Java MCQ Questions – Page 165 of 200

Q: If a concrete (non-abstract) class implements an interface in Java, what is the consequence?

B) It must provide an implementation for all abstract methods declared in the interface.. A concrete class implementing an interface is contractually obligated to provide concrete implementations for all abstract methods declared in that interface, unless it is itself declared abstract.

Q: In a traditional Java `switch` statement, what is the consequence if a `break` statement is omitted from a `case` block?

C) Execution will continue into the next `case` block (fall-through).. Omitting `break` causes 'fall-through,' meaning execution continues to the subsequent `case` blocks until a `break`, `return`, or the end of the `switch` statement is encountered.

Q: What compilation error will occur when compiling the following Java code?

A) Error: incompatible types: bad return type in lambda expression; int cannot be converted to String. The outer lambda `(i) -> ...` is expected to return `Function `, which means the inner lambda `(s) -> s.length()` should return a `String`. However, `s.length()` returns an `int`. This type mismatch between the inner lambda's actual return type (`int`) and the expected return type (`String`) causes a compilation error.

Q: Under what condition might calling the `remove()` method of a `java.util.Iterator` result in an `UnsupportedOperationException`?

B) When the underlying collection does not support the removal of elements, such as an immutable list or a read-only view.. `UnsupportedOperationException` is thrown when an optional operation, like `remove()`, is not supported by the specific collection implementation providing the iterator.

Q: What will be printed to the console when this code runs?

A) In try block. When `System.exit(0)` is called, the Java Virtual Machine is terminated immediately. This prevents the execution of any `finally` blocks or any other code that would normally run after the `try` block. Only the print statement before `System.exit(0)` is executed.

Q: What error occurs when executing the following Java code?

A) java.util.ConcurrentModificationException. Modifying an ArrayList directly (using `list.remove()`) while iterating over it with an enhanced for-loop (which implicitly uses an iterator) causes a `ConcurrentModificationException`. The iterator's internal state becomes inconsistent with the list's actual state.

Q: What does this code print?

A) Violations: 1. The `@Valid` annotation on the `List items` field ensures that each element in the list is validated. `ProductItem` has `@Min(1)` on `quantity`, which is violated by the item with quantity `0`, resulting in one violation.

Q: What compilation error will occur in this Java code?

B) The final local variable 'value' may already have been assigned. The `final` variable `value` is assigned inside the `do` block. Since the `while` condition is not a compile-time constant `false`, the compiler cannot guarantee that `value` will be assigned exactly once, leading to an error indicating it may already have been assigned.

Q: What is the output of this code?

A) AB-NoSuchElementException. The first two `next()` calls return 'A' and 'B'. The third `next()` call, when there are no more elements, throws a `NoSuchElementException`, which is caught and its simple name printed after 'AB'.

Q: Why might attempting to serialize an instance of a non-static inner class lead to an unexpected `NotSerializableException` if its enclosing class is not `Serializable`?

B) Non-static inner classes implicitly hold a reference to their enclosing instance, which must also be serializable.. A non-static inner class implicitly holds a reference to its enclosing instance. For the inner class to be serialized, this implicit reference (the enclosing instance) must also be `Serializable`.

Q3281 medium

If a concrete (non-abstract) class implements an interface in Java, what is the consequence?

Q3282 medium

In a traditional Java `switch` statement, what is the consequence if a `break` statement is omitted from a `case` block?

Q3283 hard code error

What compilation error will occur when compiling the following Java code?

java

import java.util.function.Function;

public class NestedLambdaTypeMismatch {
    public static void main(String[] args) {
        Function<Integer, Function<String, String>> f = (i) -> (s) -> s.length();
    }
}

Q3284 medium

Under what condition might calling the `remove()` method of a `java.util.Iterator` result in an `UnsupportedOperationException`?

Q3285 medium code output

What will be printed to the console when this code runs?

java

public class ExitPreventsFinally {
    public static void main(String[] args) {
        try {
            System.out.println("In try block");
            System.exit(0);
        } catch (Exception e) {
            System.out.println("In catch block");
        } finally {
            System.out.println("In finally block");
        }
        System.out.println("After try-catch-finally");
    }
}

Q3286 hard code error

What error occurs when executing the following Java code?

java

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
        list.add("Cherry");

        for (String fruit : list) {
            if ("Banana".equals(fruit)) {
                list.remove(fruit);
            }
        }
        System.out.println(list);
    }
}

Q3287 medium code output

What does this code print?

