milind k das

Thank you very much for response. I have another question.

We have Array which can store object of a class type.

for example: City[] cities = new City[10]; on a City class.

And arraylist is. ArrayList<City> cities = new ArrayList<>(); While arraylist initiates size at 0.

Why is separate ArrayList needed while Arrays themselves can work well? And what is need for conversion of Arrays to ArrayList and ArrayList to Array using Collections class? I don't know nothing about collections or generics yet. Please explain in simple terms.

package Chapter11; import java.util.ArrayList; import java.util.Scanner; import java.util.Collections; public class Problem_11_7 extends ShuffleInteger {    public static void main(String[] args) {        Scanner input = new Scanner(System.in);        ArrayList<Integer> list = new ArrayList<>();        System.out.println("Enter the numbers: ");        for(int i = input.nextInt(); i != 0; i = input.nextInt()) {            list.add(i);        }        shuffleNum(list);    } } class ShuffleInteger {    public static void shuffleNum(ArrayList<Integer> list) {        Collections.shuffle(list);        System.out.println(list);    } }

In this program, System.out.println(list); statement is printing the elements's values in list. My question is, isn't list a reference variable to ArrayList? Is it not supposed to print reference value of "list"?

Stephan van Hulst wrote:Don't use floating point values for anything other than imprecise measurements. Your account balance should be an integer and your interest rate either an integer or a BigDecimal.

IDs should be strings, not integers.

Make the ID and creation time of an account final.

Make all methods that are not intended to be overridden final. When in doubt, make it final.

Perform parameter validation in your mutators. You should not be able to withdraw or deposit negative amounts. Also, check for integer overflows.

Validate class invariants by throwing exceptions. When you try to withdraw more than the overdraft limit allows, throw an exception.

Don't use long sequences of string concatenations. Use a formatter instead.

public abstract class Account {  private final String id;  private final LocalDateTime creationTime;  private BigInteger balanceInCents = BigInteger.ZERO;  private BigDecimal annualInterestRate = BigDecimal.ZERO;  protected Account(String id, LocalDateTime creationTime) {    if (id == null)      throw new IllegalArgumentException("The ID may not be null.");    if (creationTime == null)      throw new IllegalArgumentException("The creation time may not be null.");    this.id = id.strip();    this.creationTime = creationTime;    if (this.id.isEmpty() || !this.id.equals(id))      throw new IllegalArgumentException("The ID must be a non-empty stripped string.");  }  public final String id() {    return id;  }  public final LocalDateTime creationTime() {    return creationTime;  }  public final BigInteger balanceInCents() {    return balanceInCents;  }  protected final void setBalanceInCents(BigInteger balanceInCents) {    if (balanceInCents == null)      throw new IllegalArgumentException("The balance may not be null.");    this.balanceInCents = balanceInCents;  }  public final BigDecimal annualInterestRate() {    return annualInterestRate;  }  public final void setAnnualInterestRate(BigDecimal annualInterestRate) {    if (annualInterestRate == null)      throw new IllegalArgumentException("The annual interest rate may not be null.");    this.annualInterestRate = annualInterestRate;  }  public abstract void depositCents(BigInteger amountInCents);  public abstract void withdrawCents(BigInteger amountInCents);  @Override  public String toString() {    var balanceWithTwoDecimals = new BigDecimal(balanceInCents).divide(BigDecimal.valueOf(100), 2, RoundingMode.UNNECESSARY);    var annualInterestRateAsPercentage = annualInterestRate.multiply(BigDecimal.valueOf(100)).stripTrailingZeros();    return String.format(      Locale.ROOT,      "Account { id: \"%s\", balance: %s, annualInterestRate: %%%s, creationTime: %tF %4$tT }",      id, balanceWithTwoDecimals, annualInterestRateAsPercentage, creationTime    );  } }final class CheckingAccount extends Account {  private static final BigInteger OVERDRAFT_LIMIT_IN_CENTS = BigInteger.valueOf(100_00);  CheckingAccount(String id, LocalDateTime creationTime) {    super(id, creationTime);  }  @Override  public void depositCents(BigInteger amountInCents) {    if (amountInCents == null)      throw new IllegalArgumentException("The amount may not be null.");    if (amountInCents.compareTo(BigInteger.ZERO) < 0)      throw new IllegalArgumentException("The amount may not be negative.");    setBalanceInCents(balanceInCents().add(amountInCents));  }  @Override  public void withdrawCents(BigInteger amountInCents) {    if (amountInCents == null)      throw new IllegalArgumentException("The amount may not be null.");    if (amountInCents.compareTo(BigInteger.ZERO) < 0)      throw new IllegalArgumentException("The amount may not be negative.");    var newBalanceInCents = balanceInCents().subtract(amountInCents);    if (newBalanceInCents.compareTo(OVERDRAFT_LIMIT_IN_CENTS.negate()) < 0)      throw new IllegalStateException("The current balance is too low to withdraw the given amount.");    setBalanceInCents(newBalanceInCents);  } }

