In this example, how do I initiate the object state on as soon as it's created?
public class Account {
private double balance;
public void credit(double amount){
balance = balance + amount;
}
public double getBalance(){
return balance;
}
}
Add a simple constructor, as follows:
public class Account {
private double balance;
public Account(double balance) {
this.balance = balance;
}
public void credit(double amount){
balance = balance + amount;
}
public double getBalance(){
return balance;
}
}
From the method that creates an Account instance (e.g., from your main method), you can simply call this constructor as follows:
Account account = new Account(100.5);
to initialize an Account instance with a balance value of 100.5.
This is one of the first things that you should know when learning to program in Java, so perhaps an introductory book to Java would be highly recommended. It is the fastest way to learn Java, IMHO.
Related
I am supposed to Add the following method to allow BankAccounts to transfer money to another BankAccount. Where would I add this method to make sure it works for all BankAccounts, including SavingsAccount and CheckingAccount?
The method im supposed to use is transferTo(BankAccount destinationAccount, int transferAmount)
public class BankAccount {
private String accountHolderName;
private String accountNumber;
private int balance;
public BankAccount(String accountHolder, String accountNumber) {
this.accountHolderName = accountHolder;
this.accountNumber = accountNumber;
this.balance = 0;
}
public BankAccount(String accountHolder, String accountNumber, int balance) {
this.accountHolderName = accountHolder;
this.accountNumber = accountNumber;
this.balance = balance;
}
public String getAccountHolderName() {
return accountHolderName;
}
public String getAccountNumber() {
return accountNumber;
}
public int getBalance() {
return balance;
}
// Update the balance by using the DollarAmount.Plus method
public int deposit(int amountToDeposit) {
balance = balance + amountToDeposit;
return balance;
}
// Update the balance by using the DollarAmount.Minus method
public int withdraw(int amountToWithdraw) {
balance = balance - amountToWithdraw;
return balance;
}
public int transferTo(BankAccount destinationAccount, int transferAmount) {
return balance;
}
}
Assumptions:
This is only an exercise and not a real application for a bank.
SavingsAccount and CheckingAccount are subclasses of BankAccount
The method transferTo can be implemented like the following:
public int transferTo(BankAccount destinationAccount, int transferAmount) {
this.balance -= transferAmount;
destinationAccount.deposit(transferAmount);
return balance;
}
In a real world application you need to ensure that this operation will be always atomic and thread-safe. Additionally, using int for the balance is strongly not recommended.
in this code i need to make an object of the extended class basicaccount, but i get the error message "Non-static variable cannot be referenced from a static context" what can i do better?
public class BankAccount {
private double balance;
public BankAccount() {
balance = 0;
}
public BankAccount(double initialBalance) {
balance = initialBalance;
}
public void deposit(double amount) {
double newBalance = balance + amount;
balance = newBalance;
}
public void withdraw(double amount) {
double newBalance = balance - amount;
balance = newBalance;
}
public double getBalance() {
return balance;
}
class BasicAccount extends BankAccount {
public BasicAccount(Double d) {
balance = d;
}
}
class Main {
public static void main(String args[]) {
BankAccount account = new BasicAccount(100.00);
double balance = account.getBalance(); //expected 100.00;
account.withdraw(80.00);
balance = account.getBalance(); //expected 20.00;
account.withdraw(50.00);
balance = account.getBalance(); //expected 20.00 because the amount to withdraw is larger than the balance
}
}
}
Non-static nested classes (BasicAccount and Main) cannot be instantiated without the encapsulating BankAccount object. To instantiate BasicAccount you need to call new BasicAccount(...) on an existing BankAccount object.
BankAccount account = new BankAccount().new BasicAccount(0);
However I think you may not have wanted this - if that's the case I think you should just move BasicAccount and Main from inside BankAccount.
Make Main your public class.
Remove public from BankAccount class
Move your Main class outside of your BankAccount class.
Make certain the static main(String[] args) is in the Main public class.
Here is what it should look like
class BankAccount {
private double balance;
public BankAccount() {
balance = 0;
}
public BankAccount(double initialBalance) {
balance = initialBalance;
}
public void deposit(double amount) {
double newBalance = balance + amount;
balance = newBalance;
}
public void withdraw(double amount) {
double newBalance = balance - amount;
balance = newBalance;
}
public double getBalance() {
return balance;
}
class BasicAccount extends BankAccount {
public BasicAccount(Double d) {
balance = d;
}
}
}
public class Main {
public static void main(String args[]) {
BankAccount account = new BasicAccount(100.00);
double balance = account.getBalance(); // expected 100.00;
account.withdraw(80.00);
balance = account.getBalance(); // expected 20.00;
account.withdraw(50.00);
balance = account.getBalance(); // expected 20.00 because the amount to withdraw is larger than the balance
}
}
You seem to have put your BasicAccount class inside your BankAccount class, making it an inner class. That means you can't instantiate a BasicAccount without an existing instance of BankAccount.
