Java Notes

Core Java Interview Questions

40+ Core Java interview questions with detailed answers covering JVM, data types, strings, wrapper classes, memory management, and Java fundamentals.

1. What is Java and why is it platform independent?

Answer: Java is a high-level, object-oriented programming language. It achieves platform independence through the JVM (Java Virtual Machine). Java source code is compiled into bytecode (.class file), which runs on any platform that has a JVM installed.

Code example

Source.java → javac → Source.class (bytecode) → JVM → Machine Code

The JVM acts as an abstraction layer between Java bytecode and the underlying operating system.

3. Explain JVM memory areas.

Heap	Objects, arrays (GC here)
Stack	Method frames, locals
Method Area	Class metadata, static
PC Register	Current instruction
Native Stack	Native method calls

Heap: Shared among all threads; stores all objects
Stack: Per-thread; stores method frames, local variables, references
Method Area: Class-level data, static variables, constant pool

4. What are the differences between == and .equals()?

Feature	==	.equals()
Compares	Reference (memory address)	Content/Value
For primitives	Compares values	Cannot be used
For objects	Checks same object	Checks logical equality

java example

String s1 = new String("hello");
String s2 = new String("hello");
System.out.println(s1 == s2);       // false (different objects)
System.out.println(s1.equals(s2));  // true (same content)

String s3 = "hello";
String s4 = "hello";
System.out.println(s3 == s4);       // true (String pool)

Output

false (different objects)
true (same content)
true (String pool)

5. What is the String Pool?

The String Pool (String Intern Pool) is a special memory area in the heap where Java stores string literals. When you create a string literal, Java checks the pool first:

java example

String a = "Java";  // Goes to pool
String b = "Java";  // Reuses from pool
String c = new String("Java");  // Creates new object in heap

System.out.println(a == b);  // true (same pool reference)
System.out.println(a == c);  // false (pool vs heap)
System.out.println(a == c.intern());  // true (intern returns pool ref)

Output

true (same pool reference)
false (pool vs heap)
true (intern returns pool ref)

6. Why is String immutable in Java?

Security: Strings used in network connections, class loading, etc.
String Pool: Immutability enables safe sharing in pool
Thread Safety: Immutable objects are inherently thread-safe
Hashcode Caching: Hash can be cached since string won't change
Class Loading: Class names are strings; mutation would be a security risk

7. Difference between String, StringBuilder, and StringBuffer?

Feature	String	StringBuilder	StringBuffer
Mutability	Immutable	Mutable	Mutable
Thread-safe	Yes (immutable)	No	Yes (synchronized)
Performance	Slow for concatenation	Fastest	Slower than StringBuilder
Use when	Value won't change	Single-threaded modification	Multi-threaded modification

java example

// String — creates new object each time
String s = "Hello";
s = s + " World";  // Original "Hello" is now garbage

// StringBuilder — modifies same object
StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");  // Same object modified

8. What is autoboxing and unboxing?

Autoboxing: Automatic conversion from primitive to wrapper class. Unboxing: Automatic conversion from wrapper to primitive.

java example

// Autoboxing
int num = 10;
Integer obj = num;  // int → Integer (autoboxing)

// Unboxing
Integer obj2 = Integer.valueOf(20);
int num2 = obj2;  // Integer → int (unboxing)

// Trap!
Integer a = 127;
Integer b = 127;
System.out.println(a == b);  // true (cached range: -128 to 127)

Integer c = 128;
Integer d = 128;
System.out.println(c == d);  // false (beyond cache range)

Output

true (cached range: -128 to 127)
false (beyond cache range)

9. What is the difference between final, finally, and finalize?

Keyword	Type	Purpose
`final`	Modifier	Variable (constant), method (no override), class (no inherit)
`finally`	Block	Always executes after try-catch (cleanup)
`finalize`	Method	Called by GC before object destruction (deprecated)

java example

final int MAX = 100;           // Cannot reassign
final class Utility {}          // Cannot extend
final void display() {}         // Cannot override

try { ... } 
catch (Exception e) { ... }
finally { connection.close(); } // Always runs

