Java MCQ Questions – Page 198 of 200

Q: What does this code print?

A) 2 [20, 25, 30, 35]. The 'subSet' creates a view. Initially, elements in the range [20, 40) are 20 and 30, so size is 2. Adding 25 to 'sub' works as it's within range. Adding 35 to 'original' also reflects in 'sub' as it's within its range. The set maintains sorted order.

Q: What kind of error will occur when executing the following Java code?

A) java.lang.UnsupportedOperationException. The iterators returned by `PriorityQueue` are not guaranteed to traverse the elements in any particular order and do not support the `remove()` operation. Calling `it.remove()` will throw an `UnsupportedOperationException`.

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

C) java.lang.NullPointerException. Elements of an object array are initialized to `null` by default. Calling a method like `toUpperCase()` on a `null` reference will result in a `java.lang.NullPointerException` at runtime.

Q: Which statement most accurately describes a potential pitfall or nuance when using `StringBuffer` in a complex multi-threaded application, despite its internal synchronization?

B) Even with `StringBuffer`, external synchronization (e.g., `synchronized(sb) { ... }`) might still be necessary to ensure atomicity for sequences of operations.. While `StringBuffer`'s methods are individually synchronized, this only guarantees atomicity for *single* method calls. If a sequence of operations (e.g., `append`, then `delete`, then `insert`) needs to be atomic as a whole, external synchronization using a `synchronized` block on the `StringBuffer` instance itself (or another monitor) is still required to prevent other threads from interleaving operations.

Q: What error will this code produce?

A) Compile-time error: ';' expected. The initialization and condition parts of the for loop header must be separated by a semicolon. In this code, 'int i = 0' and 'i < 5' are not separated, causing a syntax error.

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

A) Compilation error: The field Configuration.API_KEY is not visible.. Even though 'API_KEY' is a static field, it is declared as private. Private members, whether static or instance, cannot be accessed directly from outside their declaring class, resulting in a compilation error. A public getter method (like getApiKey()) should be used instead.

Q: What is the output of this Java code?

A) Error during serialization: java.io.NotSerializableException: MyNonSerializableObject. The `MyNonSerializableObject` class does not implement the `java.io.Serializable` interface. Attempting to serialize an instance of this class will immediately throw a `NotSerializableException` during the `writeObject` call.

Q: What happens if you attempt to insert a `null` key into a `TreeMap`?

A) It throws a `NullPointerException`.. A `TreeMap` does not permit `null` keys, whether using natural ordering or a custom comparator, because it cannot determine where to sort a `null` key. Attempting to insert one will result in a `NullPointerException`.

Q: What is the output of this code?

A) Base private greet. Private methods are not inherited and thus cannot be overridden. The `greet()` method in `Derived` is a completely new, separate method. When `b.callGreet()` is invoked, `Base`'s `callGreet()` method is called, which in turn calls `Base`'s own `private greet()` method.

Q: What does this code print?

A) true, false, true. `renameTo()` successfully renames the file, returning `true`. After renaming, the old file no longer exists, and the new file does.

Q3941 hard code output

What does this code print?

java

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

public class App {
    public static void main(String[] args) {
        TreeSet<Integer> original = new TreeSet<>();
        original.add(10); original.add(20); original.add(30); original.add(40); original.add(50);

        Set<Integer> sub = original.subSet(20, true, 40, false); // [20, 40)
        System.out.println(sub.size());
        sub.add(25); // Add to subset
        original.add(35); // Add to original
        System.out.println(sub);
    }
}

Q3942 hard code error

What kind of error will occur when executing the following Java code?

java

import java.util.Iterator;
import java.util.PriorityQueue;
import java.util.Queue;

public class QError6 {
    public static void main(String[] args) {
        Queue<Integer> pq = new PriorityQueue<>();
        pq.add(10);
        pq.add(20);
        Iterator<Integer> it = pq.iterator();
        while (it.hasNext()) {
            it.next();
            it.remove();
        }
    }
}

Q3943 medium code error

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

java

public class ArrayError {
    public static void main(String[] args) {
        String[] words = new String[3];
        // words[0] is null by default
        System.out.println(words[0].toUpperCase());
    }
}

Q3944 hard

Which statement most accurately describes a potential pitfall or nuance when using `StringBuffer` in a complex multi-threaded application, despite its internal synchronization?

