Interview Questions

Java Core

• What are the differences between JRE, JVM, and JDK?
• What access modifiers exist?
• What does the final keyword indicate?
• What are the default values for uninitialized variables?
• What do you know about the main() function?
• What logical operations and operators do you know?
• What is the ternary conditional operator?
• What bitwise operations do you know?
• Where and for what is the abstract modifier used?
• Define the term “interface”. What are the default modifiers for fields and methods in interfaces?
• How does an abstract class differ from an interface? When should an abstract class be used, and when should an interface be used?
• Why are methods not defined in some interfaces at all?
• Why can’t a method in an interface be declared as final?
• What has a higher level of abstraction - a class, an abstract class, or an interface?
• Can an object access a class member declared as private? If so, how?
• What is the order of invocation for constructors and initialization blocks, considering the class hierarchy?
• Why are initialization blocks needed and what types are there?
• To which constructs in Java is the static modifier applicable?
• Why are static initialization blocks used in Java?
• What happens if an exception occurs in an initialization block?
• What exception is thrown when an error occurs in a class initialization block?
• Can a static method be overridden or overloaded?
• Can non-static methods overload static ones?
• Is it possible to narrow the access level or return type when overriding a method?
• In overriding a method, is it possible to change: the access modifier, return type, argument type or their count, argument names or their order; remove, add, or change the order of elements in the throws section?
• How to access overridden methods of a parent class?
• Can a method be declared as both abstract and static at the same time?
• What is the difference between an instance member of a class and a static member of a class?
• Where is the initialization of static/non-static fields allowed?
• What types of classes are there in Java?
• Tell me about nested classes. When are they used?
• What is a “static class”?
• What features exist in the use of nested classes: static and inner? What is the difference between them?
• What is a “local class”? What are its features?
• What are “anonymous classes”? Where are they used?
• How can a nested class access the field of its outer class?
• What is the purpose of the assert operator?
• What are Heap and Stack memory in Java? What is the difference between them?
• Is the statement true that primitive data types are always stored on the stack, and instances of reference data types are on the heap?
• How are variables passed to methods: by value or by reference?
• What is the purpose of a garbage collector?
• How does a garbage collector work?
• What types of garbage collectors are implemented in the HotSpot virtual machine?
• Describe the algorithm of any garbage collector implemented in the HotSpot virtual machine.
• What is a “string pool”?
• What is finalize()? What is it needed for?
• What happens to the garbage collector if the method finalize() takes a significant amount of time, or an exception is thrown during execution?
• What are the differences between final, finally, and finalize()?
• Talk about type casting. What is narrowing and widening of type?
• When could an application throw a ClassCastException?
• What are literals?
• What is autoboxing in Java and what are the rules for wrapping primitive types in wrapper classes?
• What are the features of the String class?
• Why is the String class immutable and final?
• Why is char[] preferred over String for storing a password?
• Why is a string a popular key in HashMap in Java?
• What does the method intern() do in the String class?
• Can strings be used in a switch statement?
• What is the main difference between String, StringBuffer, and StringBuilder?
• What is the Object class? What methods does it have?
• Define the term “constructor”.
• What is a “default constructor”?
• What are the differences between a default constructor, a copy constructor, and a parameterized constructor?
• Where and how can you use a private constructor?
• Tell me about class loaders and dynamic class loading.
• What is Reflection?
• What is the purpose of equals(). How does it differ from the == operation?
• If you want to override equals(), what conditions must be met?
• What are the properties of the equivalence relation generated by equals()?
• Rules for overriding the method Object.equals().
• What is the relationship between hashCode() and equals()?
• If equals() is overridden, are there any other methods that should be overridden?
• What happens if you override equals() without overriding hashCode()? What issues may arise?
• How are the methods hashCode() and equals() implemented in the Object class?
• What is the purpose of the hashCode() method?
• What are the rules for overriding the method Object.hashCode()?
• Are there any recommendations on which fields to use when calculating hashCode()?
• Can different objects have the same hashCode()?
• If a class Point{int x, y;} implements the method equals(Object that) {(return this.x == that.x && this.y == that.y)} but makes the hash code int hashCode() {return x;}, can such points be correctly stored and extracted from a HashSet?
• Can different objects (ref0 != ref1) have ref0.equals(ref1) == true?
• Can different references to the same object (ref0 == ref1) have ref0.equals(ref1) == false?
• Can the equals(Object that) {return this.hashCode() == that.hashCode()} method be implemented like this?
• In equals(), it is required to check that the argument equals(Object that) is of the same type as the object itself. What is the difference between this.getClass() == that.getClass() and that instanceof MyClass?
• Is it possible to implement the equals() method for the MyClass class like this: class MyClass {public boolean equals(MyClass that) {return this == that;}}?
• There is a class Point{int x, y;}. Why is a hash code like 31 * x + y preferable to x + y?
• Tell me about object cloning.
• What is the difference between shallow and deep cloning?
• What cloning method is preferable?
• Why is the clone() method declared in the Object class and not in the Cloneable interface?