10. What is the difference between throw and throws?

Feature	throw	throws
Purpose	Actually throws exception	Declares possible exception
Location	Inside method body	In method signature
Count	One exception at a time	Multiple comma-separated

java example

// throw — creates and throws exception
if (age < 0) {
    throw new IllegalArgumentException("Age cannot be negative");
}

// throws — declares that method might throw
public void readFile(String path) throws IOException, FileNotFoundException {
    // code that may throw these exceptions
}

11. Explain checked vs unchecked exceptions.

Feature	Checked	Unchecked
Checked at	Compile time	Runtime
Must handle	Yes (try-catch or throws)	No
Extends	Exception	RuntimeException
Examples	IOException, SQLException	NullPointerException, ArrayIndexOutOfBounds

12. What is the difference between abstract class and interface?

Feature	Abstract Class	Interface (Java 8+)
Methods	Abstract + concrete	Abstract + default + static
Variables	Any type	Only public static final
Constructor	Yes	No
Inheritance	Single	Multiple
Access modifiers	All	Public only (methods)
Use when	IS-A relationship	CAN-DO capability

13. What is method overloading vs overriding?

Feature	Overloading	Overriding
Class	Same class	Parent-child classes
Method name	Same	Same
Parameters	Different	Same
Return type	Can differ	Same or covariant
Access modifier	Can differ	Same or wider
Static methods	Can be overloaded	Cannot be overridden
Binding	Compile-time (static)	Runtime (dynamic)

14. What is the `static` keyword?

Example

public class Counter { static int count = 0; // Class variable (shared) int instanceVar = 0; // Instance variable (per object) static void increment() { // Class method count++; // instanceVar++; ❌ Cannot access instance var from static } static { // Static block (runs once on class loading) System.out.println("Class loaded"); } }

Output

Class loaded

Static members belong to the class, not to any instance.

15. Explain pass-by-value in Java.

Java is always pass-by-value. However:

For primitives: the value is copied
For objects: the reference (memory address) is copied (not the object)

java example

void modify(int x, StringBuilder sb) {
    x = 100;              // Changes local copy only
    sb.append(" World");  // Modifies original object (same reference)
    sb = new StringBuilder("New");  // Changes local reference only
}

int num = 5;
StringBuilder text = new StringBuilder("Hello");
modify(num, text);
System.out.println(num);   // 5 (unchanged)
System.out.println(text);  // "Hello World" (modified via reference)

Output

5 (unchanged)
"Hello World" (modified via reference)

16. What is garbage collection?

GC automatically reclaims memory from unreachable objects. You cannot force GC, only request it:

java example

System.gc();        // Request (not guaranteed)
Runtime.getRuntime().gc();  // Same thing

GC Algorithms: Serial GC, Parallel GC, G1 GC, ZGC

An object becomes eligible for GC when no live reference points to it.

17. What are access modifiers?

Modifier	Class	Package	Subclass	World
public	✅	✅	✅	✅
protected	✅	✅	✅	❌
default	✅	✅	❌	❌
private	✅	❌	❌	❌

18. What is a ClassLoader?

ClassLoader loads classes into JVM memory. Three built-in class loaders:

Bootstrap ClassLoader — Loads core Java classes (java.lang.*)
Extension ClassLoader — Loads extension classes (javax.*)
Application ClassLoader — Loads your application classes

They follow delegation hierarchy — child delegates to parent first.

19. What is a marker interface?

A marker interface has NO methods. It "marks" a class for special treatment:

java example

public interface Serializable {}  // Marks class as serializable
public interface Cloneable {}     // Marks class as cloneable

The JVM/framework checks instanceof to determine behavior.

