Top Java 17 Interview Questions

Java 17, released in September 2021, is a long-term support (LTS) release and brings a variety of new features, enhancements, and improvements to the language and the platform. As Java 17 is a significant release, it is essential for developers to be familiar with its new features and changes, especially when preparing for job interviews. This blog post covers some of the most commonly asked Java 17 interview questions to help you prepare effectively.

1. What are the new features introduced in Java 17?

Answer: Java 17 introduced several new features and enhancements, including:

• Pattern Matching for instanceof (JEP 394): Simplifies the common pattern of combining instanceof and type casting.
• Sealed Classes (JEP 409): Provides a more declarative way to restrict which classes or interfaces can extend or implement them.
• Enhanced Pseudo-Random Number Generators (JEP 356): Adds new interfaces and implementations for pseudorandom number generators (PRNGs).
• Deprecation of the Applet API (JEP 398): Marks the Applet API for future removal.
• Removal of the Experimental AOT and JIT Compiler (JEP 410): Removes the experimental ahead-of-time (AOT) and just-in-time (JIT) compiler.
• Strong Encapsulation of JDK Internals (JEP 403): Strongly encapsulates JDK internals, except for critical internal APIs such as sun.misc.Unsafe.
• Foreign Function & Memory API (Incubator, JEP 412): Introduces an API for interacting with code and data outside of the Java runtime.
• Vector API (Second Incubator, JEP 414): Introduces an API to express vector computations.

2. What are Sealed Classes in Java 17, and how do they work?

Answer: Sealed classes and interfaces restrict which other classes or interfaces may extend or implement them. This feature enhances the ability to model data and provides more control over the class hierarchy.

Example:

public abstract sealed class Shape permits Circle, Rectangle {
    // Class implementation
}

public final class Circle extends Shape {
    // Class implementation
}

public final class Rectangle extends Shape {
    // Class implementation
}

Explanation:

• sealed: Used to declare a sealed class or interface.
• permits: Specifies the classes or interfaces that are allowed to extend or implement the sealed class/interface.
• final: Used to declare that no further subclassing is allowed.
• non-sealed: Used to declare that further subclassing is allowed.

3. Explain Pattern Matching for instanceof in Java 17.

Answer: Pattern Matching for instanceof simplifies the common pattern of combining instanceof and type casting. It eliminates the need for explicit casting by introducing a pattern variable.

Example:

public class PatternMatchingExample {
    public static void main(String[] args) {
        Object obj = "Hello, Java 17";

        if (obj instanceof String s) {
            System.out.println(s.toUpperCase());
        }
    }
}

Explanation:

• instanceof String s: Checks if obj is an instance of String and simultaneously declares and initializes a pattern variable s.

4. What is the Foreign Function & Memory API in Java 17?

Answer: The Foreign Function & Memory API, introduced as an incubator feature in Java 17, provides an API for interacting with code and data outside of the Java runtime. It enables Java programs to call native libraries and access native data without the need for JNI.

Example:

import jdk.incubator.foreign.*;

public class ForeignFunctionExample {
    public static void main(String[] args) {
        try (MemorySegment segment = MemorySegment.allocateNative(100)) {
            MemoryAccess.setIntAtOffset(segment, 0, 42);
            int value = MemoryAccess.getIntAtOffset(segment, 0);
            System.out.println("Value: " + value);
        }
    }
}

Explanation:

• MemorySegment: Represents a contiguous region of memory.
• MemoryAccess: Provides static methods to access memory segments.

5. How has the Random Number Generation been enhanced in Java 17?

Answer: Java 17 introduced new interfaces and implementations for pseudorandom number generators (PRNGs) as part of JEP 356. This enhancement provides more flexibility and better support for stream-based programming.

Example:

import java.util.random.RandomGenerator;
import java.util.random.RandomGeneratorFactory;

public class RandomGeneratorExample {
    public static void main(String[] args) {
        RandomGenerator random = RandomGeneratorFactory.of("L64X128MixRandom").create();
        random.ints(5).forEach(System.out::println);
    }
}

Explanation:

• RandomGenerator: A new interface for random number generators.
• RandomGeneratorFactory: Provides methods to create instances of RandomGenerator.

6. What is the Vector API in Java 17?

Answer: The Vector API, introduced as a second incubator feature in Java 17, provides an API to express vector computations. It enables developers to write complex vector operations that can be optimized at runtime.

Example:

import jdk.incubator.vector.*;

public class VectorAPIExample {
    public static void main(String[] args) {
        VectorSpecies<Integer> SPECIES = IntVector.SPECIES_PREFERRED;
        int[] a = {1, 2, 3, 4};
        int[] b = {5, 6, 7, 8};
        int[] c = new int[4];

        IntVector va = IntVector.fromArray(SPECIES, a, 0);
        IntVector vb = IntVector.fromArray(SPECIES, b, 0);
        IntVector vc = va.add(vb);
        vc.intoArray(c, 0);

        for (int i : c) {
            System.out.println(i);
        }
    }
}

Explanation:

• VectorSpecies: Describes the species of vector, which includes the element type and vector length.
• IntVector: A concrete class that provides vector operations for integers.

7. What is the purpose of deprecating the Applet API in Java 17?

Answer: The Applet API has been deprecated for removal in Java 17 as part of JEP 398. The use of applets has declined significantly, and modern web technologies such as HTML5, CSS, and JavaScript have replaced them. Deprecating the Applet API simplifies the Java platform by removing obsolete features.

8. How do you enable strong encapsulation of JDK internals in Java 17?

Answer: Java 17 enforces strong encapsulation of JDK internals by default as part of JEP 403. This means that access to internal APIs is restricted unless explicitly allowed. You can enable access to internal APIs using the --add-opens JVM option.

Example:

java --add-opens java.base/java.lang=ALL-UNNAMED -jar myapp.jar

Explanation:

• --add-opens: Opens a module for deep reflection by other modules.

9. What are the improvements in garbage collection in Java 17?

Answer: Java 17 includes several improvements in garbage collection, such as:

• Improved G1 garbage collector performance and scalability.
• Enhanced ZGC with lower latency and better support for large heaps.
• Continued enhancements to the Shenandoah garbage collector.

10. How do you use the new text blocks in Java 17?

Answer: Text blocks, introduced in Java 13 and finalized in Java 15, allow you to define multi-line string literals more easily and readably.

Example:

public class TextBlocksExample {
    public static void main(String[] args) {
        String textBlock = """
            {
                "name": "Java 17",
                "type": "LTS"
            }
            """;

        System.out.println(textBlock);
    }
}

Explanation:

• Triple quotes ("""): Delimit the beginning and end of a text block.
• Text blocks preserve the indentation and line breaks, making it easier to work with multi-line strings.

Conclusion

Java 17 is a significant release with many new features and enhancements that improve developer productivity and the performance of Java applications. Understanding these features is crucial for any Java developer, especially when preparing for an interview. This blog post covered some of the most commonly asked Java 17 interview questions, helping you prepare effectively for your next interview. By mastering these concepts, you will be well-equipped to tackle any Java 17-related challenges you may encounter.