Resource Safety, Unit Testing, and Maintainable Project Structure

Required tools

• JDK (21 LTS or newer)
• Apache Maven (3.9+ (or Gradle 8+, adjust commands accordingly))
• A terminal (any)

Setup

1. Create a Maven project: `mvn archetype:generate -DgroupId=com.visaspark.library -DartifactId=library-loans-tested -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false`.
2. Add the JUnit 5 dependency to pom.xml (junit-jupiter, test scope) and configure maven-surefire-plugin for JUnit 5, or use your own build setup if you're not using Maven.
3. Place production classes under src/main/java/com/visaspark/library/, and tests under src/test/java/com/visaspark/library/, mirroring the same package structure.

Project structure

library-loans-tested/
  pom.xml
  src/
    main/java/com/visaspark/library/
      Borrowable.java
      Book.java
      LoanValidationException.java
      Loan.java
    test/java/com/visaspark/library/
      LoanTest.java

Starter files

package com.visaspark.library;

public class LoanValidationException extends RuntimeException {
    // TODO: add a constructor(String message) that calls super(message)
}

package com.visaspark.library;

public class Loan {
    private final Borrowable item;
    private final String borrowerName;

    public Loan(Borrowable item, String borrowerName) {
        // TODO: throw LoanValidationException if item is null or borrowerName is null/blank
        // TODO: otherwise assign both fields
        this.item = item;
        this.borrowerName = borrowerName;
    }

    public String summary() {
        return borrowerName + " borrowed " + item.describe();
    }
}

package com.visaspark.library;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class LoanTest {

    // TODO: @Test method: constructing a Loan with a valid item and borrower name succeeds,
    // and summary() contains the borrower's name.

    // TODO: @Test method: constructing a Loan with a null item throws LoanValidationException
    // (use assertThrows).

    // TODO: @Test method: constructing a Loan with a blank borrower name ("", or "   ")
    // throws LoanValidationException.
}

Requirements

• LoanValidationException extends RuntimeException with a message-taking constructor.
• Loan's constructor validates both arguments and throws LoanValidationException with a specific, useful message for each invalid case.
• LoanTest has at least 3 @Test methods: one success case and at least two distinct failure cases, each asserting the correct exception type.
• All tests pass when run via the build tool's test command.

Commands to run

• Run the full test suite

  mvn test

• Compile and package (also re-runs tests)

  mvn package

Expected behavior

`mvn test` compiles the project and runs LoanTest, reporting all tests passed (BUILD SUCCESS), with the test output showing at least 3 tests executed and 0 failures.

Verify it yourself

• mvn test

  Expected: BUILD SUCCESS, with a summary line reporting 3 or more tests run and 0 failures/errors

• mvn test -Dtest=LoanTest#aTestMethodNameYouWrote

  Expected: Runs just that one test method and reports it passing

Troubleshooting

• `package org.junit.jupiter.api does not exist` — The JUnit 5 dependency is missing from pom.xml, or the surefire plugin isn't configured for JUnit 5 — check both.
• Tests run but assertThrows reports the wrong exception type or none at all — Confirm the validation check happens BEFORE any field assignment in the constructor, and that it actually throws LoanValidationException, not a different exception type or a silent return.
• `BUILD FAILURE` with a NullPointerException inside a test, not an assertion failure — This usually means a test called a method on an object that was never constructed successfully — check the test's arrange step ran without throwing when it wasn't supposed to.

Extension challenge

Add a fourth test asserting that the exception's message actually contains a useful, specific string (e.g. assertTrue(exception.getMessage().contains("borrowerName"))) by capturing the thrown exception from assertThrows' return value, rather than only checking the exception type.