Java MCQ Questions – Page 33 of 200

Q: Which interface does `java.util.HashSet` primarily implement?

C) java.util.Set. `HashSet` is a concrete implementation of the `Set` interface in Java, providing basic `Set` operations like adding, removing, and checking for existence of unique elements.

Q: Which class is primarily used to read a serialized object from an `InputStream`?

B) java.io.ObjectInputStream. `ObjectInputStream` is the class responsible for reading byte streams and reconstructing them back into Java objects during deserialization.

Q: What is the output of this code?

A) A Static B Instance. Static methods are resolved at compile time based on the reference type (A), while instance methods are resolved at runtime based on the actual object type (B). This is method hiding vs. method overriding.

Q: What does this code print?

A) Caught specific: Detail specific error. When multiple catch blocks are present, Java tries to match the exception to the most specific catch block first. Here, `CustomError` is caught by the `catch (CustomError e)` block.

Q: What is the output of this code?

B) null. HashMap allows `null` values. The `put` method successfully stores `null` as the value for the 'Font' key, and `get` retrieves this `null` value.

Q: What compilation error will occur in the `DataLoader` class?

A) Unhandled exception: CustomDataException.. `CustomDataException` extends `Exception`, making it a checked exception. The `load` method throws this exception but does not declare `throws CustomDataException` in its signature, resulting in a compile-time error.

Q: What is the error in the following Java code?

A) Compile-time error: Incompatible types: possible lossy conversion from double to int.. Assigning a double value to an int variable without an explicit cast is a narrowing conversion that can lead to loss of precision, which Java prevents with a compile-time error.

Q: Consider a class `ConcreteClass` that implements `InterfaceA` but does not implement `InterfaceB`. If you have `Object obj = new ConcreteClass();`, which of the following cast attempts will compile but throw a `ClassCastException` at runtime?

D) `(InterfaceB) obj`. Since `obj` is declared as `Object`, it can potentially hold any object, allowing casts to any interface or class type to compile. However, at runtime, `obj` holds an instance of `ConcreteClass`, which does not implement `InterfaceB`. Therefore, attempting to cast it to `InterfaceB` will result in a `ClassCastException`.

Q: What does this Java code snippet print to the console?

A) Sub action. The `Sub` class overrides the `action` method. An overridden method can throw a narrower (or no) checked exception than the method it overrides. `FileNotFoundException` is a subclass of `IOException`, so this is valid. Polymorphism ensures `Sub`'s method is called, and since it doesn't actually throw an exception in this snippet, 'Sub action' is printed.

Q: What compile-time error will occur when compiling this Java code?

A) Compile-time error: Incompatible types. Found: 'boolean', Required: 'char, byte, short, int, Character, Byte, Short, Integer, String, or an enum'. The 'switch' statement in Java does not support 'boolean' as a controlling expression type. It only accepts char, byte, short, int, their wrapper types, String, or enums.

Q641 easy

Which interface does `java.util.HashSet` primarily implement?

Q642 easy

Which class is primarily used to read a serialized object from an `InputStream`?

Q643 hard code output

What is the output of this code?

java

class A {
    public static void print() { System.out.println("A Static"); }
    public void display() { System.out.println("A Instance"); }
}
class B extends A {
    public static void print() { System.out.println("B Static"); }
    public void display() { System.out.println("B Instance"); }
}
public class Test {
    public static void main(String[] args) {
        A obj = new B();
        obj.print();
        obj.display();
    }
}

Q644 easy code output

What does this code print?

java

class CustomError extends Exception {
    public CustomError(String msg) {
        super(msg);
    }
}

public class Main {
    public static void main(String[] args) {
        try {
            throw new CustomError("Detail specific error");
        } catch (CustomError e) {
            System.out.println("Caught specific: " + e.getMessage());
        } catch (Exception e) {
            System.out.println("Caught generic: " + e.getMessage());
        }
    }
}

Q645 easy code output

What is the output of this code?

java

import java.util.HashMap;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        Map<String, String> userPreferences = new HashMap<>();
        userPreferences.put("Background", "Blue");
        userPreferences.put("Font", null);
        System.out.println(userPreferences.get("Font"));
    }
}