20. What is the difference between Array and ArrayList?

Feature	Array	ArrayList
Size	Fixed	Dynamic
Type	Primitives + Objects	Objects only
Generics	No	Yes
Performance	Faster (no overhead)	Slightly slower
Methods	.length	.size(), add(), remove()
Multi-dimensional	Yes	Not directly

21. What is type casting in Java?

java example

// Widening (implicit) — smaller to larger
int i = 100;
long l = i;       // int → long (automatic)
double d = l;     // long → double (automatic)

// Narrowing (explicit) — larger to smaller
double x = 9.78;
int y = (int) x;  // double → int (manual cast, loses .78)

// Object casting
Animal animal = new Dog();       // Upcasting (automatic)
Dog dog = (Dog) animal;          // Downcasting (explicit, needs instanceof check)

22. Explain the `this` keyword.

java example

public class Student {
    private String name;
    
    public Student(String name) {
        this.name = name;        // Distinguish field from parameter
    }
    
    public Student() {
        this("Unknown");         // Call another constructor
    }
    
    public Student getThis() {
        return this;             // Return current object
    }
}

23. What is the `super` keyword?

java example

class Parent {
    int x = 10;
    Parent(int x) { this.x = x; }
    void display() { System.out.println("Parent"); }
}

class Child extends Parent {
    int x = 20;
    
    Child() {
        super(100);              // Call parent constructor (must be first line)
    }
    
    void display() {
        super.display();         // Call parent method
        System.out.println("Child x=" + this.x + ", Parent x=" + super.x);
    }
}

24. What happens if main() method is missing?

Since Java 7+, the program won't run and throws: Error: Main method not found in class.

Before Java 7, you could use a static block to execute code without main(), but the JVM would still throw an error after the static block ran.

25. Can we override static methods?

No. Static methods belong to the class, not the instance. You can hide a static method in a subclass, but it's not true overriding:

java example

class Parent {
    static void greet() { System.out.println("Parent"); }
}
class Child extends Parent {
    static void greet() { System.out.println("Child"); }  // Method hiding
}

Parent p = new Child();
p.greet();  // "Parent" — resolved at compile time, not runtime

26. What is a singleton class?

A class that allows only ONE instance to be created:

java example

public class Singleton {
    private static volatile Singleton instance;
    
    private Singleton() {}  // Private constructor
    
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

27. What is the difference between Comparable and Comparator?

Feature	Comparable	Comparator
Package	java.lang	java.util
Method	`compareTo(T o)`	`compare(T o1, T o2)`
Modifies class	Yes (implements)	No (separate class)
Sorting	Natural order (one way)	Custom order (multiple ways)

28. What is an enum in Java?

java example

public enum Day {
    MONDAY("Start of work week"),
    TUESDAY("Second day"),
    WEDNESDAY("Mid week"),
    THURSDAY("Almost Friday"),
    FRIDAY("TGIF"),
    SATURDAY("Weekend!"),
    SUNDAY("Rest day");
    
    private final String description;
    
    Day(String description) {
        this.description = description;
    }
    
    public String getDescription() { return description; }
}

Enums are type-safe constants with methods and fields.

29. What is a transient variable?

transient prevents a field from being serialized:

java example

public class User implements Serializable {
    private String username;
    private transient String password;  // NOT saved during serialization
}

30. What is the volatile keyword?

volatile ensures a variable is always read from main memory, not from CPU cache:

java example

private volatile boolean running = true; // Visible to all threads immediately

It provides visibility but NOT atomicity. For atomic operations, use AtomicInteger, etc.

31. What is the difference between Iterator and ListIterator?

Feature	Iterator	ListIterator
Direction	Forward only	Forward + Backward
Collections	Any Collection	Only List
Methods	hasNext(), next(), remove()	+ hasPrevious(), previous(), add(), set()
Index	No	nextIndex(), previousIndex()

32. Can a constructor be private?

Yes. Used in:

Singleton pattern
Utility classes (all static methods)
Factory method pattern
Builder pattern

33. What is covariant return type?

A subclass can override a method and return a subtype of the parent's return type:

java example

class Animal { Animal create() { return new Animal(); } }
class Dog extends Animal { Dog create() { return new Dog(); } }  // Covariant

34. What is the difference between Error and Exception?

Feature	Error	Exception
Recovery	Usually not recoverable	Can be handled
Cause	JVM/System issues	Application logic issues
Examples	OutOfMemoryError, StackOverflowError	IOException, NullPointerException
Handling	Should NOT catch	Should catch and handle

35. What are varargs?

java example

public int sum(int... numbers) {  // Variable number of arguments
    int total = 0;
    for (int n : numbers) total += n;
    return total;
}

sum(1, 2);        // Works
sum(1, 2, 3, 4);  // Works
sum();            // Works (empty array)

Rules: Only one vararg per method, must be the last parameter.

