Java MCQ Questions – Page 73 of 200

Q: Which error will occur when running this Java code?

C) java.io.IOException: Stream closed at runtime.. Once a `FileReader` stream is closed using `reader.close()`, it cannot be used for further read operations. Attempting to `read()` from a closed stream will result in an `IOException` with the message 'Stream closed'.

Q: What is a possible output of this Java code?

A) Main execution continues. Anon Runnable thread: Thread-0. An anonymous inner class can implement the `Runnable` interface and be passed to a `Thread` constructor. Calling `start()` creates and executes this `Runnable` in a new thread. The output order is not guaranteed, so option A is one of the possible outcomes.

Q: What will be the output of running this Java code?

B) Runtime error: java.lang.ClassCastException. Although a `Cat` is an `Animal`, it is not a `Dog`. Attempting to downcast an `Animal` reference that actually points to a `Cat` object to a `Dog` type at runtime will result in a `java.lang.ClassCastException`.

Q: What is the output of this Java code?

A) Reading file... Error: File not found End of main.. The `readFile` method prints a message and then throws a checked `IOException`. The `main` method's `try-catch` block successfully catches this exception, printing the error message. Finally, the program continues to execute the line after the `try-catch` block.

Q: What error will this code produce at compile time?

A) incompatible types: java.lang.String cannot be converted to java.lang.Integer. The enhanced for loop attempts to iterate over a List but tries to assign each element to an Integer variable, which is an incompatible type at compile time.

Q: What is the output of this code?

A) Error: NonSerializableDependency. When a 'Serializable' class contains a field that is neither 'transient' nor 'Serializable' itself, attempting to serialize the object will result in a 'NotSerializableException' for the non-serializable field's class.

Q: If a `break` statement is encountered inside a `do-while` loop in Java, what is its immediate effect?

B) It terminates the loop immediately and execution continues with the statement following the loop.. The `break` statement is used to exit a loop prematurely, causing execution to resume at the statement immediately after the loop.

Q: Consider a long-running `while` loop that periodically creates new, large objects (e.g., `byte[]` arrays) within its body. If these objects are not stored in collections that persist beyond the loop iteration or explicitly nulled, what is the most likely impact on Garbage Collection during the loop's execution?

A) Increased frequency of minor GCs due to rapid allocation in the Eden space.. New objects are typically allocated in the Eden space of the young generation. Rapid, frequent allocation within a loop will quickly fill Eden, triggering frequent minor garbage collections to reclaim these short-lived objects.

Q: What is the scope of the `this` keyword inside a lambda expression compared to an anonymous inner class?

B) `this` refers to the enclosing instance of the class that defined the lambda, unlike an anonymous inner class.. Lambdas do not introduce a new scope for `this`. `this` inside a lambda refers to the instance of the enclosing class, whereas in an anonymous inner class, `this` refers to the anonymous class instance itself.

Q: What happens if a class attempts to override a `final` method inherited from its superclass?

B) A compile-time error occurs, as `final` methods cannot be overridden.. Methods declared as `final` cannot be overridden in a subclass. Attempting to do so will result in a compile-time error, ensuring the method's implementation remains unchanged.

Q1441 easy code error

Which error will occur when running this Java code?

java

import java.io.FileReader;
import java.io.IOException;
import java.io.File;
import java.io.FileWriter;

public class FileReaderIssue {
    public static void main(String[] args) throws IOException {
        File tempFile = File.createTempFile("test", ".txt");
        FileWriter writer = new FileWriter(tempFile);
        writer.write("Data");
        writer.close();
        
        FileReader reader = new FileReader(tempFile);
        reader.close(); // Stream is closed here
        int data = reader.read(); // Attempt to read from closed stream
        tempFile.deleteOnExit();
    }
}

Q1442 medium code output

What is a possible output of this Java code?

java

public class Main {
    public static void main(String[] args) {
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("Anon Runnable thread: " + Thread.currentThread().getName());
            }
        });
        t.start();
        System.out.println("Main execution continues.");
    }
}

Q1443 hard code error

What will be the output of running this Java code?

java

class Animal {}
class Dog extends Animal {}
class Cat extends Animal {}

public class Zoo {
    public static void main(String[] args) {
        Animal a = new Cat();
        Dog d = (Dog) a; 
        System.out.println("Casted successfully");
    }
}

Q1444 easy code output

What is the output of this Java code?

