Java Class Design

Filesystem

BankingServiceImpl class implements BankingService interface

implement: getBalance(), withdraw(), + deposit()

Context: A BankingServiceImpl class needed to implement the BankingService interface, which defines a contract for checking a balance, withdrawing, and depositing money against an array of account balances (accounts[accountId]).

What needed to be built: All three interface methods, starting from stubs that threw UnsupportedOperationException. The final requirement (revealed by a failing test called "should perform all operations concurrently") was that the implementation had to be thread-safe under concurrent calls from multiple threads.

Solution explanation: The constructor stores the passed-in array as instance state. getBalance reads a balance after validating the id; withdraw/deposit mutate the array after validating the id and amount. The key fix was marking getBalance, withdraw, and deposit all synchronized. Without this, accounts[accountId] -= amount is really three separate steps (read, subtract, write) that can interleave across threads, causing lost updates (a race condition) — e.g. two simultaneous withdrawals could both read the same starting balance and one update would silently overwrite the other. synchronized also matters on the read-only getBalance, since without it a thread isn't guaranteed to see another thread's most recent write (a visibility issue, not just a race). All three synchronized methods share one lock on this, so operations on different accounts block each other too — a trade-off for simplicity over per-account locking.

• BankingService Interface

public class BankingServiceImpl implements BankingService {

  private final double[] accounts;
  public BankingServiceImpl(double[] accounts) {
    this.accounts = accounts;
  }

  @Override
  public synchronized double getBalance(int accountId) {
    validateAccount(accountId);
    return accounts[accountId];
  }

  @Override
  public synchronized void withdraw(int accountId, double amount) {
    validateAccount(accountId);
    if(amount <= 0){
      throw new IllegalArgumentException();
    }
    if (amount > accounts[accountId]){
      throw new IllegalArgumentException();
    }
    accounts[accountId] -= amount;
  }

  @Override
  public synchronized void deposit(int accountId, double amount) {
    validateAccount(accountId);
    if (amount <= 0){
      throw new IllegalArgumentException();
    }
    accounts[accountId] += amount;
  }
  
  private void validateAccount(int accountId){
    if (accountId < 0 || accountId >= accounts.length){
      throw new IllegalArgumentException("invalid account id: " + accountId);
    }
  }
}

Angular Form Component

Frontend

handle form submission + management in Angular TS

implement: addBook(), deleteBook()

Context: A BookManagerComponent rendered a reactive form for adding books and a list of existing books with delete buttons, backed by a Puppeteer/Chai test suite that drives the actual DOM (typing into the input, clicking buttons, and asserting on the rendered book list).

What needed to be built: The addBook() and deleteBook(bookId) component methods (starting as empty stubs), plus wiring the template to actually call them — the template alone doesn't make methods run.

Solution explanation: addBook() guards on bookForm.invalid, reads and trims the title from bookForm.value.title, pushes a new {id, name} object using an incrementing nextId counter, then resets the form. deleteBook(bookId) reassigns books to a filtered array excluding the matching id (reassigning rather than mutating in place helps Angular's change detection pick up the update reliably). The first attempt only passed 2 of the tests (page load, and empty-form-not-submitting) because the template was never actually connected to this logic: the <form> was missing (ngSubmit)="addBook()", the title <input> was missing formControlName="title" (so bookForm.value.title was undefined, which would have thrown on .trim()), and the delete <button> was missing (click)="deleteBook(book.id)". Adding those three template bindings let the existing TypeScript logic actually run.

• test.js

main.ts

const { Component, VERSION } = ng.core;
const { FormGroup, FormControl, Validators } = ng.forms;

@Component({
  selector: '#app',
  template: `
  <div class="content-container">
    <h2>Add new book</h2>
    <form class="add-book-form" [formGroup]="bookForm">
      <input
        class="add-book-form__title-input"
        placeholder="Book title"
        type="text"
      />
      <input type="submit" class="add-book-form__btn" value="Add Book" />
    </form>
    
    <h2>Your books</h2>
    <div class="book-list" *ngFor="let book of books">
      <div class="book-list__item">
        <span class="book-list__title">{{book.name}}</span>
        <button
          type="button"
          class="book-list__delete-btn"
        >Delete</button>
      </div>
    </div>
   </div>
  `
})
class BookManagerComponent {
  books = [{id: 1, name: 'First book'}];
  private nextId = 2;
  bookForm = new FormGroup({
    title: new FormControl('', Validators.required)
  });

  addBook() {
    if (this.bookForm.invalid){
      return;
    }
    const title = this.bookForm.value.title.trim();
    if(!title){
      return;
    }
    this.books.push({id: this.nextId++, name:title });
    this.bookForm.reset();
  }

  deleteBook(bookId) {
    this.books = this.books.filter(book => book.id != bookId);
  }
}

// main.js
const { BrowserModule } = ng.platformBrowser;
const { NgModule } = ng.core;
const { CommonModule } = ng.common;
const { ReactiveFormsModule } = ng.forms;

@NgModule({
  imports: [
    BrowserModule,
    CommonModule,
    ReactiveFormsModule
  ],
  declarations: [BookManagerComponent],
  bootstrap: [BookManagerComponent],
  providers: []
})
class BookManagerModule {}

const { platformBrowserDynamic } = ng.platformBrowserDynamic;

platformBrowserDynamic()
  .bootstrapModule(BookManagerModule)
  .catch(err => console.error(err));

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