Java 8 introduced one of the most important changes to interfaces:
- Default methods
- Static methods inside interfaces
- Multiple inheritance conflict resolution
- Functional interfaces
- Method references
- Date-Time API
- Lambda exception handling
Q1. What is the output?
interface Vehicle {
default void start() {
System.out.print(“Vehicle “);
}
}
class Car implements Vehicle {
}
public class Test {
public static void main(String[] args) {
new Car().start();
}
}
A. Vehicle
B. Car
C. Vehicle Car
D. Compilation Error
Q2. What is the output?
interface A {
default void show() {
System.out.println(“A”);
}
}
class Test implements A {
public static void main(String[] args) {
new Test().show();
}
}
A. A
B. Test
C. Exception
D. Compilation Error
Q3. What is the output?
interface Demo {
static void display() {
System.out.println(“Java”);
}
}
public class Test {
public static void main(String[] args) {
Demo.display();
}
}
A. Java
B. Demo
C. Exception
D. Compilation Error
Q4. What happens?
interface A {
default void show() {
System.out.println(“A”);
}
}
interface B {
default void show() {
System.out.println(“B”);
}
}
class Test implements A, B {
}
A. Prints A
B. Prints B
C. Compilation Error
D. Runtime Exception
Q5. What is the output?
@FunctionalInterface
interface Demo {
void show();
}
Demo d =
() -> System.out.println(“Java”);
d.show();
A. Java
B. Demo
C. Exception
D. Error
Q6. What is the output?
List<String> list =
Arrays.asList(“A”,”B”);
list.forEach(System.out::print);
A. AB
B. A B
C. [A, B]
D. Exception
Q7. Insert missing code.
interface Vehicle {
__________
}
Desired behavior:
Vehicle
A.
default void start() {
System.out.println(“Vehicle”);
}
B.
void start();
C.
abstract void start();
D.
static start();
Q8. Exception Question
try {
Runnable r =
() -> {
throw new RuntimeException();
};
r.run();
}
catch(Exception e) {
System.out.println(“Exception”);
}
A. RuntimeException
B. Exception
C. Compilation Error
D. Nothing
Q9. Date-Time API
LocalDate d =
LocalDate.of(2025,6,20);
System.out.println(
d.plusMonths(2));
A. 2025-08-20
B. 2025-07-20
C. Exception
D. 2025-06-22
Q10. Which annotation indicates a functional interface?
A. @Override
B. @FunctionalInterface
C. @Lambda
D. @Interface
Answers – SET 15
| Q | Answer | Explanation |
| 1 | A | The implementing class inherits the default method from the interface. Since Car does not override start(), Vehicle.start() executes. |
| 2 | A | Default methods become part of the implementing class if not overridden. |
| 3 | A | Static interface methods belong to the interface and must be called using InterfaceName.method(). |
| 4 | C | Java cannot decide which default method to inherit. The class must override show(). |
| 5 | A | A lambda expression provides the implementation for the single abstract method. |
| 6 | A | Method reference prints both elements without spaces. |
| 7 | A | Default methods allow interfaces to contain behavior. |
| 8 | B | RuntimeException propagates from the lambda and is caught by the catch block. |
| 9 | A | LocalDate is immutable. plusMonths() returns a new object. |
| 10 | B | This annotation tells the compiler that the interface must contain exactly one abstract method. |
SET 16 – Parallel Streams and Collectors
Q1. What is the output?
List<Integer> list =
Arrays.asList(1,2,3);
System.out.println(
list.stream()
.reduce(0, Integer::sum));
A. 3
B. 6
C. 9
D. Exception
Q2. What is the output?
List<String> list =
Arrays.asList(“A”,”B”,”C”);
String s =
list.stream()
.collect(Collectors.joining());
System.out.println(s);
A. ABC
B. A B C
C. [A,B,C]
D. Exception
Q3. What is the output?
List<Integer> list =
Arrays.asList(10,20,30);
System.out.println(
list.parallelStream()
.count());
A. 0
B. 1
C. 3
D. Exception
Q4. What is the output?
List<Integer> list =
Arrays.asList(1,2,3,4);
System.out.println(
list.stream()
.max(Integer::compareTo)
.get());
A. 1
B. 2
C. 3
D. 4
Q5. What is the output?
List<Integer> list =
Arrays.asList(10,20,30);
System.out.println(
list.stream()
.min(Integer::compareTo)
.get());
A. 10
B. 20
C. 30
D. Exception
Q6. What is the output?
List<String> list =
Arrays.asList(“java”,”spring”);
list.stream()
.map(String::toUpperCase)
.forEach(System.out::print);
A. javaspring
B. JAVASPRING
C. JAVA SPRING
D. Exception
Q7. Insert missing code.
List<Integer> list =
Arrays.asList(1,2,3);
___________
Desired output: 6
A.
