Top 10 Java interview questions

1. Explain the difference between == and equals() in Java.

== checks for reference equality, while equals() checks for content equality.

String s1 = new String(“hello”);
String s2 = new String(“hello”);
System.out.println(s1 == s2); // false
System.out.println(s1.equals(s2)); // true

2. How can you make a class immutable in Java?

Make its fields private and final, provide no setter methods, and ensure deep copies in constructors and getters if necessary.

public final class ImmutableClass {
    private final int value;

    public ImmutableClass(int value) {
        this.value = value;
    }

    public int getValue() {
        return value;
    }
}

3. Describe the difference between ArrayList and LinkedList.

ArrayList is backed by an array, while LinkedList is a doubly-linked list.

ArrayList<Integer> arrList = new ArrayList<>();
LinkedList<Integer> linkList = new LinkedList<>();

4. How can you prevent a method from being overridden?

Use the final keyword.

public class MyClass {
    public final void myMethod() {
        // …
    }
}

5. How do you create a thread in Java?

Either by extending the Thread class or implementing the Runnable interface.

class MyThread extends Thread {
    public void run() {
        // …
    }
}
class MyRunnable implements Runnable {
    public void run() {
        // …
    }
}

6. What is the difference between throw and throws in Java?

throw is used to explicitly throw an exception, while throws declares exceptions a method might throw.

public void myMethod() throws MyException {
    if (condition) {
        throw new MyException("Error occurred");
    }
}

7. How can you execute a block of code regardless of whether an exception is thrown?

Use the finally block.

try {
    // risky code
} catch (Exception e) {
    // handle exception
} finally {
    // code to run regardless
}

8. How do you use Java Streams to filter and transform a list?

Use the filter() and map() methods.

List<String> list = Arrays.asList("a", "ab", "abc");
List<String> result = list.stream()
                            .filter(s -> s.length() > 1)
                            .map(s -> s.toUpperCase())
                            .collect(Collectors.toList());

9. How can you ensure thread safety when updating a counter?

Use synchronized or java.util.concurrent utilities.

private int counter = 0;
public synchronized void increment() {
    counter++;
}

10. How can you use Java 8’s Optional to handle potential null values?

Use the Optional class to wrap potential null values and provide alternatives.

Optional<String> opt = Optional.ofNullable(getNullableString());
String result = opt.orElse("default");

In closing, the intricacies of Java backend engineering are both challenging and rewarding. These foundational questions and insights serve as stepping stones for both budding and seasoned developers. As the tech landscape evolves, so should our understanding and adaptability. Here’s to continuous learning and coding excellence.

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