Java MCQ Questions – Page 128 of 200

Q: What is wrong with this Java code?

C) Method reference syntax `::` does not allow arguments to be passed directly like `(" World")`.. Method references (`::`) point to a method, not a method call. Arguments cannot be passed directly within the method reference syntax. You would typically use a lambda `s -> greeting.concat(s)` or `s -> s.concat(" World")`.

Q: Which exception will be thrown when executing this Java code?

A) java.lang.NullPointerException. Calling the equals() method on a null reference (s1) will result in a NullPointerException. To avoid this, call equals() on the known non-null string, e.g., `s2.equals(s1)`.

Q: What is the result of compiling this Java code?

A) A compilation error occurs. The 'continue' statement can only be used inside a loop. Using it within a regular method block, even if the method is called from a loop, causes a compile-time error.

Q: What is the most likely error when executing this Java code snippet?

B) Configuration Error: Buffer size <= 0. The `BufferedWriter` constructor requires a buffer size greater than zero. Passing 0 or a negative value will result in an `IllegalArgumentException` at runtime.

Q: What does this Java code print?

A) Processing long: 15. The integer literal `15` is an `int`. It can be widened to `long` or `double`. Java prefers the 'narrower' widening conversion, so `int` to `long` is chosen over `int` to `double`.

Q: Predict the runtime error for the given Java program.

A) java.lang.ClassCastException. The `putAll` method attempts to insert all mappings from the source map into the target map. The `anotherMap` expects `Integer` keys, but `map` contains `String` keys. When `anotherMap` tries to process the `String` keys, it will attempt to compare them as `Integer` (or naturally compare them if no specific `Comparator` is given, which would be `String` vs `Integer`), resulting in a `ClassCastException`.

Q: What compile-time error will occur in the `faultyMethod`?

A) error: unreported exception MyCheckedException; must be caught or declared to be thrown. `MyCheckedException` is a checked exception. A method that throws a checked exception must either catch it or declare it in its `throws` clause. `faultyMethod` does neither, leading to a compile-time error.

Q: What does this code print?

A) 10.0. When an `int` and a `double` are involved in an arithmetic operation, the `int` is promoted to a `double` before the operation to avoid loss of precision. So, `7 + 3.0` becomes `7.0 + 3.0`, resulting in `10.0`.

Q: What is the output of this code?

A) Before close: After close: Data one.. When `writer.write()` is called, data is stored in `BufferedWriter`'s internal buffer, not immediately in the underlying `StringWriter`. Therefore, `stringWriter.toString()` is empty before the `try-with-resources` block closes (which implicitly flushes). After the block, `close()` is called, flushing the buffer, and then `stringWriter.toString()` contains the written data.

Q: What is the error in this Java code?

A) If an `IOException` occurs during `new FileWriter()`, `writer` will be `null` and `writer.close()` will throw a `NullPointerException`.. If the `FileWriter` constructor fails (e.g., due to file permissions), `writer` will remain `null`. In the `finally` block, `writer.close()` would then attempt to dereference a `null` object, leading to a `NullPointerException` at runtime. A null check (`if (writer != null)`) is required before closing.

Q2541 easy code error

What is wrong with this Java code?

java

import java.util.function.Function;

public class Test {
    public static void main(String[] args) {
        String greeting = "Hello";
        Function<String, String> appender = greeting::concat(" World");
    }
}

Q2542 medium code error

Which exception will be thrown when executing this Java code?

java

public class StringError4 {
    public static void main(String[] args) {
        String s1 = null;
        String s2 = "test";
        boolean result = s1.equals(s2);
        System.out.println(result);
    }
}

Q2543 easy code error

What is the result of compiling this Java code?

java

public class Main {
    public static void someMethod() {
        continue;
    }
    public static void main(String[] args) {
        someMethod();
    }
}

Q2544 hard code error

What is the most likely error when executing this Java code snippet?

java

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

public class Test {
    public static void main(String[] args) {
        try {
            BufferedWriter bw = new BufferedWriter(new StringWriter(), 0);
            bw.write("Hello");
            bw.flush();
        } catch (IOException e) {
            System.err.println("Error: " + e.getMessage());
        } catch (IllegalArgumentException e) {
            System.err.println("Configuration Error: " + e.getMessage());
        }
    }
}

Q2545 easy code output

What does this Java code print?

java

public class OverloadDemo4 {
    void process(long l) {
        System.out.println("Processing long: " + l);
    }
    void process(double d) {
        System.out.println("Processing double: " + d);
    }
    public static void main(String[] args) {
        OverloadDemo4 obj = new OverloadDemo4();
        obj.process(15);
    }
}

Q2546 medium code error

Predict the runtime error for the given Java program.

