Java MCQ Questions – Page 21 of 200

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

A) java.lang.IllegalThreadStateException. The deprecated `resume()` method throws an `IllegalThreadStateException` if the target thread has not been suspended. It expects the thread to be in a suspended state to resume it.

Q: What is the approximate time complexity for `add()`, `remove()`, and `contains()` operations in a `TreeSet`?

B) O(log n) (logarithmic time). Due to its Red-Black tree implementation, `TreeSet` guarantees `log(n)` time complexity for basic operations like `add`, `remove`, and `contains`.

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

B) Error: Illegal forward reference.. In Java, fields are initialized in order of declaration. Here, `BASE_URL` is initialized by calling `getBaseUrl()`, which attempts to use `API_KEY`. However, `API_KEY` is declared and initialized *after* `BASE_URL`, leading to an 'illegal forward reference' compile-time error for `API_KEY` when `getBaseUrl()` tries to access it during `BASE_URL`'s initialization.

Q: What is the error in the execution of this Java code snippet?

A) NegativeArraySizeException. The `ensureCapacity` method requires a non-negative argument for `minimumCapacity`. Passing a negative value, such as -5, results in a `NegativeArraySizeException` at runtime.

Q: Which of the following primitive type casting scenarios would result in a compile-time error?

C) Casting `boolean` to `int`.. A `boolean` type cannot be directly cast to any other primitive data type, including numeric types like `int`. This operation is fundamentally incompatible and will cause a compile-time error.

Q: Given `int a = 5; int b = a;` and `MyObject obj1 = new MyObject(); MyObject obj2 = obj1;`, what is the fundamental difference in how `b` gets its value from `a` compared to `obj2` getting its value from `obj1`?

B) For primitives (`a`, `b`), the actual value is copied. For objects (`obj1`, `obj2`), the *reference* to the same object is copied.. Primitive type assignment copies the literal value. Reference type assignment copies the memory address (reference), so both reference variables point to the same object.

Q: What is the primary difference between `FileReader` and `FileInputStream`?

B) `FileReader` reads characters, while `FileInputStream` reads bytes.. `FileReader` is a character stream class designed to read characters (applying encoding), whereas `FileInputStream` is a byte stream class designed to read raw bytes without encoding conversion.

Q: What is the output of this code?

B) java.util.ConcurrentModificationException. An enhanced for-loop internally uses an iterator. Modifying the list directly (e.g., `list.add(4)`) while iterating with this implicit iterator causes a `ConcurrentModificationException`.

Q: Which of the following is NOT a direct advantage of encapsulation?

C) Ability to achieve polymorphism and inheritance.. Polymorphism and inheritance are other pillars of OOP, distinct from encapsulation, although they can be used together. Encapsulation primarily focuses on data hiding and controlled access.

Q: What error will prevent the following Java code from compiling?

A) Compilation error: Local variables referenced from a lambda expression must be final or effectively final.. Local variables accessed from within a lambda expression (or an anonymous inner class) must be final or effectively final. The attempt to increment `counter` makes it non-effectively final, causing a compilation error.

Q401 medium code error

What error will occur when running the following Java code?

java

public class ResumeWithoutSuspend {
    public static void main(String[] args) {
        Thread t = new Thread(() -> {
            System.out.println("Thread running");
            try { Thread.sleep(100); } catch (InterruptedException e) {}
        });
        t.start();
        // The thread was never suspended
        t.resume(); // Deprecated, but still behaves this way
    }
}

Q402 medium

What is the approximate time complexity for `add()`, `remove()`, and `contains()` operations in a `TreeSet`?

Q403 hard code error

What is the compilation error in the following Java code?

java

public class Configuration {
    private final String API_KEY = "key_123";
    private final String BASE_URL = getBaseUrl();

    private String getBaseUrl() {
        // This method depends on API_KEY but is called before API_KEY is initialized.
        return "https://api.example.com/" + API_KEY.substring(0, 3);
    }

    public String getFullUrl() {
        return BASE_URL;
    }
}

Q404 hard code error

What is the error in the execution of this Java code snippet?

java

public class BuilderError {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Data");
        sb.ensureCapacity(-5);
        System.out.println(sb.capacity());
    }
}

Q405 medium

Which of the following primitive type casting scenarios would result in a compile-time error?

Q406 hard

Given `int a = 5; int b = a;` and `MyObject obj1 = new MyObject(); MyObject obj2 = obj1;`, what is the fundamental difference in how `b` gets its value from `a` compared to `obj2` getting its value from `obj1`?