36-40. Quick Fire Questions

36. Can we have multiple public classes in one file? No. Only one public class per file, and its name must match the filename.

37. What is the default value of local variables? Local variables have NO default value — must be initialized before use.

38. Can abstract class have constructor? Yes. Called when subclass is instantiated.

39. What is the Object class? Root of all Java classes. Every class implicitly extends java.lang.Object. Methods: toString(), equals(), hashCode(), clone(), getClass(), finalize(), wait(), notify().

40. What is diamond problem? How does Java solve it? When a class inherits from two classes with same method. Java doesn't allow multiple class inheritance. With interfaces (Java 8+ default methods), the class must explicitly override the conflicting method.

Summary

In this chapter, we learned about Core Java Interview Questions in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

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Java Notes

Core Java Interview Questions

40+ Core Java interview questions with detailed answers covering JVM, data types, strings, wrapper classes, memory management, and Java fundamentals.

1. What is Java and why is it platform independent?

Code example

Source.java → javac → Source.class (bytecode) → JVM → Machine Code

The JVM acts as an abstraction layer between Java bytecode and the underlying operating system.

3. Explain JVM memory areas.

Heap	Objects, arrays (GC here)
Stack	Method frames, locals
Method Area	Class metadata, static
PC Register	Current instruction
Native Stack	Native method calls

Heap: Shared among all threads; stores all objects
Stack: Per-thread; stores method frames, local variables, references
Method Area: Class-level data, static variables, constant pool

4. What are the differences between == and .equals()?

Feature	==	.equals()
Compares	Reference (memory address)	Content/Value
For primitives	Compares values	Cannot be used
For objects	Checks same object	Checks logical equality

java example

String s1 = new String("hello");
String s2 = new String("hello");
System.out.println(s1 == s2);       // false (different objects)
System.out.println(s1.equals(s2));  // true (same content)

String s3 = "hello";
String s4 = "hello";
System.out.println(s3 == s4);       // true (String pool)

Output

false (different objects)
true (same content)
true (String pool)

5. What is the String Pool?

The String Pool (String Intern Pool) is a special memory area in the heap where Java stores string literals. When you create a string literal, Java checks the pool first:

java example

String a = "Java";  // Goes to pool
String b = "Java";  // Reuses from pool
String c = new String("Java");  // Creates new object in heap

System.out.println(a == b);  // true (same pool reference)
System.out.println(a == c);  // false (pool vs heap)
System.out.println(a == c.intern());  // true (intern returns pool ref)

Output

true (same pool reference)
false (pool vs heap)
true (intern returns pool ref)

6. Why is String immutable in Java?

Security: Strings used in network connections, class loading, etc.
String Pool: Immutability enables safe sharing in pool
Thread Safety: Immutable objects are inherently thread-safe
Hashcode Caching: Hash can be cached since string won't change
Class Loading: Class names are strings; mutation would be a security risk

7. Difference between String, StringBuilder, and StringBuffer?

Feature	String	StringBuilder	StringBuffer
Mutability	Immutable	Mutable	Mutable
Thread-safe	Yes (immutable)	No	Yes (synchronized)
Performance	Slow for concatenation	Fastest	Slower than StringBuilder
Use when	Value won't change	Single-threaded modification	Multi-threaded modification

java example

// String — creates new object each time
String s = "Hello";
s = s + " World";  // Original "Hello" is now garbage

// StringBuilder — modifies same object
StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");  // Same object modified

8. What is autoboxing and unboxing?

Autoboxing: Automatic conversion from primitive to wrapper class. Unboxing: Automatic conversion from wrapper to primitive.

java example

// Autoboxing
int num = 10;
Integer obj = num;  // int → Integer (autoboxing)

// Unboxing
Integer obj2 = Integer.valueOf(20);
int num2 = obj2;  // Integer → int (unboxing)

