Develop Reference

How can we create an instance for a nested class in array of objects in java?

import java.util.Arrays;
import java.util.Scanner;
public class employee{
public String name;
public class employee_address{
String street_name;
String city;
String zipcode;
String state;
String country;
}
public static void main(String []args){
Scanner user_input = new Scanner(System.in);
int no_of_employees = user_input.nextInt();
employee[] employees_list = new employee[no_of_employees];
for(int i = 0;i < no_of_employees;i++){
employees_list[i].name = user_input.nextLine();
employees_list[I].employee_address = // this is it ?
}
}
}
In the code above I do understand that the employee_address is a class and can't be accessed
directly without an instance being created like in the code, that makes no sense. but how can I create an instance of the employee_address class that is associate with each employee.
like in the code above 'employee_address' is associated with every employee but how can the class 'employee_address' be initialised and how can I set the street_name, city and the rest of the members in the address class. any ideas would be appreciated.

You can't directly create an instance of inner class, the reason because since it is the property of another instance we always need to use it though the instance of parent variable.
Let's say you have a class, which have two propeties:
public class Employee {
public String name;
public EmployeeAddress emAddress;
}
to access emAddress you need to use through the instance of Employee class, for example -
Employee object = new Employee();
EmployeeAddress empAdd = object.new EmployeeAddress();
Full code:
public class Employee {
public String name;
public EmployeeAddress emAddress;
public class EmployeeAddress {
String street_name;
String city;
String zipcode;
String state;
String country;
public String getStreet_name() {
return street_name;
}
public void setStreet_name(String street_name) {
this.street_name = street_name;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getZipcode() {
return zipcode;
}
public void setZipcode(String zipcode) {
this.zipcode = zipcode;
}
public String getState() {
return state;
}
public void setState(String state) {
this.state = state;
}
public String getCountry() {
return country;
}
public void setCountry(String country) {
this.country = country;
}
#Override
public String toString() {
return "EmployeeAddress [street_name=" + street_name + ", city=" + city + ", zipcode=" + zipcode
+ ", state=" + state + ", country=" + country + "]";
}
}
public static void main(String[] args) {
Scanner user_input = new Scanner(System.in);
int no_of_employees = user_input.nextInt(); // let's say no_of_employees = 1
Employee[] employees = new Employee[no_of_employees];
for (int i = 0; i < no_of_employees; i++) {
Employee object = new Employee();
object.setName("Virat Kohli");
EmployeeAddress empAdd = object.new EmployeeAddress();
empAdd.setCity("New Delhi");
empAdd.setCountry("India");
empAdd.setState("Delhi");
empAdd.setStreet_name("Chandni Chalk");
empAdd.setZipcode("741124");
object.setEmAddress(emAddress);
employees[i] = object;
}
System.out.println(employees[0]);
user_input.close();
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public EmployeeAddress getEmAddress() {
return emAddress;
}
#Override
public String toString() {
return "Employee [name=" + name + ", emAddress=" + emAddress + "]";
}
public void setEmAddress(EmployeeAddress emAddress) {
this.emAddress = emAddress;
}
}
I have modified your code to sonar standard.

Below code uses Java naming conventions (which your code does not).
Notes after the code.
import java.util.Scanner;
public class Employee {
private String name;
private EmployeeAddress address;
public class EmployeeAddress {
String streetName;
String city;
String zipcode;
String state;
String country;
}
public static void main(String[] args) {
Scanner userInput = new Scanner(System.in);
int noOfEmployees = userInput.nextInt();
Employee[] employeesList = new Employee[noOfEmployees];
for (int i = 0; i < noOfEmployees; i++) {
employeesList[i] = new Employee();
employeesList[i].name = userInput.nextLine();
EmployeeAddress employeeAddress = employeesList[i].new EmployeeAddress();
employeesList[i].address = employeeAddress;
employeesList[i].address.streetName = userInput.nextLine();
}
}
}
An inner class is a normal class. It is not a member of its enclosing class. If you want class Employee to have an [employee] address, as well as a [employee] name, you need to add another member variable to class Employee whose type is EmployeeAdress.
Employee[] employeesList = new Employee[noOfEmployees];
The above line creates an array but every element in the array is null. Hence you need to first create a Employee object and assign it to an element of the array. Hence the following line in my code, above:
employeesList[i] = new Employee();
Since EmployeeAddress is not a static class, in order to create a new instance, you first need an instance of the enclosing class, i.e. Employee. Hence the following line in the above code.
EmployeeAddress employeeAddress = employeesList[i].new EmployeeAddress();
Since all your code is in class Employee, in method main you can directly access the members of both class Employee and EmployeeAddress. Nonetheless you need to be aware of the different access modifiers in java.