You probably didn't want to do that. I suggest that you avoid putting classes inside other classes before you understand the effects.
Move your BasicAccount and Main classes outside of the definition of the BankAccount class.
I am working on a program to hold bank accounts as objects. The accounts have an interest rate, balance, ID, and date created data to go along. In what I have made, the default balance, id, and interest is 0 from what I understand. The interest rate is undefined by default. The book I am learning form shows that a no-arg constructor is done with "Circle() { }".I used "account() { }" in the account class. When I run the program in jGRASP, I get error "invalid method declaration; return type required" for both of my constructors. It is recognizing what I intend to be constructors as methods. What do I need to understand so I can make my constructors not be recognized as methods?
When running the first constructor, I understand we create an Account object called account with the default values. When we run the second constructor, we are changing the values of the account object to something with specified
public class Bank{
public static void main(String[] args){
Account account = new Account(1122, 20000);
account.setAnnualInterestRate(4.5);
account.withdraw(2500);
account.deposit(3000);
System.out.println("Balance is " + account.getBalance());
System.out.println("Monthly interest is " + account.getMonthlyInterest());
System.out.println("This account was created at " + account.getDateCreated());
}
}
class Account {
private int id = 0;
private double balance = 0;
private double annualInterestRate = 0;
private String dateCreated;
account(){
}
account(int newID, double newBalance){
id = newID;
balance = newBalance;
}
//accessor for ID
public int getID(){
return id;
}
//acessor for balance
public double getBalance(){
return balance;
}
//accessor for interest rate
public double getAnnualInterest(){
return annualInterestRate;
}
//mutator for ID
public void setID(int IDset){
id = IDset;
}
//mutator for balance
public void setBalance(int BalanceSet){
balance = BalanceSet;
}
//mutator for annual interest
public void setAnnualInterestRate(double InterestSet){
annualInterestRate = InterestSet;
}
//accessor for date created
public String getDateCreated(){
return dateCreated;
}
//method that converts annual interest into monthly interest and returns the value
public double getMonthlyInterest(){
double x = annualInterestRate / 12;
return x;
}
//method that witdraws from account
public double withdraw(double w){
balance -= w;
return balance;
}
//method that deposits into account
public double deposite(double d){
balance += d;
return balance;
}
}
Constructor names must match the class names in a case sensitive way. In the Account class, change
account(){
to
Account(){
and likewise for your other constructor.
You need to capitalize your a in both constructors. Java is case sensitive.
Account(){
}
Account(int newID, double newBalance){
id = newID;
balance = newBalance;
}
Otherwise, Java sees this as a method with no return type. Remember, a constructor does not have or needs a return type.
Constructors should be camelized(as a convention, same with the class name) and it return only the type of itself
see my example :)
Account() {
return;
}
Okay guys, im having trouble with these constructors for this code and some logic. I did most of it, just confused on how to finish this off.
Here is the main code i have. (it might be a little sloppy but its "good enough")
public class Accountdrv {
public static void main (String[] args) {
Account account = new Account(1122, 20000, 4.5);
account.withdraw(2500);
account.deposit(3000);
System.out.println("Balance is " + account.getBalance());
System.out.println("Monthly interest is " +
account.getMonthlyInterest());
System.out.println("This account was created at " +
account.getDateCreated());
}
}
class Account {
private int id;
private double balance;
private double annualInterestRate;
private java.util.Date dateCreated;
public Account() {
dateCreated = new java.util.Date();
}
public Account(int id, double balance, double annualInterestRate) {
this.id = id;
this.balance = balance;
this.annualInterestRate = annualInterestRate;
dateCreated = new java.util.Date();
}
public int getId() {
return this.id;
}
public double getBalance() {
return balance;
}
public double getAnnualInterestRate() {
return annualInterestRate;
}
public void setId(int id) {
this.id =id;
}
public void setBalance(double balance) {
this.balance = balance;
}
public void setAnnualInterestRate(double annualInterestRate) {
this.annualInterestRate = annualInterestRate;
}
public double getMonthlyInterest() {
return balance * (annualInterestRate / 1200);
}
public java.util.Date getDateCreated() {
return dateCreated;
}
public void withdraw(double amount) {
balance -= amount;
}
public void deposit(double amount) {
balance += amount;
}
}
Now, i wanted to create a Savings, and Checking account. I need help with what i need to add for constructors, i put in the comments the parts i know im missing but confused on how to do them.
Savings:
class Savings extends Account{
//need to create a constructor
public Savings(int id, double balance, double annualInterestRate) {
//need to invoke the constructor for Account
super(id, balance, annualInterestRate);
}
// need to override the withdraw method in Account
public void withdraw(double amount) {
// place logic to prevent the account from being overdrawn
// that is do not allow a negative balance
}
}
You just need to check id the amount < balance Or whatever logic you have (eg: you want
some threshold amount to be present in the account).