// Trap!
Integer a = 127;
Integer b = 127;
System.out.println(a == b);  // true (cached range: -128 to 127)

Integer c = 128;
Integer d = 128;
System.out.println(c == d);  // false (beyond cache range)

Output

true (cached range: -128 to 127)
false (beyond cache range)

9. What is the difference between final, finally, and finalize?

Keyword	Type	Purpose
`final`	Modifier	Variable (constant), method (no override), class (no inherit)
`finally`	Block	Always executes after try-catch (cleanup)
`finalize`	Method	Called by GC before object destruction (deprecated)

java example

final int MAX = 100;           // Cannot reassign
final class Utility {}          // Cannot extend
final void display() {}         // Cannot override

try { ... } 
catch (Exception e) { ... }
finally { connection.close(); } // Always runs

10. What is the difference between throw and throws?

Feature	throw	throws
Purpose	Actually throws exception	Declares possible exception
Location	Inside method body	In method signature
Count	One exception at a time	Multiple comma-separated

java example

// throw — creates and throws exception
if (age < 0) {
    throw new IllegalArgumentException("Age cannot be negative");
}

// throws — declares that method might throw
public void readFile(String path) throws IOException, FileNotFoundException {
    // code that may throw these exceptions
}

11. Explain checked vs unchecked exceptions.

Feature	Checked	Unchecked
Checked at	Compile time	Runtime
Must handle	Yes (try-catch or throws)	No
Extends	Exception	RuntimeException
Examples	IOException, SQLException	NullPointerException, ArrayIndexOutOfBounds

12. What is the difference between abstract class and interface?

Feature	Abstract Class	Interface (Java 8+)
Methods	Abstract + concrete	Abstract + default + static
Variables	Any type	Only public static final
Constructor	Yes	No
Inheritance	Single	Multiple
Access modifiers	All	Public only (methods)
Use when	IS-A relationship	CAN-DO capability

13. What is method overloading vs overriding?

Feature	Overloading	Overriding
Class	Same class	Parent-child classes
Method name	Same	Same
Parameters	Different	Same
Return type	Can differ	Same or covariant
Access modifier	Can differ	Same or wider
Static methods	Can be overloaded	Cannot be overridden
Binding	Compile-time (static)	Runtime (dynamic)

14. What is the `static` keyword?

Example

Output

Class loaded

Static members belong to the class, not to any instance.

15. Explain pass-by-value in Java.

Java is always pass-by-value. However:

For primitives: the value is copied
For objects: the reference (memory address) is copied (not the object)

java example

void modify(int x, StringBuilder sb) {
    x = 100;              // Changes local copy only
    sb.append(" World");  // Modifies original object (same reference)
    sb = new StringBuilder("New");  // Changes local reference only
}

int num = 5;
StringBuilder text = new StringBuilder("Hello");
modify(num, text);
System.out.println(num);   // 5 (unchanged)
System.out.println(text);  // "Hello World" (modified via reference)

Output

5 (unchanged)
"Hello World" (modified via reference)

16. What is garbage collection?

GC automatically reclaims memory from unreachable objects. You cannot force GC, only request it:

java example

System.gc();        // Request (not guaranteed)
Runtime.getRuntime().gc();  // Same thing

GC Algorithms: Serial GC, Parallel GC, G1 GC, ZGC

An object becomes eligible for GC when no live reference points to it.

17. What are access modifiers?

Modifier	Class	Package	Subclass	World
public	✅	✅	✅	✅
protected	✅	✅	✅	❌
default	✅	✅	❌	❌
private	✅	❌	❌	❌

18. What is a ClassLoader?

ClassLoader loads classes into JVM memory. Three built-in class loaders:

Bootstrap ClassLoader — Loads core Java classes (java.lang.*)
Extension ClassLoader — Loads extension classes (javax.*)
Application ClassLoader — Loads your application classes

They follow delegation hierarchy — child delegates to parent first.

19. What is a marker interface?

A marker interface has NO methods. It "marks" a class for special treatment:

java example

public interface Serializable {}  // Marks class as serializable
public interface Cloneable {}     // Marks class as cloneable

The JVM/framework checks instanceof to determine behavior.