Thank you very much. this has been helpful.

package Chapter11; import java.util.ArrayList; import java.util.Scanner; import java.util.Collections; public class Problem_11_7 extends ShuffleInteger {    public static void main(String[] args) {        Scanner input = new Scanner(System.in);        ArrayList<Integer> list = new ArrayList<>();        System.out.println("Enter the numbers: ");        for(int i = input.nextInt(); i != 0; i = input.nextInt()) {            list.add(i);        }        shuffleNum(list);    } } class ShuffleInteger {    public static void shuffleNum(ArrayList<Integer> list) {        Collections.shuffle(list);        System.out.println(list);    } }


In this program, System.out.println(list); statement is printing the elements's values in list. My question is, isn't list a reference variable to ArrayList? Is it not supposed to print reference value of "list"?

Carey Brown wrote:Typo error. Check your method names.

ooops!! Thank you very much!

package Chapter9; import java.util.Date; public class Account {    private int id;    private double balance;    private double annualInterestRate;    private Date dateCreated;    public Account() {        this.id = 0;        this.balance = 0;        this.annualInterestRate = 0;        this.dateCreated = new Date();    }    public Account(int id, double initialBalance) {        this.id = id;        this.balance = initialBalance;        this.dateCreated = new Date();    }    public Date getDateCreated() {        return dateCreated;    }    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public double getBalance() {        return balance;    }    public void setBalance(double balance) {        this.balance = balance;    }    public double getAnnualInterestRate() {        return annualInterestRate;    }    public void setAnnualInterestRate(double annualInterestRate) {        this.annualInterestRate = annualInterestRate;    }    public double getMonthlyInterestRate() {        return (annualInterestRate)/12;    }    public double getMonthlyInteest() {        return getMonthlyInterestRate()*balance;    }    public void withdaraw(double amount) {        this.balance = this.balance-amount;    }    public void deposit(double amount) {        this.balance = this.balance+amount;    }    @Override    public String toString() {        return "Account{" +                "id=" + id +                ", balance=" + balance +                ", annualInterestRate=" + annualInterestRate +                ", dateCreated=" + dateCreated +                '}';    } } package Chapter11; import Chapter9.Account; public class Problem_11_3 { } class CheckingAccount extends Account {    private double overdraftLimit;    public CheckingAccount() {        this.overdraftLimit = 100;    }    @Override    public void withdraw(double amount) {        if(amount < overdraftLimit) {            setBalance(getBalance() -amount);        }    } }

In the above code, I have Account class in different Package - Chapter9 and class CheckingAccount is in package Chapter11. I have imported the Account and all methods in Account are public. However, in class CheckingAccount, even though the withdraw method I want to override has same signature, @Override is showing error "Method does not override method from its superclass". Please let me know why its not letting me override? What's wrong here?

@Stephan van Hulst Thank you for explanation. Your explanation gave me some clarity. Still confused. But I will reading again this topic more detail. Please such some source with explanation in detail including how JVM handles.

@Campbell Ritchie Thank you for posting that. I was about to post my second question on this. Can you elaborate on this why it's not applied on data fields and static methods? Thank you

Hello I started reading a java book. Introduction to Java programming by liang. I have question in polymorphism section of the book. Below are the statements from the book.

An object of a subclass can be used wherever its superclass object is
used. This is commonly known as polymorphism (from a Greek word meaning “many
forms”). In simple terms, polymorphism means that a variable of a supertype can refer to
a subtype object.

Polymorphism means that a variable of a supertype can refer to a subtype object.

I believe superclass cannot be of subclass type. While subclass can be of superclass type and holds superclass type object too with subclass object.

If this is the case, how can superclass type variable can hold subclass type object? Is it not contradicting? Sorry if its basic, please explain in simple terms

Consider below example:

public class MasterExperiment {    private static boolean accessedProfile;    private static double rateSkillSet[][];    public MasterExperiment() {        MasterExperiment.accessedProfile = true;        System.out.println("I'm MasterExperiment");    }    MasterExperiment(double rating[][]) {    }    public void setAccessedProfile(boolean accessed) {        accessedProfile = accessed;    }    public boolean getAccessedProfile() {        return  accessedProfile;    }    public MasterExperiment masterObject() {        return this;    } } public class Experiment extends MasterExperiment {    private static double skillSetRate;    Experiment () {        System.out.println("I'm Experiment");    }    public void rateSkillSet(double rating) {        setAccessedProfile(true);    } } public class TestExperiment {    public static void main(String[] args) {        Experiment experiment = (Experiment) new MasterExperiment();        MasterExperiment masterExperiment = new Experiment(); //        System.out.println("Experiment Ref: "+experiment+" & MasterExperiment Ref: "+experiment.masterObject());    } }


In the code, may I know what is the difference in two statements,
       Experiment experiment = (Experiment) new MasterExperiment();        MasterExperiment masterExperiment = new Experiment();

And how are the objects allocated and their behavior in above two statements.