Errors and Testing

try-with-resources

Anything that must be closed, such as files, sockets, or connections, belongs in a try-with-resources header. It is closed automatically when the block exits, even on an exception. Any AutoCloseable works, and multiple resources close in reverse order.

static long countErrors(Path log) throws IOException {
    try (BufferedReader reader = Files.newBufferedReader(log)) {
        return reader.lines()
                .filter(line -> line.contains("ERROR"))
                .count();
    } // reader.close() runs automatically
}

Custom exceptions

Domain-specific exceptions let callers tell failures apart. Extend Exception when callers must plan for the failure, RuntimeException otherwise, and accept a cause so wrapped stack traces are preserved.

public class OrderNotFoundException extends RuntimeException {
    public OrderNotFoundException(long orderId) {
        super("order not found: " + orderId);
    }

    public OrderNotFoundException(long orderId, Throwable cause) {
        super("order not found: " + orderId, cause);
    }
}

Testing with JUnit

JUnit is the standard testing framework for Java, and tests use the Jupiter API introduced with JUnit 5. With Maven, add junit-jupiter as a test-scoped dependency (the previous chapter shows Gradle); test classes go in the same package under src/test/java.

<dependency>
  <groupId>org.junit.jupiter</groupId>
  <artifactId>junit-jupiter</artifactId>
  <version>${junit.version}</version>
  <scope>test</scope>
</dependency>

Annotate test methods with @Test and verify results with the static methods in Assertions. assertThrows checks that an exception is thrown and hands it back, so you can inspect its message too.

package com.example.config;

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

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.ValueSource;

class ConfigLoaderTest {
    @Test
    @DisplayName("a valid port is parsed to a number")
    void parsesValidPort() {
        assertEquals(8080, ConfigLoader.parsePort("8080"));
    }

    @Test
    void rejectsOutOfRangePort() {
        var e = assertThrows(IllegalArgumentException.class,
                () -> ConfigLoader.parsePort("70000"));
        assertTrue(e.getMessage().contains("70000"));
    }

    @ParameterizedTest
    @ValueSource(strings = {"0", "-1", "65536"})
    void rejectsInvalidPorts(String value) {
        assertThrows(IllegalArgumentException.class, () -> ConfigLoader.parsePort(value));
    }
}

Run the suite with mvn test or ./gradlew test. @ParameterizedTest repeats a check over many inputs, and IDEs can run or debug a single test from the editor.

Summary

• Checked exceptions must be caught or declared; unchecked exceptions extend RuntimeException.
• Open anything that needs closing in a try-with-resources statement.
• Give custom exceptions a cause constructor so wrapped failures keep their history.
• Verify behavior with JUnit Jupiter's @Test, assertEquals, and assertThrows, and run tests through your build tool.