java

import jakarta.validation.*;
import jakarta.validation.constraints.Min;
import java.util.*;

public class Test {
    static class ProductItem { @Min(1) public int quantity; public ProductItem(int q) { this.quantity = q; } }
    static class ShoppingCart {
        @Valid public List<ProductItem> items;
        public ShoppingCart(List<ProductItem> i) { this.items = i; }
    }
    public static void main(String[] args) {
        Validator validator = Validation.buildDefaultValidatorFactory().getValidator();
        ShoppingCart cart = new ShoppingCart(Arrays.asList(new ProductItem(0))); // quantity < 1
        Set<ConstraintViolation<ShoppingCart>> violations = validator.validate(cart);
        System.out.println("Violations: " + violations.size());
    }
}

Q3288 hard code error

What compilation error will occur in this Java code?

java

class Test {
    public static void main(String[] args) {
        final int value;
        do {
            value = 10;
        } while (Math.random() < 0.5); // Condition might cause loop to run more than once
        System.out.println(value);
    }
}

Q3289 easy code output

What is the output of this code?

java

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<String> letters = new ArrayList<>();
        letters.add("A");
        letters.add("B");
        Iterator<String> it = letters.iterator();
        System.out.print(it.next());
        System.out.print(it.next());
        try {
            System.out.print(it.next());
        } catch (Exception e) {
            System.out.print("-" + e.getClass().getSimpleName());
        }
    }
}

Q3290 hard

Why might attempting to serialize an instance of a non-static inner class lead to an unexpected `NotSerializableException` if its enclosing class is not `Serializable`?

Q3291 hard code output

What is the output of this code?

java

public class DataTypeChallenge {
    public static void main(String[] args) {
        char c1 = 'A'; // ASCII 65
        char c2 = 1;
        int result = c1 + c2;
        char c3 = (char) (c1 + c2);
        System.out.println(result + ", " + c3);
    }
}

Q3292 easy code output

What is the output of this code?

java

import java.util.function.Consumer;

public class LambdaTest {
    public static void main(String[] args) {
        Consumer<String> printer = s -> System.out.println(s + "!");
        printer.accept("Java");
    }
}

Q3293 medium

How does encapsulation contribute to creating immutable objects in Java?

Q3294 medium code output

What is the output of this code?

java

public class Main {
    public static void main(String[] args) {
        String msg = "Hello from Runnable!";
        Runnable task = () -> {
            System.out.println(msg);
        };
        new Thread(task).start();
        System.out.println("Main thread is done.");
    }
}

Q3295 hard code error

What is the runtime error encountered when executing this Java code snippet?

java

import java.util.HashSet;
import java.util.Objects;

class KeyWithNullableIntField {
    Integer id;
    public KeyWithNullableIntField(Integer id) { this.id = id; }
    @Override
    public int hashCode() {
        return id.hashCode(); // Potential NPE
    }
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        KeyWithNullableIntField that = (KeyWithNullableIntField) o;
        return Objects.equals(id, that.id);
    }
}

public class Main {
    public static void main(String[] args) {
        HashSet<KeyWithNullableIntField> set = new HashSet<>();
        set.add(new KeyWithNullableIntField(null)); // Object with null 'id' field added
    }
}

Q3296 easy

Which of these classes is designed to be mutable for efficient string manipulation?

Q3297 hard code output

What does this code print?

java

class A {}
class B extends A {}
class C extends A {}
public class Test {
    public static void main(String[] args) {
        A obj1 = new B();
        A obj2 = new A();
        B b1 = (B) obj1;
        try {
            C c1 = (C) obj2;
            System.out.println("Cast successful");
        } catch (ClassCastException e) {
            System.out.println("ClassCastException caught");
        }
        System.out.println("Program continues");
    }
}

Q3298 hard code error

What error will this Java code produce when compiled and run?

java

public class ArrayMistake {
    public static void main(String[] args) {
        int[] numbers = new int[3];
        numbers[0] = 1;
        numbers[1] = 2;
        // Attempt to re-initialize using initializer list after declaration
        numbers = {4, 5, 6};
        System.out.println(numbers[0]);
    }
}

Q3299 easy

Which access modifier is commonly used to achieve data hiding as part of encapsulation in Java?

Q3300 hard code error

What compile-time error will this Java code produce?

java

public class OperatorError4 {
    public static void main(String[] args) {
        int a = 10;
        int b = 20;
        int c = (a + b)++; // Error line
        System.out.println(c);
    }
}

☕ Java MCQ Questions – Page 165