Java MCQ Questions – Page 129 of 200

Q: What is the implicit return type of a Java constructor?

D) Constructors do not have an explicit or implicit return type.. Constructors do not have any return type, not even `void`. If you specify a return type, it becomes a regular method, not a constructor. They implicitly return an instance of the class they initialize.

Q: What is the consequence of passing a `null` `Boolean` object to an `if` condition, such as `if (myBooleanObject)`?

C) A `NullPointerException` is thrown at runtime during unboxing.. Java's `if` statement expects a `boolean` primitive. When a `Boolean` object is used, it undergoes unboxing. If `myBooleanObject` is `null`, attempting to unbox it to a primitive `boolean` results in a `NullPointerException`.

Q: What is the error encountered when running this Java code?

B) IOException: Stream closed. Calling the 'ready()' method on a FileReader instance after it has been closed will result in an IOException with a message indicating the stream is closed, similar to attempting a read operation on a closed stream.

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

B) java.lang.StringIndexOutOfBoundsException. The `insert(int offset, String str)` method requires the `offset` to be between `0` and `length()` inclusive. Since `sb.length() + 1` (which is 6) exceeds the valid range, a `StringIndexOutOfBoundsException` is thrown.

Q: What is the runtime behavior of this Java `TreeMap` code?

A) java.lang.ClassCastException. Even though `get` takes an `Object`, `TreeMap` internally attempts to compare the provided key with existing keys using its `Comparator` (or natural ordering). Since the `TreeMap` is for `String` keys, it will try to cast `nonStringKey` (an `Integer`) to `String` for comparison, resulting in a `ClassCastException`.

Q: Where is the `throws` keyword used in a Java program?

C) In a method's signature to declare potential checked exceptions.. The `throws` keyword is used in a method's signature to declare which checked exceptions the method might throw. This informs callers about the exceptions they need to handle.

Q: Is it generally possible for `FileWriter.write(char[] cbuf, int off, int len)` to successfully write *fewer* than `len` characters to the underlying file without throwing an `IOException`?

A) No, `FileWriter` (as a `Writer`) guarantees that either all `len` characters are successfully written, or an `IOException` is thrown to indicate failure.. For standard `Writer` implementations like `FileWriter`, a single `write(char[], int, int)` operation is designed to be atomic with respect to the requested characters. It either completely writes all `len` characters or throws an `IOException` if a problem occurs before completion. It does not return the count of characters written.

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

A) Runtime Error: java.lang.IllegalMonitorStateException. The `wait()` method can only be called from within a `synchronized` block or method. If the current thread does not own the object's monitor (lock), calling `wait()` will throw a `java.lang.IllegalMonitorStateException`.

Q: What does this Java code print to the console?

A) Hello Java. `sb.replace(5, 10, " Java")` replaces the characters from index 5 (inclusive, 'W') to 10 (exclusive, end of 'HelloWorld') with ' Java'. This results in 'Hello' + ' Java' which is 'Hello Java'.

Q: If you want to create a custom exception that callers are *not required* to handle or declare, which class should it extend?

D) java.lang.RuntimeException. Extending `java.lang.RuntimeException` makes a custom exception an unchecked exception, meaning its handling is optional at compile time.

Q2561 medium

What is the implicit return type of a Java constructor?

Q2562 hard

What is the consequence of passing a `null` `Boolean` object to an `if` condition, such as `if (myBooleanObject)`?

Q2563 hard code error

What is the error encountered when running this Java code?

java

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

public class FileReaderError10 {
    public static void main(String[] args) {
        File file = new File("test_fr10.txt");
        try {
            file.createNewFile();
            try (FileReader fr = new FileReader(file)) {
                fr.close(); // Close the stream
                boolean readyStatus = fr.ready(); // Check ready on a closed stream
                System.out.println("Ready status: " + readyStatus);
            }
        } catch (IOException e) {
            System.err.println(e.getClass().getSimpleName() + ": " + e.getMessage());
        } finally {
            file.delete();
        }
    }
}

Q2564 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("Start"); // Length 5
        String toInsert = "End";
        // Attempt to insert at index 6 (length + 1)
        sb.insert(sb.length() + 1, toInsert);
        System.out.println(sb);
    }
}

Q2565 medium code error

What is the runtime behavior of this Java `TreeMap` code?

java

import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        TreeMap<String, Integer> map = new TreeMap<>();
        map.put("One", 1);
        map.put("Two", 2);
        Object nonStringKey = new Integer(3);
        map.get(nonStringKey);
    }
}

Q2566 easy

Where is the `throws` keyword used in a Java program?

