Java MCQ Questions – Page 56 of 200

Q: What error will this Java code produce when executed?

A) ArrayIndexOutOfBoundsException. The inner loop condition `j <= matrix[i].length` attempts to access `matrix[i][matrix[i].length]`, which is one index beyond the valid range (0 to `length-1`). This will cause an `ArrayIndexOutOfBoundsException`.

Q: What error will this code produce at runtime?

A) java.util.ConcurrentModificationException. Modifying a collection (removing an element) while iterating over it using an enhanced for loop leads to a `ConcurrentModificationException` at runtime.

Q: Which of the following is NOT a direct benefit of applying encapsulation in Java programming?

D) Automatic prevention of memory leaks by clearing unused objects.. Encapsulation improves maintainability, reduces coupling, and protects state integrity. Automatic memory leak prevention is handled by Java's garbage collector, which is unrelated to encapsulation.

Q: Consider the declaration: `int[][] matrix = new int[3][];` Which statement about the state of `matrix` after this declaration is true?

B) `matrix` is an array of 3 array references, and each of these 3 inner array references is `null`.. When only the first dimension of a multi-dimensional array is specified, Java creates an array whose elements are references to other arrays. These inner array references are initialized to `null` by default; they do not point to actual arrays until explicitly instantiated.

Q: What is the output of this Java program?

A) Name: Alice, Age: 30. Serialization captures the state of the object at the time `writeObject()` is called. Changing the original object after serialization does not affect the data that was written to the stream, which is then restored upon deserialization.

Q: What is the compile-time or runtime error in the following Java code snippet when `main` method is executed?

A) java.io.NotSerializableException: SerializationQuestion5$$Lambda$1/XXXXXXXXX (or similar generated lambda class name). Lambda expressions in Java are generally not serializable by default, as they often capture context (variables from the enclosing scope) that might not be serializable or reliably recreatable. Attempting to serialize a class that has a non-transient field initialized with a lambda expression will result in a `NotSerializableException` for the generated lambda class.

Q: What compile-time error will this Java code produce?

A) Incompatible types: cannot convert from String to char. Character literals must be enclosed in single quotes (e.g., `'J'`). Double quotes create a `String` literal. Attempting to assign a `String` to a `char` variable results in an 'incompatible types' compile-time error.

Q: What is the compile-time error in the provided Java code snippet?

A) Compile-time error: 'incompatible types: long cannot be converted to int'. The `switch` statement in Java does not support `long`, `boolean`, `float`, or `double` as selector types. Attempting to use a `long` results in a compile-time error because it cannot be implicitly converted to `int`, which is the widest primitive type supported besides `String`, `char`, `byte`, `short`, and their wrapper classes.

Q: What error will occur after deserializing the `User` object, given the `password` field is `transient`?

A) NullPointerException. A `transient` field is not serialized. Upon deserialization, its value will be its default (null for object types, 0/false for primitives). Attempting to call a method (`.length()`) on a `null` `password` field will result in a `NullPointerException`.

Q: How does Java 8+ allow for a form of 'multiple inheritance of implementation' and what mechanism resolves potential conflicts?

B) Through `default` methods in interfaces, with specific rules for resolving diamond problem ambiguities by requiring explicit overriding or preferring class implementations.. `default` methods in interfaces allow classes to inherit implementations from multiple interfaces. Java resolves diamond problem ambiguities by prioritizing concrete class methods over interface `default` methods, and requiring explicit override if multiple interfaces provide a `default` method with the same signature.

Q1101 easy code error

What error will this Java code produce when executed?

java

public class ArrayError {
    public static void main(String[] args) {
        int[][] matrix = {{1, 2}, {3, 4}};
        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j <= matrix[i].length; j++) { // Loop condition error
                System.out.print(matrix[i][j]);
            }
        }
    }
}

Q1102 hard code error

What error will this code produce at runtime?

java

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

public class ConcurrentModLoop {
    public static void main(String[] args) {
        List<String> items = new ArrayList<>();
        items.add("Apple");
        items.add("Banana");
        items.add("Cherry");

        for (String item : items) {
            if (item.equals("Banana")) {
                items.remove(item);
            }
        }
        System.out.println(items);
    }
}

Q1103 medium

Which of the following is NOT a direct benefit of applying encapsulation in Java programming?

Q1104 hard

Consider the declaration: `int[][] matrix = new int[3][];` Which statement about the state of `matrix` after this declaration is true?

