAward = award;
courseLeader = leader;
courseDuration = duration;
courseSandwich = sandwich;
}
}
public class Student
{
private int studentNumber;
private String studentName;
private int studentPhone;
// This is where you keep the course object associated to student
public Course studentCourse;
public Student(int number, String name, int phone, Course course)
{
studentNumber = number;
studentName = name;
studentPhone = phone;
studentCourse = course;
}
}
Example usage would be something like this:
Course course = new Course("ASD", "TITLE", null, "ME", 50, true);
Student student = new Student(1, "JOHN", "5551234", course);
And then, get the course information you need from student via, i.e.:
student.studentCourse.courseTitle;
Since now student.studentCourse will be a course object with all of its properties.
Cheers,

Maybe you do not need to add the course name to student. What I would do is add Students to some datastructure in Course. This is cleaner and reduces the coupling between Course and Student. This would also allow you to have Students being in more than one course. For example:
public class Course extends Student{
private Award courseAward;
private String courseCode;
public String courseTitle;
private Student courseLeader;//change to a student Object
private int courseDuration;
private boolean courseSandwich;
private Set<Student> students;//have course hold a collection of students
/**
* Constructor for objects of class Course
*/
public Course(String code, String title, Award award, Student leader, int duration, boolean sandwich){
courseCode = code;
courseTitle = title;
courseAward = award;
courseLeader = leader;
courseDuration = duration;
courseSandwich = sandwich;
this.students=new HashSet<Student>();
}
public boolean addStudent(Student student){
return students.add(student);
}
public Set<Student> getStudents(){
return students;
}
}

As mentioned, stay away from the "extends" for this. In general, you shouldn't use it unless the "is-a" relationship makes sense.
You should probably provide getters for the methods on the Course class:
public class Course {
...
public String getTitle() {
return title;
}
}
And then if the Student class needs that, it would somehow get a hold of the course (which is up to you in your design), and call the getter:
public class Student {
private Set<Course> courses = new HashSet<Course>();
public void attendCourse(Course course) {
courses.add(course);
}
public void printCourses(PrintStream stream) {
for (Course course : courses) {
stream.println(course.getTitle());
}
}
}

Here below find out the solution of your problem and if you want to check below code on your machine then create a file named Test.java and paste the below codes:
package com;
class Course
{
private Award courseAward;
private String courseCode;
public String courseTitle;
private String courseLeader;
private int courseDuration;
private boolean courseSandwich;
public Course(String code, String title, Award award, String leader, int duration, boolean sandwich)
{
courseAward = award;
courseCode = code;
courseTitle = title;
courseLeader = leader;
courseDuration = duration;
courseSandwich = sandwich;
}
public Award getCourseAward() {
return courseAward;
}
public void setCourseAward(Award courseAward) {
this.courseAward = courseAward;
}
public String getCourseCode() {
return courseCode;
}
public void setCourseCode(String courseCode) {
this.courseCode = courseCode;
}
public String getCourseTitle() {
return courseTitle;
}
public void setCourseTitle(String courseTitle) {
this.courseTitle = courseTitle;
}
public String getCourseLeader() {
return courseLeader;
}
public void setCourseLeader(String courseLeader) {
this.courseLeader = courseLeader;
}
public int getCourseDuration() {
return courseDuration;
}
public void setCourseDuration(int courseDuration) {
this.courseDuration = courseDuration;
}
public boolean isCourseSandwich() {
return courseSandwich;
}
public void setCourseSandwich(boolean courseSandwich) {
this.courseSandwich = courseSandwich;
}
}
class Student
{
private int studentNumber;
private String studentName;
private int studentPhone;
private Course studentCourse;
/**
* Constructor for objects of class Student
*/
public Student(int number, String name, int phone, Course course)
{
studentNumber = number;
studentName = name;
studentPhone = phone;
studentCourse = course;
}
public int getStudentNumber() {
return studentNumber;
}
public void setStudentNumber(int studentNumber) {
this.studentNumber = studentNumber;
}
public String getStudentName() {
return studentName;
}
public void setStudentName(String studentName) {
this.studentName = studentName;
}
public int getStudentPhone() {
return studentPhone;
}
public void setStudentPhone(int studentPhone) {
this.studentPhone = studentPhone;
}
public Course getStudentCourse() {
return studentCourse;
}
public void setStudentCourse(Course studentCourse) {
this.studentCourse = studentCourse;
}
}
class Award{
private long awardId;
private String awardName;
Award(long awardId, String awardName){
this.awardId = awardId;
this.awardName = awardName;
}
public long getAwardId() {
return awardId;
}
public void setAwardId(long awardId) {
this.awardId = awardId;
}
public String getAwardName() {
return awardName;
}
public void setAwardName(String awardName) {
this.awardName = awardName;
}
}
public class Test{
public static void main(String ar[]){
// use your all classes here
}
}