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Java Interface

2026-03-28 00:43

Tags: #java

Author: Duke Hsu

Key Concept

  • interface is a template that can be applied to a class.
  • interface similar to inheritance, but specifies what a class has/ must do.
  • classes can apply more than one interface,

java_interface_banksys

Code Example:

  • Bank Class
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/**
 * 
 */
package bankingsystem;

/**
 * 
 */
public class Bank {

    /**
     * @param args
     */
    public static void main(String[] args) {

        //initd object
        SavingAccount  dkAccount = new SavingAccount("duke", 5000);

        // call methods - interface BankAccount and InteresEarning
        dkAccount.deposit(9000);
        dkAccount.deposit(100000);
        dkAccount.withdraw(5000);
        System.out.println("**********************************");
        dkAccount.printStatement();

        System.out.println();
        System.out.println("==================================");
        dkAccount.applyInterest();
        System.out.println("InterestRate is:  "+dkAccount.getInterestRate());

    }

}
  • SavingAccount Class
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package bankingsystem;


//class SavingAccount can apply more than one interface -  BankAccount,InteresEarning
public class SavingAccount implements BankAccount,InteresEarning{

    private double balance;
    private String accountHoulder;
    private double interestRate =0.03;

    //constructor 
    public SavingAccount(String name, double initialBanlance) {
        this.accountHoulder = name;
        this.balance = initialBanlance;
    }


    @Override
    public void deposit(double amount) {
        balance += amount;
        System.out.println("Account:  " + accountHoulder + " Deposited: " + amount);
    }


    @Override
    public void withdraw(double amount) {
        if (balance >= amount) {

            balance -= amount;
            System.out.println("Account: " +accountHoulder + " Withdraw: " + amount);

        }else {

            System.out.println("Isufficient funds!!!");

        }

    }


    @Override
    public double getBalance() {

        return balance;
    }



    @Override
    public void printStatement() {

        System.out.println("=== Savings Account ===");
        System.out.println("Holder: " + accountHoulder);
        System.out.println("Balance: $" + balance);

    }


    @Override
    public void applyInterest() {
        double interest = balance * interestRate;
        balance += interest;
        System.out.println("  Interest applied: $" + interest);
        System.out.println("  New balance: $" + balance);

    }


    @Override
    public double getInterestRate() {
        return interestRate;
    }

}
  • CheckingAccount Class
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package bankingsystem;

public class CheckingAccount implements BankAccount {
    private double balance;
    private String accountHolder;

    public CheckingAccount(String name, double initialBalance) {
        this.accountHolder = name;
        this.balance = initialBalance;
    }

    @Override
    public void deposit(double amount) {
        balance += amount;
        System.out.println("💰 " + accountHolder + " deposited $" + amount);
    }

    @Override
    public void withdraw(double amount) {
        // overdraft
        balance -= amount;
        System.out.println("💸 " + accountHolder + " withdrew $" + amount);
    }

    @Override
    public double getBalance() {
        return balance;
    }

    @Override
    public void printStatement() {
        System.out.println("=== Checking Account ===");
        System.out.println("Holder: " + accountHolder);
        System.out.println("Balance: $" + balance);
    }
}
  • Interface BankAccount 
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package bankingsystem;

//this interface can implements to class
public interface BankAccount {

    void deposit(double amount); //deposit money
    void withdraw(double amount); // withdraw money 
    double getBalance(); //balance check 
    void printStatement(); //print statements

}
  • Interface InterestEarning
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package bankingsystem;


// this interface can implements to class
public interface InteresEarning {
    void applyInterest();
    double getInterestRate();

}

Note

Use Command + Shift + V paste the code from Eclipse

References