A few hints:
stick to naming conventions: class names in Java start with capital letters
use (class) definitions before using them (collect them at the top if not inconventient)
if you are sure you want to use inner classes, set them static, unless you want them to be entangled in generics.
Usually normal classes in each their own file are a lot more flexible and far easier to use
if you use objects that only carry public data, try to use final keyword and initialize them ASAP
use proper objects first, and after finishing them assign them to arrays. avan better would be the use of ArrayList and the like
if Employee contains EmployeeAddress, it should initialize it if conventient. so an object is always responsible for its own stuff
Use try/resrouce/catch
scanner.nextInt() can be problematic with newline/line breaks. For user input better readLine() and parse input
Code:
import java.io.PrintStream;
import java.util.ArrayList;
import java.util.Scanner;
public class Employee {
static public class EmployeeAddress {
public final String street_name;
public final String city;
public final String zipcode;
public final String state;
public final String country;
public EmployeeAddress(final Scanner pScanner, final PrintStream pOutPS) {
street_name = readLine(pScanner, pOutPS, "Please enter Street Name:");
city = readLine(pScanner, pOutPS, "Please enter City Name:");
zipcode = readLine(pScanner, pOutPS, "Please enter Zip Code:");
state = readLine(pScanner, pOutPS, "Please enter State:");
country = readLine(pScanner, pOutPS, "Please enter Country:");
}
}
static public String readLine(final Scanner pScanner, final PrintStream pOutPS, final String pPrompt) {
pOutPS.print(pPrompt);
final String value = pScanner.nextLine();
pOutPS.println();
return value;
}
static public int readInt(final Scanner pScanner, final PrintStream pOutPS, final String pPrompt) {
return Integer.parseInt(readLine(pScanner, pOutPS, pPrompt));
}
public final String name;
public final EmployeeAddress address;
public Employee(final Scanner pScanner, final PrintStream pOutPS) {
name = readLine(pScanner, pOutPS, "Please enter Employee Name: ");
System.out.println("Name: " + name);
address = new EmployeeAddress(pScanner, pOutPS);
}
public static void main(final String[] args) {
try (final Scanner user_input = new Scanner(System.in);
final PrintStream output = System.out;) {
final int no_of_employees = readInt(user_input, output, "Please enter number of users: ");
final Employee[] employees_list = new Employee[no_of_employees]; // either this line
final ArrayList<Employee> employees = new ArrayList<>(); // or this line
for (int i = 0; i < no_of_employees; i++) {
output.println("Creating user #" + (i + 1) + "...");
final Employee newEmployeeWithAddress = new Employee(user_input, output);
employees_list[i] = newEmployeeWithAddress; // either this line
employees.add(newEmployeeWithAddress); // or this line
}
}
}
}

I'm having a hard time understanding and implementing a constructor and I'm looking for some guidance

I'm learning about constructors, but the videos that I've watched don't seem to help and everything I find on google seems to describe it in an advanced way.
I want to write a simple program which takes two inputs, a name (String) and an id (integer) and then just outputs it as "id" - "name". So for example:
01 - hello
This is the program that I'm trying to fix:
import java.util.Scanner;
public class ConstructorTest {
public static void main(String[] args) {
ConstructorTest();
toString(null);
}
//Constructor
public ConstructorTest(){
Scanner name = new Scanner(System.in);
Scanner id = new Scanner(System.in);
}
// Method
public String toString(String name, int id) {
System.out.print(id + " - " + name);
return null;
}
}
The errors that I get, are saying that my methods and constructors are undefined.

A constructor creates ("constructs") a new object. You can then call methods against that object.
Here's a simple object:
public class MyObject {
private int id;
private String name;
public MyObject(int id, String name) {
this.id = id;
this.name = name;
}
// Other methods here, for example:
public void print() {
System.out.println(id + " " + name);
}
}
You would call this constructor like this:
MyObject thing = new MyObject(1, "test");
And then you could call its method like this:
thing.print();
So for your example, what you'd do in your main method is first prompt the user for id and name, then create an object using a constructor, and then call a method on the constructor.
public class ConstructorTest {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
// get the id and name from the scanner (I would suggest using prompts)
String name = in.nextLine();
int id = in.nextInt();
// create an object:
ConstructorTest myObject = new ConstructorTest(id, name);
// call the method:
String myString = myObject.toString();
// print the result:
System.out.println(myString);
}
// private variables, effectively the "properties" stored by the object:
private int id;
private String name;
// constructor:
public ConstructorTest(int id, String name) {
this.id = id;
this.name = name;
}
// method
#Override // because this is a method in java.lang.Object and we're overriding it
public String toString() {
return id + " - " + name;
}
}

Try this:
import java.util.Scanner;
public class ConstructorTest {
private int id;
private String name;
public static void main(String[] args) {
String name = args[0];
int id = Integer.valueOf(args[1]);
ConstructorTest ct = new ConstructorTest(name, id);
System.out.println(ct);
}
public ConstructorTest(String n, int i) {
this.id = i;
this.name = n;
}
// Method
public String toString() {
return String.format("%d - %s", id, name);
}
}
Never, ever put I/O in a constructor.