Q2567 hard

Is it generally possible for `FileWriter.write(char[] cbuf, int off, int len)` to successfully write *fewer* than `len` characters to the underlying file without throwing an `IOException`?

Q2568 easy code error

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

java

public class MyMonitor {
    public void myMethod() {
        try {
            this.wait(); // Calling wait() without holding the monitor
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

    public static void main(String[] args) {
        MyMonitor monitor = new MyMonitor();
        new Thread(monitor::myMethod).start();
    }
}

Q2569 medium code output

What does this Java code print to the console?

java

public class StringBufferTest {
    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("HelloWorld");
        sb.replace(5, 10, " Java");
        System.out.print(sb);
    }
}

Q2570 easy

If you want to create a custom exception that callers are *not required* to handle or declare, which class should it extend?

Q2571 medium code error

Which exception will be thrown when executing this Java code?

java

public class StringError7 {
    public static void main(String[] args) {
        String val = "3.14.15";
        double d = Double.parseDouble(val);
        System.out.println(d);
    }
}

Q2572 medium

If a class has two methods: `void calculate(int a, int b)` and `void calculate(int... numbers)`, which method will be invoked by the call `calculate(10, 20)`?

Q2573 medium

A thread instance is created using `new Thread()`. What is its initial state before the `start()` method is invoked?

Q2574 hard code output

What is the output of this Java code?

java

import java.io.FileWriter;
import java.io.IOException;
import java.io.FileReader;
import java.io.BufferedReader;
import java.nio.file.Files;
import java.nio.file.Path;

public class FileWriterCharArrayOffset {
    public static void main(String[] args) {
        String filename = "char_array_offset.txt";
        char[] data = {'J', 'a', 'v', 'a', 'I', 'O', 'S', 't', 'r', 'e', 'a', 'm'};
        try (FileWriter fw = new FileWriter(filename)) {
            fw.write(data, 4, 3);
            fw.write(" is ");
            fw.write(data, 0, 4);
        } catch (IOException e) { System.out.println("Error: " + e.getMessage()); }

        try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
            System.out.println(br.readLine());
        } catch (IOException e) { System.out.println("Error reading: " + e.getMessage());
        } finally { try { Files.deleteIfExists(Path.of(filename)); } catch (IOException ignored) {} }
    }
}

Q2575 medium code output

What does this code print?

java

import java.io.BufferedReader;
import java.io.StringReader;
import java.io.IOException;

public class BufferedReaderTest {
    public static void main(String[] args) {
        String data = "HelloWorld";
        char[] buffer = new char[5];
        try (BufferedReader br = new BufferedReader(new StringReader(data))) {
            int charsRead = br.read(buffer, 0, 5);
            System.out.println(charsRead);
            System.out.println(new String(buffer));
            charsRead = br.read(buffer, 0, 3);
            System.out.println(charsRead);
            System.out.println(new String(buffer, 0, charsRead));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Q2576 hard

When running a Java application with a restrictive `SecurityManager`, which type of exception is most likely to be thrown by `java.io.File` methods like `delete()`, `mkdir()`, or `exists()` if the required permissions are not granted?

Q2577 medium

Which method must be implemented by a class that implements the `Runnable` interface?

Q2578 hard

If a HashMap is initialized with a capacity of 17 and a load factor of 0.75, what will be its actual internal initial capacity and when does the first automatic rehashing operation typically occur?

Q2579 medium

A method `foo()` declares `throws CustomCheckedException`. What must a calling method do if it invokes `foo()`?

Q2580 medium code output

What does this Java code snippet using a switch expression with `yield` print?

java

public class SwitchTest {
    public static void main(String[] args) {
        int month = 4; // April
        String quarter = switch (month) {
            case 1, 2, 3 -> {
                yield "Q1";
            }
            case 4, 5, 6 -> {
                yield "Q2";
            }
            case 7, 8, 9 -> {
                yield "Q3";
            }
            default -> {
                yield "Q4";
            }
        };
        System.out.println(quarter);
    }
}

☕ Java MCQ Questions – Page 129