Q407 easy

What is the primary difference between `FileReader` and `FileInputStream`?

Q408 hard code output

What is the output of this code?

java

import java.util.LinkedList;

public class Test {
    public static void main(String[] args) {
        LinkedList<Integer> list = new LinkedList<>();
        list.add(1);
        list.add(2);
        list.add(3);
        for (Integer num : list) {
            if (num == 2) {
                list.add(4); // Modifying list during enhanced for-loop
            }
        }
        System.out.println(list);
    }
}

Q409 easy

Which of the following is NOT a direct advantage of encapsulation?

Q410 medium code error

What error will prevent the following Java code from compiling?

java

public class Main {
    public static void main(String[] args) {
        int counter = 0;
        Runnable r = () -> {
            counter++; // Attempt to modify 'counter'
            System.out.println(counter);
        };
        Thread t = new Thread(r);
        t.start();
    }
}

Q411 hard code output

What is the output of this code?

java

public class DataTypeChallenge {
    public static void main(String[] args) {
        float f = 0.7f;
        double d = 0.7;
        System.out.println(f == d);
    }
}

Q412 hard

Under what specific circumstance might a `SecurityException` be thrown during the construction of a `FileReader` instance, even if the file exists and the path is valid?

Q413 hard code output

What is the output of this code?

java

public class WhileTrueBreak {
    public static void main(String[] args) {
        int x = 0;
        int y = 0;
        while (true) {
            x++;
            if (x % 2 == 0) {
                y++;
            }
            if (x > 5) {
                if (y >= 2) {
                    break;
                }
                y = 0; 
            }
        }
        System.out.println(x + ", " + y);
    }
}

Q414 easy code error

What compile-time error will this code produce?

java

class MyTask implements Runnable {
    // No run() method implemented
}

public class Main {
    public static void main(String[] args) {
        MyTask task = new MyTask();
        Thread thread = new Thread(task);
        thread.start();
    }
}

Q415 medium code error

What is the primary error in the following Java code when attempting to compile it?

java

public class MyTask implements Runnable {
    // No run() method implemented
}

public class Main {
    public static void main(String[] args) {
        MyTask task = new MyTask();
        System.out.println("Task created.");
    }
}

Q416 medium code output

What is the output of this Java code?

java

import java.util.TreeSet;
import java.util.Set;

public class Test {
    public static void main(String[] args) {
        Set<Integer> uniqueNumbers = new TreeSet<>();
        uniqueNumbers.add(10);
        uniqueNumbers.add(20);
        uniqueNumbers.add(10);
        uniqueNumbers.add(30);
        uniqueNumbers.add(20);
        System.out.println(uniqueNumbers.size());
    }
}

Q417 hard code output

What is the output of this code?

java

enum MySingleton {
    INSTANCE;
    private MySingleton() {
        System.out.println("MySingleton ctor.");
    }
    public void doSomething() {
        System.out.println("Singleton action.");
    }
}
class Utility {
    private Utility() { throw new AssertionError("No instances!"); }
    public static void info() { System.out.println("Utility info."); }
}
public class Main {
    public static void main(String[] args) {
        Utility.info();
        MySingleton s1 = MySingleton.INSTANCE;
        s1.doSomething();
        MySingleton s2 = MySingleton.INSTANCE;
        System.out.println(s1 == s2);
    }
}

Q418 easy code error

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

java

public class MyClass {
    public static void main(String[] args) {
        int value; // Uninitialized local variable
        switch (value) {
            case 1:
                System.out.println("One");
                break;
            default:
                System.out.println("Other");
        }
    }
}

Q419 easy code error

What compile-time error will occur in the `main` method?

java

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

public class Main {
    public static void main(String[] args) {
        try {
            throw MyCustomException; // Attempting to throw a class name, not an instance
        } catch (MyCustomException e) {
            System.out.println(e.getMessage());
        }
    }
}

Q420 medium code output

What is the output of the following Java code snippet?

java

import java.io.BufferedWriter;
import java.io.StringWriter;
import java.io.IOException;

public class Main {
    public static void main(String[] args) throws IOException {
        StringWriter sw = new StringWriter();
        BufferedWriter bw = new BufferedWriter(sw);
        bw.write("First");
        bw.newLine(); // Uses default system separator
        System.setProperty("line.separator", "---");
        bw.write("Second");
        bw.newLine(); // Uses new system separator
        bw.write("Third");
        bw.close();
        System.out.print(sw.toString());
    }
}

☕ Java MCQ Questions – Page 21