20. What is the difference between Array and ArrayList?

Feature	Array	ArrayList
Size	Fixed	Dynamic
Type	Primitives + Objects	Objects only
Generics	No	Yes
Performance	Faster (no overhead)	Slightly slower
Methods	.length	.size(), add(), remove()
Multi-dimensional	Yes	Not directly

21. What is type casting in Java?

java example

// Widening (implicit) — smaller to larger
int i = 100;
long l = i;       // int → long (automatic)
double d = l;     // long → double (automatic)

// Narrowing (explicit) — larger to smaller
double x = 9.78;
int y = (int) x;  // double → int (manual cast, loses .78)

// Object casting
Animal animal = new Dog();       // Upcasting (automatic)
Dog dog = (Dog) animal;          // Downcasting (explicit, needs instanceof check)

22. Explain the `this` keyword.

java example

public class Student {
    private String name;
    
    public Student(String name) {
        this.name = name;        // Distinguish field from parameter
    }
    
    public Student() {
        this("Unknown");         // Call another constructor
    }
    
    public Student getThis() {
        return this;             // Return current object
    }
}

23. What is the `super` keyword?

java example

class Parent {
    int x = 10;
    Parent(int x) { this.x = x; }
    void display() { System.out.println("Parent"); }
}

class Child extends Parent {
    int x = 20;
    
    Child() {
        super(100);              // Call parent constructor (must be first line)
    }
    
    void display() {
        super.display();         // Call parent method
        System.out.println("Child x=" + this.x + ", Parent x=" + super.x);
    }
}

24. What happens if main() method is missing?

Since Java 7+, the program won't run and throws: Error: Main method not found in class.

Before Java 7, you could use a static block to execute code without main(), but the JVM would still throw an error after the static block ran.

25. Can we override static methods?

No. Static methods belong to the class, not the instance. You can hide a static method in a subclass, but it's not true overriding:

java example

class Parent {
    static void greet() { System.out.println("Parent"); }
}
class Child extends Parent {
    static void greet() { System.out.println("Child"); }  // Method hiding
}

Parent p = new Child();
p.greet();  // "Parent" — resolved at compile time, not runtime

26. What is a singleton class?

A class that allows only ONE instance to be created:

java example

public class Singleton {
    private static volatile Singleton instance;
    
    private Singleton() {}  // Private constructor
    
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

27. What is the difference between Comparable and Comparator?

Feature	Comparable	Comparator
Package	java.lang	java.util
Method	`compareTo(T o)`	`compare(T o1, T o2)`
Modifies class	Yes (implements)	No (separate class)
Sorting	Natural order (one way)	Custom order (multiple ways)

28. What is an enum in Java?

java example

public enum Day {
    MONDAY("Start of work week"),
    TUESDAY("Second day"),
    WEDNESDAY("Mid week"),
    THURSDAY("Almost Friday"),
    FRIDAY("TGIF"),
    SATURDAY("Weekend!"),
    SUNDAY("Rest day");
    
    private final String description;
    
    Day(String description) {
        this.description = description;
    }
    
    public String getDescription() { return description; }
}

Enums are type-safe constants with methods and fields.

29. What is a transient variable?

transient prevents a field from being serialized:

java example

public class User implements Serializable {
    private String username;
    private transient String password;  // NOT saved during serialization
}

30. What is the volatile keyword?

volatile ensures a variable is always read from main memory, not from CPU cache:

java example

private volatile boolean running = true; // Visible to all threads immediately

It provides visibility but NOT atomicity. For atomic operations, use AtomicInteger, etc.

