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Tricky Core Java Interview Questions & Answers

This guide addresses advanced scenarios in Core Java, including exception handling, design patterns, and Java 8 features.

1. How can you break a Singleton Design Pattern?​

A Singleton pattern ensures one instance per JVM, but it can be broken using:

  • Reflection: By changing the constructor's accessibility (setAccessible(true)), you can create a second instance.
  • Serialization: If you serialize and then deserialize an object, a new instance is created with a different hashcode.
  • Cloning: If the singleton class implements Cloneable, calling clone() creates a new instance.
  • Executor Service: (Advanced multi-threading scenarios).

2. ClassNotFoundException vs. NoClassDefFoundError​

FeatureClassNotFoundExceptionNoClassDefFoundError
TypeException (Checked)Error
OccurrenceOccurs when an application tries to load a class at runtime using Class.forName() or loadClass(), but the class is not in the classpath.Occurs when the class was present at compile-time but is missing at runtime (e.g., the .class file was deleted).
SourceThrown by the application (reflection methods).Thrown by the Java Runtime System.

3. Which predefined classes can be used as Keys in a Map?​

Apart from String, you can use Integer and other wrapper classes like Byte, Character, Short, Boolean, Long, Double, and Float. These are natural candidates because they are immutable and have well-defined equals() and hashCode() methods.

4. Java 8 Stream Operations on Employee List​

Given a list of Employees, here is how you perform common sorting and filtering tasks:

Sort by Salary (Descending)​

List<Employee> sortedList = empList.stream()
    .sorted((e1, e2) -> (int)(e2.getSalary() - e1.getSalary()))
    .collect(Collectors.toList());

Fetch Top 3 Salaried Employees​

List<Employee> top3 = empList.stream()
    .sorted((e1, e2) -> (int)(e2.getSalary() - e1.getSalary()))
    .limit(3)
    .collect(Collectors.toList());

Fetch Employees with Salary less than the 3rd Highest​

List<Employee> others = empList.stream()
    .sorted((e1, e2) -> (int)(e2.getSalary() - e1.getSalary()))
    .skip(3)
    .collect(Collectors.toList());

5. Why use Character Array over String for Passwords?​

  • Immutability: Strings are immutable. Once created, they stay in the String Constant Pool until Garbage Collection occurs.
  • Security Risk: If a memory dump is taken, sensitive data like passwords stored in the String pool can be extracted and exploited.
  • Mutability (Safe Alternative): A character array (char[]) is mutable. You can overwrite the data (e.g., set every index to a blank space) as soon as you are done with it, ensuring the password no longer exists in memory.