java

import java.util.TreeMap;
import java.util.Comparator;

public class Main {
    public static void main(String[] args) {
        TreeMap<String, Integer> map = new TreeMap<>();
        map.put("one", 1);
        map.put("two", 2);
        TreeMap<Integer, String> anotherMap = new TreeMap<>();
        anotherMap.putAll(map); // Intentional error point
    }
}

Q2547 easy code error

What compile-time error will occur in the `faultyMethod`?

java

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

public class Main {
    public void faultyMethod() { 
        // This method declares no 'throws' clause
        throw new MyCheckedException("Error in method"); 
    }

    public static void main(String[] args) {
        Main obj = new Main();
        try {
            obj.faultyMethod();
        } catch (MyCheckedException e) {
            System.out.println(e.getMessage());
        }
    }
}

Q2548 easy code output

What does this code print?

java

public class DataTypeTest {
    public static void main(String[] args) {
        int value1 = 7;
        double value2 = 3.0;
        double sum = value1 + value2;
        System.out.println(sum);
    }
}

Q2549 easy code output

What is the output of this code?

java

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

public class Main {
    public static void main(String[] args) {
        StringWriter stringWriter = new StringWriter();
        try (BufferedWriter writer = new BufferedWriter(stringWriter)) {
            writer.write("Data one.");
            System.out.println("Before close: " + stringWriter.toString().replace("\n", "\\n"));
        } catch (IOException e) {
            System.out.println("Error: " + e.getMessage());
        }
        System.out.println("After close: " + stringWriter.toString().replace("\n", "\\n"));
    }
}

Q2550 easy code error

What is the error in this Java code?

java

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

public class FileWriterError6 {
    public static void main(String[] args) {
        FileWriter writer = null;
        try {
            writer = new FileWriter("somefile.txt");
            writer.write('A'); // Writing a single character
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            writer.close(); // Potential NullPointerException here
        }
    }
}

Q2551 easy code error

What is wrong with this Java code?

java

import java.util.function.Supplier;

public class Test {
    public static void main(String[] args) {
        Supplier<String> printer = () -> System.out.println("Hello");
    }
}

Q2552 medium

Which of the following primitive data types CANNOT be used as the argument for a Java `switch` statement or expression?

Q2553 hard code error

What is the result of compiling and running the following Java code snippet?

java

public class StringError {
    public static void main(String[] args) {
        String s = "developer";
        System.out.println(s.substring(5, 2));
    }
}

Q2554 medium

What is the main purpose of the `serialVersionUID` field in a `Serializable` class?

Q2555 medium code error

What exception will be thrown when running this Java code snippet?

java

import java.util.PriorityQueue;

class CustomObject { /* No Comparable implementation */ }

public class QueueError {
    public static void main(String[] args) {
        PriorityQueue<CustomObject> pq = new PriorityQueue<>();
        pq.add(new CustomObject());
        pq.add(new CustomObject()); // This will cause an error
    }
}

Q2556 medium code output

What is the output of this Java code?

java

import java.util.function.Supplier;

class MyClass {
    String message = "Default";
    MyClass() {
        System.out.println("MyClass constructor called.");
    }
    public String getMessage() { return message; }
}

public class MethodRefTest {
    public static void main(String[] args) {
        Supplier<MyClass> factory = MyClass::new;
        MyClass obj = factory.get();
        System.out.println(obj.getMessage());
    }
}

Q2557 medium code output

What does the following Java code print to the console?

java

public class SwitchTest {
    public static void main(String[] args) {
        int value = 0;
        String message = "Start";
        switch (value) {
            case 0:
                message += "-Zero";
            case 1:
                message += "-One";
                break;
            case 2:
                message += "-Two";
                break;
            default:
                message += "-Default";
        }
        System.out.println(message);
    }
}

Q2558 medium code error

What is the result of attempting to run the following Java code?

java

public class Main {
    public static void main(String[] args) {
        Object lock = new Object();
        try {
            lock.wait(); // This line
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
        System.out.println("Finished waiting.");
    }
}

Q2559 hard code error

What error will this code throw when `thread1` and `thread2` try to execute their run methods?

java

import java.util.concurrent.locks.ReentrantLock;

public class DeadlockExample {
    private static ReentrantLock lock1 = new ReentrantLock();
    private static ReentrantLock lock2 = new ReentrantLock();

    public static void main(String[] args) {
        Thread thread1 = new Thread(() -> {
            lock1.lock();
            try { Thread.sleep(100); } catch (InterruptedException e) {}
            lock2.lock(); // Tries to acquire lock2
        });

        Thread thread2 = new Thread(() -> {
            lock2.lock();
            try { Thread.sleep(100); } catch (InterruptedException e) {}
            lock1.lock(); // Tries to acquire lock1
        });

        thread1.start();
        thread2.start();
    }
}

Q2560 medium

What is the significance of the semicolon at the end of a `do-while` loop's `while` condition (e.g., `do { /* ... */ } while (condition);`)?

☕ Java MCQ Questions – Page 128