System.out.println(
list.stream()
.reduce(0,Integer::sum));
B.
list.sum();
C.
sum(list);
D.
list.total();
Q8. Exception Question
Stream<Integer> s =
Stream.of(1,2,3);
s.count();
s.forEach(System.out::println);
A. 3
B. Exception
C. 0
D. Compilation Error
Q9. Date-Time API
LocalDateTime dt =
LocalDateTime.of(
2025,1,1,10,30);
System.out.println(
dt.plusHours(5));
A. 2025-01-01T15:30
B. 2025-01-01T10:30
C. Exception
D. 2025-01-01T05:30
Q10. Which collector converts a stream into a List?
A. Collectors.sum()
B. Collectors.toList()
C. Collectors.list()
D. Collectors.array()
Answers – SET 16
| Q | Answer | Explanation |
| 1 | B | reduce() starts with identity value 0 and adds all elements: 0+1+2+3 = 6. |
| 2 | A | Collectors.joining() concatenates all strings without separators. |
| 3 | C | parallelStream() changes execution strategy but not the result. |
| 4 | D | max() returns the largest value wrapped inside Optional. |
| 5 | A | min() returns the smallest element. |
| 6 | B | map() transforms each string before printing. |
| 7 | A | reduce() is commonly used to aggregate values in streams. |
| 8 | B | Streams can only have one terminal operation. After count(), the stream is closed. |
| 9 | A | LocalDateTime is immutable. plusHours() returns a new object. |
| 10 | B | Collectors.toList() is the standard collector for producing lists. |
SET 17 – Advanced Streams, Collectors and Date-Time API
Q1. What is the output?
List<Integer> list =
Arrays.asList(1,2,3,4,5);
long count =
list.stream()
.filter(x -> x > 2)
.count();
System.out.println(count);
A. 2
B. 3
C. 4
D. 5
Q2. What is the output?
List<String> list =
Arrays.asList(“A”,”B”,”C”);
System.out.println(
list.stream()
.collect(Collectors.joining(“,”)));
A. ABC
B. A,B,C
C. [A,B,C]
D. Exception
Q3. What is the output?
List<Integer> list =
Arrays.asList(10,20,30);
System.out.println(
list.stream()
.findFirst()
.get());
A. 10
B. 20
C. 30
D. Exception
Q4. What is the output?
List<Integer> list =
Arrays.asList(10,20,30);
System.out.println(
list.stream()
.anyMatch(x -> x > 25));
A. true
B. false
C. Exception
D. null
Q5. What is the output?
List<Integer> list =
Arrays.asList(1,2,3);
System.out.println(
list.stream()
.allMatch(x -> x > 0));
A. true
B. false
C. Exception
D. Error
Q6. What is the output?
List<String> list =
Arrays.asList(“java”,”spring”);
System.out.println(
list.stream()
.map(String::length)
.reduce(0,Integer::sum));
A. 10
B. 11
C. 12
D. Exception
Q7. Insert missing code.
List<String> list =
Arrays.asList(“java”,”spring”);
____________
Desired output:
[4, 6]
A.
System.out.println(
list.stream()
.map(String::length)
.collect(Collectors.toList()));
B.
list.length();
C.
collect();
D.
map();
Q8. Exception Question
Optional<String> op =
Optional.empty();
System.out.println(op.get());
A. null
B. NoSuchElementException
C. NullPointerException
D. IOException
Q9. Date-Time API
Period p =
Period.ofDays(10);
LocalDate d =
LocalDate.of(2025,1,1);
System.out.println(
d.plus(p));
A. 2025-01-11
B. 2025-01-10
C. Exception
D. 2025-01-01
Q10. Which collector groups data?
A. Collectors.toList()
B. Collectors.joining()
C. Collectors.groupingBy()
D. Collectors.counting()
Answers – SET 17
| Q | Answer | Explanation |
| 1 | B | Three elements (3,4,5) satisfy the condition x > 2. |
| 2 | B | joining(”,”) inserts commas between values. |
| 3 | A | findFirst() returns an Optional containing the first element. |
| 4 | A | 30 satisfies the condition x > 25, so anyMatch returns true. |
| 5 | A | All values are greater than zero. |
| 6 | A | Lengths are 4 and 6. Their sum is 10. |
| 7 | A | map() transforms strings to lengths and collect() gathers results into a list. |
| 8 | B | Calling get() on an empty Optional throws NoSuchElementException. |
| 9 | A | Period adds 10 days to the LocalDate. |
| 10 | C | groupingBy() is commonly used to group stream elements by a key. |
These three sets complete most important Java 8 interview topics:
✅ Lambdas
✅ Functional Interfaces
✅ Predicate, Function, Consumer, Supplier
✅ Streams
✅ Intermediate Operations
✅ Terminal Operations
✅ Optional
✅ Method References
✅ Default Methods
✅ Static Interface Methods
✅ Parallel Streams
✅ Collectors
✅ Reduce
✅ Date-Time API
✅ Exception Handling
✅ Missing Code Questions
✅ Output Prediction Questions