31. What is the difference between Iterator and ListIterator?

Feature	Iterator	ListIterator
Direction	Forward only	Forward + Backward
Collections	Any Collection	Only List
Methods	hasNext(), next(), remove()	+ hasPrevious(), previous(), add(), set()
Index	No	nextIndex(), previousIndex()

32. Can a constructor be private?

Yes. Used in:

Singleton pattern
Utility classes (all static methods)
Factory method pattern
Builder pattern

33. What is covariant return type?

A subclass can override a method and return a subtype of the parent's return type:

java example

class Animal { Animal create() { return new Animal(); } }
class Dog extends Animal { Dog create() { return new Dog(); } }  // Covariant

34. What is the difference between Error and Exception?

Feature	Error	Exception
Recovery	Usually not recoverable	Can be handled
Cause	JVM/System issues	Application logic issues
Examples	OutOfMemoryError, StackOverflowError	IOException, NullPointerException
Handling	Should NOT catch	Should catch and handle

35. What are varargs?

java example

public int sum(int... numbers) {  // Variable number of arguments
    int total = 0;
    for (int n : numbers) total += n;
    return total;
}

sum(1, 2);        // Works
sum(1, 2, 3, 4);  // Works
sum();            // Works (empty array)

Rules: Only one vararg per method, must be the last parameter.

36-40. Quick Fire Questions

36. Can we have multiple public classes in one file? No. Only one public class per file, and its name must match the filename.

37. What is the default value of local variables? Local variables have NO default value — must be initialized before use.

38. Can abstract class have constructor? Yes. Called when subclass is instantiated.

Summary

In this chapter, we learned about Core Java Interview Questions in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Core Java Interview Questions.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

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Continue learning this concept

PreviousCollections Interview Questions NextJDBC Interview Questions

1. What is Java and why is it platform independent?

3. Explain JVM memory areas.

4. What are the differences between == and .equals()?

5. What is the String Pool?

6. Why is String immutable in Java?

7. Difference between String, StringBuilder, and StringBuffer?

8. What is autoboxing and unboxing?

9. What is the difference between final, finally, and finalize?

10. What is the difference between throw and throws?

11. Explain checked vs unchecked exceptions.

12. What is the difference between abstract class and interface?

13. What is method overloading vs overriding?

14. What is the static keyword?

15. Explain pass-by-value in Java.

16. What is garbage collection?

17. What are access modifiers?

18. What is a ClassLoader?

19. What is a marker interface?

20. What is the difference between Array and ArrayList?

21. What is type casting in Java?

22. Explain the this keyword.

23. What is the super keyword?

24. What happens if main() method is missing?

25. Can we override static methods?

26. What is a singleton class?

27. What is the difference between Comparable and Comparator?

28. What is an enum in Java?

29. What is a transient variable?

30. What is the volatile keyword?

31. What is the difference between Iterator and ListIterator?

32. Can a constructor be private?

33. What is covariant return type?

34. What is the difference between Error and Exception?

35. What are varargs?

36-40. Quick Fire Questions

Summary

Continue learning this concept

1. What is Java and why is it platform independent?

3. Explain JVM memory areas.

4. What are the differences between == and .equals()?

5. What is the String Pool?

6. Why is String immutable in Java?

7. Difference between String, StringBuilder, and StringBuffer?

8. What is autoboxing and unboxing?

9. What is the difference between final, finally, and finalize?

10. What is the difference between throw and throws?

11. Explain checked vs unchecked exceptions.

12. What is the difference between abstract class and interface?

13. What is method overloading vs overriding?

14. What is the static keyword?

15. Explain pass-by-value in Java.

16. What is garbage collection?

17. What are access modifiers?

18. What is a ClassLoader?

19. What is a marker interface?

20. What is the difference between Array and ArrayList?

21. What is type casting in Java?

22. Explain the this keyword.

23. What is the super keyword?

24. What happens if main() method is missing?

25. Can we override static methods?

26. What is a singleton class?

27. What is the difference between Comparable and Comparator?

28. What is an enum in Java?

29. What is a transient variable?

30. What is the volatile keyword?

31. What is the difference between Iterator and ListIterator?

32. Can a constructor be private?

33. What is covariant return type?

34. What is the difference between Error and Exception?

35. What are varargs?

36-40. Quick Fire Questions

Summary

Continue learning this concept

14. What is the `static` keyword?

22. Explain the `this` keyword.

23. What is the `super` keyword?

14. What is the `static` keyword?

22. Explain the `this` keyword.

23. What is the `super` keyword?