java

import java.io.IOException;

public class MyClass {
    public static void readFile() throws IOException {
        System.out.println("Reading file...");
        throw new IOException("File not found");
    }

    public static void main(String[] args) {
        try {
            readFile();
        } catch (IOException e) {
            System.out.println("Error: " + e.getMessage());
        }
        System.out.println("End of main.");
    }
}

Q1445 hard code error

What error will this code produce at compile time?

java

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

public class EnhancedForError {
    public static void main(String[] args) {
        List<String> words = new ArrayList<>();
        words.add("Java");
        words.add("ForLoop");

        for (Integer num : words) {
            System.out.println(num);
        }
    }
}

Q1446 hard code output

What is the output of this code?

java

import java.io.*;

class NonSerializableDependency {
    String value;
    public NonSerializableDependency(String v) { this.value = v; }
    public String toString() { return "Dep: " + value; }
}

class ParentObject implements Serializable {
    private static final long serialVersionUID = 1L;
    int id;
    NonSerializableDependency dependency;

    public ParentObject(int id, String depValue) {
        this.id = id;
        this.dependency = new NonSerializableDependency(depValue);
    }

    public String toString() {
        return "ID: " + id + ", Dependency: " + dependency;
    }
}

public class SerializationTest {
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        ParentObject obj = new ParentObject(1, "Data");
        
        try (ByteArrayOutputStream bos = new ByteArrayOutputStream();
             ObjectOutputStream oos = new ObjectOutputStream(bos)) {
            oos.writeObject(obj);
            oos.close();
            
            ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
            ObjectInputStream ois = new ObjectInputStream(bis);
            ParentObject deserializedObj = (ParentObject) ois.readObject();
            ois.close();
            System.out.println(deserializedObj);
        } catch (NotSerializableException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

Q1447 medium

If a `break` statement is encountered inside a `do-while` loop in Java, what is its immediate effect?

Q1448 hard

Consider a long-running `while` loop that periodically creates new, large objects (e.g., `byte[]` arrays) within its body. If these objects are not stored in collections that persist beyond the loop iteration or explicitly nulled, what is the most likely impact on Garbage Collection during the loop's execution?

Q1449 medium

What is the scope of the `this` keyword inside a lambda expression compared to an anonymous inner class?

Q1450 hard

What happens if a class attempts to override a `final` method inherited from its superclass?

Q1451 medium code error

What is the compile-time error in this Java code?

java

import java.util.function.Function;

public class LambdaError {
    public static void main(String[] args) {
        Function<String, Integer> stringLength = (str) -> {
            return str.length();
        };
        
        // Incorrect usage
        Function<Integer, String> intToString = (num) -> num;
    }
}

Q1452 medium code output

What does this Java code print to the console?

java

import java.util.ArrayList;

public class Test {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");
        names.remove(1);
        System.out.println(names);
    }
}

Q1453 medium code error

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

java

public class ArrayError {
    public static void main(String[] args) {
        int size = -10;
        int[] data = new int[size];
        System.out.println(data.length);
    }
}

Q1454 easy code output

What is the output of this code?

java

interface Calculator {
    int operate(int a, int b);
}

public class LambdaTest {
    public static void main(String[] args) {
        Calculator adder = (x, y) -> x + y;
        System.out.println(adder.operate(5, 3));
    }
}

Q1455 easy

Which of the following is NOT a standard state in the Java Thread lifecycle (java.lang.Thread.State enum)?

Q1456 easy code error

What will happen when this Java code is compiled and executed?

java

class MyClass {
    public void emptyThrow() throws Exception {
        throw;
    }
}

Q1457 medium

Can a `private` method be overloaded in Java?

Q1458 easy code error

What is the error in this Java code?

java

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

public class FileWriterError8 {
    public static void main(String[] args) throws IOException {
        FileWriter writer = new FileWriter(123);
        writer.write("Number as file.");
        writer.close();
    }
}

Q1459 easy

How can you obtain a reference to the currently executing `Thread` object?

Q1460 easy code output

What does this code print to the console?

java

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

public class FileWriterTest {
    public static void main(String[] args) {
        try (FileWriter writer = new FileWriter("/nonexistent/path/output.txt")) {
            writer.write("Hello");
        } catch (IOException e) {
            System.out.println("Failed to write: " + e.getMessage());
        }
    }
}

☕ Java MCQ Questions – Page 73