Q3945 easy code error

What error will this code produce?

java

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

Q3946 medium code error

What is the error in the following Java code?

java

class Configuration {
    private static String API_KEY = "abcd123";

    public static String getApiKey() {
        return API_KEY;
    }
}

public class ExternalService {
    public static void main(String[] args) {
        System.out.println(Configuration.API_KEY); // Direct access to private static field
    }
}

Q3947 medium code error

What is the output of this Java code?

java

import java.io.*;

class MyNonSerializableObject {
    String name;
    public MyNonSerializableObject(String name) { this.name = name; }
}

public class DeserializationError1 {
    public static void main(String[] args) {
        byte[] data = null;
        try (ByteArrayOutputStream bos = new ByteArrayOutputStream();
             ObjectOutputStream oos = new ObjectOutputStream(bos)) {
            oos.writeObject(new MyNonSerializableObject("Test"));
            data = bos.toByteArray();
        } catch (IOException e) {
            System.out.println("Error during serialization: " + e.getClass().getName() + ": " + e.getMessage());
        }

        if (data != null) {
            try (ByteArrayInputStream bis = new ByteArrayInputStream(data);
                 ObjectInputStream ois = new ObjectInputStream(bis)) {
                MyNonSerializableObject obj = (MyNonSerializableObject) ois.readObject();
                System.out.println("Deserialized: " + obj.name);
            } catch (Exception e) {
                System.out.println("Error during deserialization: " + e.getClass().getName() + ": " + e.getMessage());
            }
        }
    }
}

Q3948 easy

What happens if you attempt to insert a `null` key into a `TreeMap`?

Q3949 hard code output

What is the output of this code?

java

class Base {
    private void greet() { System.out.println("Base private greet"); }
    public void callGreet() { greet(); }
}
class Derived extends Base {
    private void greet() { System.out.println("Derived private greet"); }
}
public class Test {
    public static void main(String[] args) {
        Base b = new Derived();
        b.callGreet();
    }
}

Q3950 medium code output

What does this code print?

java

import java.io.File;
import java.io.IOException;

public class FileRename {
    public static void main(String[] args) throws IOException {
        File oldFile = new File("original.txt");
        oldFile.createNewFile();
        File newFile = new File("renamed.txt");
        boolean renamed = oldFile.renameTo(newFile);
        boolean oldExists = oldFile.exists();
        boolean newExists = newFile.exists();
        
        // Cleanup
        newFile.delete();
        
        System.out.println(renamed + ", " + oldExists + ", " + newExists);
    }
}

Q3951 medium code output

What does this code print?

java

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

public class Test {
    public static void main(String[] args) {
        List<String> items = new ArrayList<>(List.of("A", "B", "C"));
        Iterator<String> it = items.iterator();
        try {
            it.remove(); // Attempt to remove before calling next()
        } catch (IllegalStateException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

Q3952 hard code output

What is the output of this code?

java

public class ReentrancyTest {
    public synchronized void outerMethod() {
        System.out.println(Thread.currentThread().getName() + " inside outerMethod.");
        innerMethod(); // Calls another synchronized method on the same object
        System.out.println(Thread.currentThread().getName() + " exiting outerMethod.");
    }

    public synchronized void innerMethod() {
        System.out.println(Thread.currentThread().getName() + " inside innerMethod.");
    }

    public static void main(String[] args) throws InterruptedException {
        ReentrancyTest test = new ReentrancyTest();
        Thread t = new Thread(test::outerMethod, "TestThread");
        t.start();
        t.join();
    }
}

Q3953 easy code output

What is the output of this Java code snippet?

java

public class ArrayTest {
    public static void main(String[] args) {
        int[] numbers = {10, 20, 30, 40};
        System.out.println(numbers[2]);
    }
}

Q3954 easy

What is an 'infinite loop' in the context of a `while` loop?

Q3955 hard code error

Does this Java code compile? If not, what is the compile-time error?

java

import java.io.IOException;
public class MainMethodThrows {
    public static void doSomething() throws IOException {
        throw new IOException("Something went wrong");
    }
    public static void main(String[] args) {
        doSomething();
        System.out.println("Done.");
    }
}

Q3956 easy code output

What does this code print?

java

public class MyClass {
    public static void checkValue(int value) {
        if (value < 0) {
            throw new IndexOutOfBoundsException("Value cannot be negative");
        }
        System.out.println("Value is: " + value);
    }

    public static void main(String[] args) {
        try {
            checkValue(-5);
        } catch (IndexOutOfBoundsException e) {
            System.out.println("Handled error: " + e.getMessage());
        }
    }
}

Q3957 easy code error

What is wrong with this Java code?

java

import java.util.function.Runnable;

public class Test {
    public static void main(String[] args) {
        Runnable r = -> System.out.println("Running");
    }
}

Q3958 easy code output

What is the output of this Java code?

java

public class LoopTest {
    public static void main(String[] args) {
        for (int i = 0; i < 3; i++) {
            if (i == 1) {
                continue;
            }
            System.out.print(i);
        }
    }
}

Q3959 hard code output

What is the output of this code?

java

public class PrePostContinueWhile {
    public static void main(String[] args) {
        int i = 0;
        int sum = 0;
        while (i++ < 5) { 
            if (i % 2 == 0) { 
                continue;
            }
            sum += i; 
        }
        System.out.println(sum + ", " + i);
    }
}

Q3960 easy code error

What kind of error will occur when this Java code is executed?

java

public class Test {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("HelloWorld");
        sb.delete(5, 2);
        System.out.println(sb);
    }
}

☕ Java MCQ Questions – Page 198