Send array data from one class to another JAVA

(I'm a beginner so this may sound obvious/lack information.) I have an ArrayList of attributes for different pets including attributes such as their given-name, common-name, the price of the animal, sex, date bought and date sold. this information is generated from a separate class that adds an array of information to an array of arrays of the already existing list of animals. Essentially, I want to send the array to another class (called Pets) so it can then be added to the array of arrays. I understand this may sound confusing but this is the only way I can word it, I can clarify anything if needed. Any help would be great as I'm really stuck and can't work out how to send it. This is the code that generates my values in the array (using text-boxes to input the information).
public void actionPerformed(ActionEvent e) {
ArrayList<String> NewanimalArr = new ArrayList<>();
String givenName = txtGivenname.getText();
String commonName = txtCommonName.getText();
String priceOf = txtPrice_1.getText();
String sexOf = txtSex.getText();
String colourOf = txtMaincolour.getText();
String dateOfA = txtArrivaldate.getText();
String dateSold = txtSellingdate.getText();
NewanimalArr.add(givenName);
NewanimalArr.add(commonName);
NewanimalArr.add(priceOf);
NewanimalArr.add(sexOf);
NewanimalArr.add(colourOf);
NewanimalArr.add(dateOfA);
NewanimalArr.add(dateSold);
System.out.println(NewanimalArr);
}
});
this will then print information generated that is entered for example:
[alex, Dog, 40.50, Male, Brown, 14/04/2015, 14/12/2016]
how do I then send this data to another class

Option one Constructor Injection:
public class Foo {
List<String> actionPerformed(ActionEvent e) {
List<String> newanimalArr = new ArrayList<>();
.....
return newanimalArr
}
...
public class Pets {
private final List<String> array;
public Pets(final List<String> array) {
this.array = array;
}
void bar() {
System.out.println(this.array);
}
}
....
public static void main(String[] args) {
Foo foo = new Foo();
Pets pets = new Pets(foo.actionPerformed( new ActionEvent() ) );
pets.bar();
}
Option two Getter-Setter Injection:
public class Foo {
private final List<String> newanimalArr;
public Foo() {
this.newanimalArr = new ArrayList<>();
}
public void actionPerformed(ActionEvent e) {
.....
}
public List<String> getNewanimalArr() {
return new ArrayList<String>(newanimalArr);
}
}
...
public class Pets {
private List<String> array;
public Pets() {
this.array = Collections.<String>emptyList();
}
public void setArray(final List<String> array) {
this.array = array;
}
public void bar() {
System.out.println(this.array);
}
}
....
public static void main(String[] args) {
Foo foo = new Foo();
foo.actionPerformed( new ActionEvent() );
Pets pets = new Pets();
bar.setArray( foo.getNewanimalArr() );
pets.bar();
}
See also Dependency Injection Patterns

Create a class definition of Pet, using instance variables for the fields. In Java it is custom to create a setXyz and a getXyz for each xyz field. You can also create a constructor in which you pass all the values and assign them to the fields, this minimizes the risk of fields not being filled in.
The initial ArrayList you are creating doesn't add that much use, it is easier to create the Pet instances directly:
List<Pet> newArrivals = new ArrayList<>();
// get data from view fields and if necessary transform them to other objects such as:
LocalDate arrivedOn = LocalDate.parse(txtArrivaldate.getText(), DateTimeFormatter.ofLocalizedDate(FormatStyle.FormatStyle);
// create and add a new Pet object to the list
newArrivals.add(new Pet(.....));
public class Pet {
public enum Gender {
FEMALE, MALE
}
private String givenName;
private String commonName;
private double price;
private Gender gender;
private String color;
private LocalDate arrivedOn;
private LocalDate soldOn;
public Pet() {
}
public Pet(String givenName, String commonName, double price, Gender gender, String color, LocalDate arrivedOn,
LocalDate soldOn) {
super();
this.givenName = givenName;
this.commonName = commonName;
this.price = price;
this.gender = gender;
this.color = color;
this.arrivedOn = arrivedOn;
this.soldOn = soldOn;
}
public String getGivenName() {
return givenName;
}
public void setGivenName(String givenName) {
this.givenName = givenName;
}
public String getCommonName() {
return commonName;
}
public void setCommonName(String commonName) {
this.commonName = commonName;
}
public double getPrice() {
return price;
}
public void setPrice(double price) {
this.price = price;
}
public Gender getGender() {
return gender;
}
public void setGender(Gender gender) {
this.gender = gender;
}
public String getColor() {
return color;
}
public void setColor(String color) {
this.color = color;
}
public LocalDate getArrivedOn() {
return arrivedOn;
}
public void setArrivedOn(LocalDate arrivedOn) {
this.arrivedOn = arrivedOn;
}
public LocalDate getSoldOn() {
return soldOn;
}
public void setSoldOn(LocalDate soldOn) {
this.soldOn = soldOn;
}
}

