Java MCQ Questions – Page 68 of 200

Q: What is the output of this code?

A) Observer checking main state: WAITING Main thread finished join.. When one thread calls `join()` on another thread, the calling thread (here, `mainThread`) enters the WAITING state until the joined thread (here, `childThread`) terminates.

Q: Which method in `StringBuffer` is used to insert a string at a specific index?

C) insert(). The `insert()` method allows you to add a string or other data types at a specified zero-based index within the existing character sequence of the `StringBuffer`.

Q: Consider an `abstract` method in a superclass that returns type `T`. When a concrete subclass overrides this method, which statement correctly describes the use of covariant return types?

C) The subclass's method can return a subtype `S` of `T`, preserving the method signature.. Covariant return types allow an overridden method in a subclass to return a more specific subtype of the type returned by the original method in the superclass or interface, while maintaining the same method signature.

Q: A thread executes `Thread.sleep(1000)` for one second. Which state does this thread enter during the sleep duration?

C) TIMED_WAITING. When a thread calls `Thread.sleep()` with a timeout, it enters the TIMED_WAITING state. It will automatically return to RUNNABLE after the specified time elapses, or if interrupted.

Q: What is the outcome of attempting to compile the given Java code?

A) Compile-time error. Java generics do not support primitive types (`int`, `boolean`, `char`, etc.) as type arguments. Wrapper classes (e.g., `Integer`) must be used instead.

Q: What compile-time error occurs when attempting to compile this Java code?

B) Error: Shape is abstract; cannot be instantiated. Abstract classes cannot be instantiated directly. An instance can only be created from a concrete subclass that implements all abstract methods.

Q: When does the stream pipeline processing truly begin?

C) Only when a terminal operation is invoked on the stream.. Stream pipelines are executed lazily. No actual computation happens when intermediate operations are applied; processing only starts when a terminal operation is called, which consumes the stream.

Q: What kind of error will occur when compiling or running the following Java code?

A) java.lang.NullPointerException. The `suffix` variable is explicitly assigned `null`. When `suffix.toString()` is called, it attempts to invoke a method on a null reference, leading to a `NullPointerException`.

Q: What error will be reported when attempting to compile the `MyClass` class?

B) Compile-time error: 'non-static variable instanceVar cannot be referenced from a static context'.. A static method belongs to the class itself and does not operate on a specific instance. Therefore, it cannot directly access non-static (instance) variables, which require an object instance to exist.

Q: What is the output of this code, assuming System.getProperty("line.separator") returns "\n"?

A) ABC\\nHello. The bw.write(int c) method writes a single character, interpreting the integer argument as a Unicode character value. 65 is 'A', 'B' is 'B', and 0x0043 is 'C'. bw.newLine() inserts a system line separator (assumed \n).

Q1341 medium code output

What is the output of this code?

java

public class ThreadStateDemo4 {
    public static void main(String[] args) throws InterruptedException {
        Thread childThread = new Thread(() -> {
            try { Thread.sleep(200); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
        });
        childThread.start();
        Thread mainThread = Thread.currentThread();
        Thread observer = new Thread(() -> {
            try {
                Thread.sleep(50); // Give main thread time to call join
                System.out.println("Observer checking main state: " + mainThread.getState());
            } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
        });
        observer.start();
        childThread.join(); // Main thread waits for childThread
        System.out.println("Main thread finished join.");
    }
}

Q1342 easy

Which method in `StringBuffer` is used to insert a string at a specific index?

Q1343 hard

Consider an `abstract` method in a superclass that returns type `T`. When a concrete subclass overrides this method, which statement correctly describes the use of covariant return types?

Q1344 medium

A thread executes `Thread.sleep(1000)` for one second. Which state does this thread enter during the sleep duration?

Q1345 easy code error

What is the outcome of attempting to compile the given Java code?

java

import java.util.Queue;
import java.util.LinkedList;

public class QueueError {
    public static void main(String[] args) {
        Queue<int> invalidQueue = new LinkedList<int>();
        invalidQueue.offer(10);
        System.out.println(invalidQueue.poll());
    }
}

Q1346 hard code error

What compile-time error occurs when attempting to compile this Java code?

java

abstract class Shape {
    public abstract void draw();
    public void getDescription() {
        System.out.println("This is a shape.");
    }
}

public class Main {
    public static void main(String[] args) {
        Shape s = new Shape();
    }
}

Q1347 medium

When does the stream pipeline processing truly begin?

Q1348 hard code error

What kind of error will occur when compiling or running the following Java code?

java

public class SBError {
    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("Result: ");
        String suffix = null;
        sb.append("Prefix").reverse().append(suffix.toString());
        System.out.println(sb);
    }
}

Q1349 medium code error

What error will be reported when attempting to compile the `MyClass` class?

java

class MyClass {
    int instanceVar = 10;

    static void printValue() {
        System.out.println(instanceVar); 
    }
}

public class Main {
    public static void main(String[] args) {
        MyClass.printValue();
    }
}

Q1350 hard code output

What is the output of this code, assuming System.getProperty("line.separator") returns "\n"?

java

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

public class Test {
    public static void main(String[] args) throws IOException {
        StringWriter sw = new StringWriter();
        BufferedWriter bw = new BufferedWriter(sw);
        bw.write(65);
        bw.write('B');
        bw.write(0x0043);
        bw.newLine();
        bw.write("Hello");
        bw.close();
        System.out.println(sw.toString().replace("\n", "\\n").replace("\r", "\\r"));
    }
}

Q1351 easy code output

What is the output of this code?

java

public class Main {
    public static void main(String[] args) {
        int a = 10;
        int b = 3;
        System.out.println(a / b + a % b);
    }
}

Q1352 medium code error

What is the compilation error in the provided Java code?

java

public class MyClass {
    public abstract void performAction();
    public void someMethod() {
        System.out.println("Hello");
    }
}

Q1353 easy

Which of the following is a valid argument type for a `FileReader` constructor to specify the source file?

Q1354 medium

Which statement best describes a `static` variable in Java?

Q1355 easy code error

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

java

import java.util.LinkedList;

public class MyClass {
    public static void main(String[] args) {
        LinkedList<String> list = new LinkedList<>();
        list.set(0, "First"); // Tries to replace an element at index 0
    }
}

Q1356 hard code output

What is the output of this code?

java

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

public class Test {
    public static void main(String[] args) {
        StringWriter sw = new StringWriter();
        BufferedWriter bw = new BufferedWriter(sw);
        try {
            bw.write("Data");
            bw.close();
            System.out.println("First close complete.");
            bw.close(); // Second close call
            System.out.println("Second close complete.");
        } catch (IOException e) {
            System.out.println("Error: " + e.getMessage());
        } finally {
            System.out.println("Content: " + sw.toString());
        }
    }
}

Q1357 easy code error

What is the compilation error in the `Main` class?

java

class Singleton {
    private Singleton() {
        System.out.println("Singleton instance created.");
    }
}

public class Main {
    public static void main(String[] args) {
        Singleton instance = new Singleton();
    }
}

Q1358 easy code output

What does this Java code print?

java

import java.io.File;

public class FileTest {
    public static void main(String[] args) {
        File file = new File("documents/reports/report.pdf");
        System.out.println(file.getParent());
    }
}

Q1359 medium code error

What error occurs when compiling this Java code?

java

public class LoopTest {
    public static void main(String[] args) {
        do {
            int x = 10;
            System.out.println(x);
        } while (x < 13);
    }
}

Q1360 easy

What does it mean if a class is declared with the `final` keyword?

☕ Java MCQ Questions – Page 68