Also I would recommend to look into synchronized methods as you should synchronize access to account of a given user to ensure that when a withdraw function is invoked...another withdraw on the same user account has to wait...or else it will lead to problems...I will leave that to you to figure out.
public void withdraw(double amount) {
// place logic to prevent the account from being overdrawn
// that is do not allow a negative balance
if(balance < amount)
{
// print error message or do something
}
else
{
// withdraw the money
balance -= amount;
// print message or do something
}
}
super() invokes the constructor for the super-class, in the Savings case, Account.
I apologize ahead of time if this code isn't formatted correctly, trying to paste instead of retyping each line. If it isn't right, can someone tell me an easy way to paste multiple lines of code at once?
My main question is that I keep getting an error message stating: Cannot make a static reference to the non-static field balance.
I have tried making the methods static, with no result, and making the main method non-static by removing "static" from the header, but then I get the message: java.lang.NoSuchMethodError: main Exception in thread "main"
Does anyone have any ideas? Any help is appreciated.
public class Account {
public static void main(String[] args) {
Account account = new Account(1122, 20000, 4.5);
account.withdraw(balance, 2500);
account.deposit(balance, 3000);
System.out.println("Balance is " + account.getBalance());
System.out.println("Monthly interest is " + (account.getAnnualInterestRate()/12));
System.out.println("The account was created " + account.getDateCreated());
}
private int id = 0;
private double balance = 0;
private double annualInterestRate = 0;
public java.util.Date dateCreated;
public Account() {
}
public Account(int id, double balance, double annualInterestRate) {
this.id = id;
this.balance = balance;
this.annualInterestRate = annualInterestRate;
}
public void setId(int i) {
id = i;
}
public int getID() {
return id;
}
public void setBalance(double b){
balance = b;
}
public double getBalance() {
return balance;
}
public double getAnnualInterestRate() {
return annualInterestRate;
}
public void setAnnualInterestRate(double interest) {
annualInterestRate = interest;
}
public java.util.Date getDateCreated() {
return this.dateCreated;
}
public void setDateCreated(java.util.Date dateCreated) {
this.dateCreated = dateCreated;
}
public static double withdraw(double balance, double withdrawAmount) {
double newBalance = balance - withdrawAmount;
return newBalance;
}
public static double deposit(double balance, double depositAmount) {
double newBalance = balance + depositAmount;
return newBalance;
}
}
main is a static method. It cannot refer to balance, which is an attribute (non-static variable). balance has meaning only when it is referred through an object reference (such as myAccount.balance or yourAccount.balance). But it doesn't have any meaning when it is referred through class (such as Account.balance (whose balance is that?))
I made some changes to your code so that it compiles.
public static void main(String[] args) {
Account account = new Account(1122, 20000, 4.5);
account.withdraw(2500);
account.deposit(3000);
and:
public void withdraw(double withdrawAmount) {
balance -= withdrawAmount;
}
public void deposit(double depositAmount) {
balance += depositAmount;
}
the lines
account.withdraw(balance, 2500);
account.deposit(balance, 3000);
you might want to make withdraw and deposit non-static and let it modify the balance
public void withdraw(double withdrawAmount) {
balance = balance - withdrawAmount;
}
public void deposit(double depositAmount) {
balance = balance + depositAmount;
}
and remove the balance parameter from the call
You are trying to access non static field directly from static method which is not legal in java. balance is a non static field, so either access it using object reference or make it static.
The static calls to withdraw and deposit are your problem.
account.withdraw(balance, 2500);
This line can't work , since "balance" is an instance variable of Account. The code doesn't make much sense anyway, wouldn't withdraw/deposit be encapsulated inside the Account object itself? so the withdraw should be more like
public void withdraw(double withdrawAmount)
{
balance -= withdrawAmount;
}
Of course depending on your problem you could do additional validation here to prevent negative balance etc.
Just write:
private static double balance = 0;
and you could also write those like that:
private static int id = 0;
private static double annualInterestRate = 0;
public static java.util.Date dateCreated;
To access instance variables it is a must to create an object, these are not available in the memory, before instantiation.
Therefore, you cannot make static reference to non-static fields(variables) in Java. If you still, try to do so a compile time error is generated saying “non-static variable math cannot be referenced from a static context”.
you can keep your withdraw and deposit methods static if you want however you'd have to write it like the code below.
sb = starting balance and eB = ending balance.
Account account = new Account(1122, 20000, 4.5);
double sB = Account.withdraw(account.getBalance(), 2500);
double eB = Account.deposit(sB, 3000);
System.out.println("Balance is " + eB);
System.out.println("Monthly interest is " + (account.getAnnualInterestRate()/12));
account.setDateCreated(new Date());
System.out.println("The account was created " + account.getDateCreated());