Error accessing variable of a nested class

I desperately need help. I have two classes.
One is a PatientRecord with nested classes in it.
The other is a PatientGenerator which fills a database with PatientRecords by generating random data
for each field of the PatientRecord.
My problem is that I can't access the variables of some of the nested classes but I can access others. (At least it compiles for some and fails with an error for others.)
error: cannot find symbol
temp.fname = getRFirstName();
symbol: variable fname
location: variable temp of type PatientRecord.Visitor
Here is the failing calls
private ArrayList<PatientRecord.Visitor> generateVisitors(PatientRecord p) {
int payments = rnd.nextInt(10);
ArrayList<PatientRecord.Visitor> array = new ArrayList<PatientRecord.Visitor>();
for (int i = 0; i < payments; i++) {
PatientRecord.Visitor temp = p.new Visitor();
temp.fname = getRFirstName();
temp.lname = getRLastName();
temp.relation = relations[rnd.nextInt(relations.length)];
array.add(temp);
}
return array;
}
and here is part of the class with nested class Visitor in it.
public class PatientRecord implements Serializable{
public int pId;
public String FirstName;
public String MiddleName;
public String LastName;
public boolean gender;
public Location location;
public ArrayList visitors;
public ArrayList emergencyContacts;
public DateTime discharge;
public Admission admission;
public String primaryDoctor;
public ArrayList procedures;
public ArrayList prescriptions;
public ArrayList nurseNotes;
public ArrayList doctorNotes;
public InsurancePolicy insurancePolicy;
public ArrayList billing; // Arraylist of payments
public PatientRecord() {
init();
}
public PatientRecord(int id) {
pId = id;
init();
}
private void init() {
visitors = new ArrayList<Visitor>();
emergencyContacts = new ArrayList<Contact>();
location = new Location();
discharge = new DateTime();
admission = new Admission();
procedures = new ArrayList<Procedure>();
prescriptions = new ArrayList<Prescription>();
nurseNotes = new ArrayList<Note>();
doctorNotes = new ArrayList<Note>();
billing = new ArrayList<Payment>();
insurancePolicy = new InsurancePolicy();
}
public class Visitor {
public String fname;
public String lname;
public String relation;
public Visitor() {
}
}

I have no compilation errors (after having removed some of the code):
public class PatientRecord
{
public int pId;
public String FirstName;
public String MiddleName;
public String LastName;
public boolean gender;
public ArrayList visitors;
public ArrayList emergencyContacts;
public String primaryDoctor;
public ArrayList procedures;
public ArrayList prescriptions;
public ArrayList nurseNotes;
public ArrayList doctorNotes;
public ArrayList billing; // Arraylist of payments
public PatientRecord() {
init();
}
public PatientRecord(int id) {
pId = id;
init();
}
private void init() {
visitors = new ArrayList<Visitor>();
}
public class Visitor
{
public String fname;
public String lname;
public String relation;
}
public class PatientGenerator
{
public ArrayList<PatientRecord.Visitor> generateVisitors(PatientRecord p) {
//int payments = rnd.nextInt(10);
ArrayList<PatientRecord.Visitor> array = new ArrayList<PatientRecord.Visitor>();
for (int i = 0; i < 10; i++) {
PatientRecord.Visitor temp = p.new Visitor();
temp.fname = "first";
temp.lname = "last";
temp.relation = "brother";
array.add(temp);
}
return array;
}
}
public static void main(String[] args)
{
PatientRecord t = new PatientRecord();
PatientGenerator c = t.new PatientGenerator();
c.generateVisitors(t);
}
}
Does your code look similar to this?