Q646 medium code error

What compilation error will occur in the `DataLoader` class?

java

public class CustomDataException extends Exception {
    public CustomDataException(String message) {
        super(message);
    }
}

public class DataLoader {
    public void load(String path) {
        if (path == null || path.isEmpty()) {
            throw new CustomDataException("Path cannot be empty.");
        }
        System.out.println("Loading data from: " + path);
    }

    public static void main(String[] args) {
        DataLoader loader = new DataLoader();
        loader.load(null);
    }
}

Q647 easy code error

What is the error in the following Java code?

java

public class Main {
    public static void main(String[] args) {
        double d = 10.5;
        int i = d;
        System.out.println(i);
    }
}

Q648 hard

Consider a class `ConcreteClass` that implements `InterfaceA` but does not implement `InterfaceB`. If you have `Object obj = new ConcreteClass();`, which of the following cast attempts will compile but throw a `ClassCastException` at runtime?

Q649 medium code output

What does this Java code snippet print to the console?

java

class OverrideThrows {
    static class Super {
        void action() throws java.io.IOException {
            System.out.println("Super action");
        }
    }

    static class Sub extends Super {
        @Override
        void action() throws java.io.FileNotFoundException { // Can throw narrower exception
            System.out.println("Sub action");
        }
    }

    public static void main(String[] args) {
        try {
            Super obj = new Sub();
            obj.action();
        } catch (java.io.IOException e) {
            System.out.println("Caught IO Exception");
        }
    }
}

Q650 easy code error

What compile-time error will occur when compiling this Java code?

java

public class MyClass {
    public static void main(String[] args) {
        boolean flag = true;
        switch (flag) {
            case true:
                System.out.println("It's true");
                break;
            default:
                System.out.println("It's false");
        }
    }
}

Q651 easy

How can you use an enhanced `for` loop (for-each loop) to print each element of a `String` array named `words`?

Q652 easy code error

What type of error will occur when compiling or running this Java code snippet?

java

public class ArrayTest {
    public static void main(String[] args) {
        int[] data = null;
        System.out.println(data[0]);
    }
}

Q653 easy

Which keyword is used within a subclass to explicitly call the superclass's version of an overridden method?

Q654 easy code error

What error will this code produce?

java

public class LoopError {
    public static void main(String[] args) {
        int limit = 5;
        for (int i = 0; j < limit; i++) {
            System.out.println(i);
        }
    }
}

Q655 medium

In which of the following scenarios is a `while` loop generally considered more appropriate than a `for` loop?

Q656 easy code output

What is the output of this Java code?

java

import java.util.function.Consumer;

public class Main {
    public static void printStatic(String text) {
        System.out.print("Static: " + text);
    }

    public static void main(String[] args) {
        Consumer<String> consumer = Main::printStatic;
        consumer.accept("Message");
    }
}

Q657 medium

When an `ArrayBlockingQueue` (a bounded queue) is full, what is the behavior of its `add()` method?

Q658 medium code output

What is the output of this code snippet?

java

public class StringTest {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = new String("Hello");
        String str3 = "hello";
        System.out.println(str1.equals(str2) + "," + str1.equalsIgnoreCase(str3));
    }
}

Q659 medium code output

What is the output of the given Java program?

java

import java.util.Arrays;
import java.util.List;

public class StreamMapReduceLambda {
    public static void main(String[] args) {
        List<String> words = Arrays.asList("a", "bb", "ccc");
        int totalLength = words.stream()
                                  .mapToInt(String::length) // Method reference
                                  .sum();
        System.out.println(totalLength);
    }
}

Q660 medium code error

What is the expected outcome when compiling and running the following Java code?

java

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;

public class Test {
    public static void main(String[] args) {
        try {
            BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt"), -1);
            writer.write("Content");
            writer.close();
        } catch (IOException e) {
            System.out.println(e.getClass().getSimpleName() + ": " + e.getMessage());
        } catch (IllegalArgumentException e) {
            System.out.println(e.getClass().getSimpleName() + ": " + e.getMessage());
        }
    }
}

☕ Java MCQ Questions – Page 33