How to check if an array in an arraylist contains a certain value?

I have an array list which contains arrays of type String. I create the array list and add arrays to it with the following code:
List<String[]> transaction = new ArrayList<String[]>();
String[] transactionLine = new String[7];
transactionLine[0] = "0";
transactionLine[1] = "1";
//.....
transactionLine[6] = "some value";
transactionLines.add(transactionLine);
Now I want to test if one of the arrays contain a certain value. I tried it like this, but then it checks for an array and not an element of an array:
if(transactionLines.contains("some value")) {
//Do some stuff with it
}
I know this doesn't work, but I don't now how to do it otherwise. I couldn't find any post of this already on Stackoverflow (not with the logical search terms for this problem anyway).
Note: I have chosen this structure of arrays in an arraylist, because I have a fixed number of columns (as suggested in how to create dynamic two dimensional array in java?).
Any help is greatly appreciated!

#assylias suggestion to use the object oriented way is good, but his example does not tell if the list contains a transaction where one property has a certain value. This example does:
public class Test {
public static void main(final String[] args) {
final List<TransactionLine> transaction = new ArrayList<>();
transaction.add(new TransactionLine(1, "some value"));
transaction.add(new TransactionLine(2, "another value"));
transaction.add(new TransactionLine(3, "yet another value"));
System.out.println(containsName(transaction, "some value"));
System.out.println(containsName(transaction, "non-existent value"));
}
// Iterates over all transactions until a transaction is found that has the
// same name as specified in search
private static boolean containsName(final List<TransactionLine> transaction, final String search) {
for (final TransactionLine transactionLine : transaction) {
if (transactionLine.getName().equals(search)) {
return true;
}
}
return false;
}
private static class TransactionLine {
private int id;
private String name;
public TransactionLine(final int id, final String name) {
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public void setId(final int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(final String name) {
this.name = name;
}
}
}
Here is an example with two classes (Transaction and TransactionLine):
Test:
public class Test {
public static void main(final String[] args) throws Exception {
final Transaction transaction = new Transaction();
transaction.add("some name");
transaction.add("another name");
transaction.add("yet another name");
System.out.println(transaction.containsName("some name"));
System.out.println(transaction.containsName("non-existent name"));
}
}
Transaction:
import java.util.ArrayList;
import java.util.List;
public class Transaction {
private final List<TransactionLine> transactionLines = new ArrayList<>();
public void add(final String name) {
final TransactionLine tl = new TransactionLine(transactionLines.size(), name);
transactionLines.add(tl);
}
public boolean containsName(final String name) {
for (final TransactionLine transactionLine : transactionLines) {
if (transactionLine.getName().equals(name)) {
return true;
}
}
return false;
}
}
TransactionLine:
public class TransactionLine {
private int id;
private String name;
public TransactionLine() {
}
public TransactionLine(final int id, final String name) {
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public void setId(final int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(final String name) {
this.name = name;
}
}

The object oriented way of solving your problem would be to create a class:
class Transaction {
private final int id;
private final String name;
//etc.
}
Then if you need to test if a given transaction is in the list you could implement equals and hashcode in that class, which would enable you to call:
if(transactionLines.contains(someTransaction)) { ... }
If you just need to find transactions with a specific characteristics, you would need to iterate over the list and check each transaction, for example:
Transaction result = null;
for (Transaction t : transacionLines) {
if(t.getName().equals("some value") {
result = t;
break;
}
}

public static boolean isListOfStringArraysContainsString(List<String[]> arrayList, String s) {
for (String[] arr : arrayList) {
for (String string : arr) {
if ((string != null) && (string.equals(s))) {
return true;
}
}
}
return false;
}
Provided code do exactly what you are asking about, but solution provided by #assylias is proper

I got your point. By using ArrayList you are trying to make an array of another array of strings. But you have made one simple mistake.This is how you tried to retrieved a String inside an array inside an ArrayList:
if(transactionLines.contains("some value")) {
//Do some stuff with it
}
This "some value" is a string present in String array "transactionLine" and not referred by the List "transactionLines" (which is referring to ArrayList object).
Instead this is what you should have done:
List<String[]> transactionLines = new ArrayList<String[]>();
String[] transactionLine = new String[7];
transactionLine[0] = "0";
transactionLine[1] = "1";
transactionLine[2] = "something";
transactionLine[3] = "3";
transactionLine[4] = "4";
transactionLines.add(transactionLine);
String[] mySL=transactionLines.get(0);
System.out.println(mySL[2]);
if (mySL[2].equals("something")) {
//some code
} else {
//some code
}
Hope this helps.