Q1105 easy code output

What is the output of this Java program?

java

import java.io.*;

class Person implements Serializable {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String toString() {
        return "Name: " + name + ", Age: " + age;
    }
}

public class Main {
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        Person p = new Person("Alice", 30);
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(p); 
        oos.close();

        p = new Person("Bob", 25); // Original object reference changed

        ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
        ObjectInputStream ois = new ObjectInputStream(bis);
        Person deserializedP = (Person) ois.readObject();
        ois.close();

        System.out.println(deserializedP);
    }
}

Q1106 medium code error

What is the compile-time or runtime error in the following Java code snippet when `main` method is executed?

java

import java.io.*;

class TaskContainer implements Serializable {
    // Runnable is not Serializable by default, and lambda expressions are tricky to serialize.
    private Runnable task = () -> System.out.println("Executing task");
    private String description = "Simple Task";
}

public class SerializationQuestion5 {
    public static void main(String[] args) throws IOException {
        TaskContainer container = new TaskContainer();
        try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("task.ser"))) {
            oos.writeObject(container);
        }
    }
}

Q1107 easy code error

What compile-time error will this Java code produce?

java

public class DataTypeCharStringError {
    public static void main(String[] args) {
        char initial = "J";
        System.out.println(initial);
    }
}

Q1108 hard code error

What is the compile-time error in the provided Java code snippet?

java

public class InvalidSwitchSelectorType {
    public static void main(String[] args) {
        long id = 12345L;
        String category = "Unknown";
        switch (id) { // Error expected here
            case 100L: category = "A"; break;
            case 200L: category = "B"; break;
            default: category = "Other";
        }
        System.out.println(category);
    }
}

Q1109 easy code error

What error will occur after deserializing the `User` object, given the `password` field is `transient`?

java

import java.io.*;

class User implements Serializable {
    String username;
    transient String password; 

    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }
    public String getPassword() { return password; }
}

public class Main {
    public static void main(String[] args) {
        User user = new User("admin", "secret123");
        try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("user.ser"))) {
            oos.writeObject(user);
        } catch (IOException e) { /* handle exception */ }

        try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream("user.ser"))) {
            User deserializedUser = (User) ois.readObject();
            System.out.println(deserializedUser.getPassword().length()); // Access transient field
        } catch (IOException | ClassNotFoundException e) {
            System.err.println(e.getClass().getSimpleName() + ": " + e.getMessage());
        }
    }
}

Q1110 hard

How does Java 8+ allow for a form of 'multiple inheritance of implementation' and what mechanism resolves potential conflicts?

Q1111 hard

A `TreeSet<Integer>` named `mainSet` contains `{1, 2, 3, 4, 5}`. A `headSet` view is created as `viewSet = mainSet.headSet(3, true)`. If `viewSet.remove(2)` is called, what is the state of `mainSet` afterwards?

Q1112 medium code error

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

java

public class ArrayError {
    public static void main(String[] args) {
        Object[] myObjects = new String[3];
        myObjects[0] = new Integer(123);
    }
}

Q1113 easy code error

What will happen when you try to compile this Java code?

java

class MyValue {
    private final int val;
    public MyValue(int v) {
        this.val = v;
    }
    public void attemptChange(int newVal) {
        this.val = newVal;
    }
    public int getVal() { return val; }
}
public class ImmutableInternalReassignment {
    public static void main(String[] args) {
        MyValue mv = new MyValue(100);
        mv.attemptChange(200);
        System.out.println(mv.getVal());
    }
}

Q1114 hard code error

What is the compile-time error in this Java code?

java

public class ParameterizedRun implements Runnable {
    @Override
    public void run(String message) { // Line 3
        System.out.println(message);
    }

    public static void main(String[] args) {
        // new Thread(new ParameterizedRun()).start();
    }
}

Q1115 easy

What is the default value for an uninitialized instance variable of a reference type (e.g., String, Object) in Java?

Q1116 hard code output

What is the output of this code?

java

public class Test {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("JavaIsFun");
        sb.setLength(15);
        sb.setCharAt(10, 'A');
        sb.setLength(7);
        System.out.println(sb.toString());
    }
}

Q1117 hard

You have a class `class MyKey { int id; MyKey(int id) { this.id = id; } }` which does *not* implement `Comparable`. If you create `TreeMap<MyKey, String> map = new TreeMap<>();` and then call `map.put(new MyKey(1), "value");`, what will occur?

Q1118 easy

What does a Java HashMap primarily store?

Q1119 easy code output

What is the output of this code?

java

abstract class Animal {
    abstract String makeSound();
}

class Dog extends Animal {
    @Override
    String makeSound() {
        return "Woof";
    }
}

public class Main {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        System.out.println(myDog.makeSound());
    }
}

Q1120 easy

What are the default values for elements in a newly initialized `int[][]` array in Java?

☕ Java MCQ Questions – Page 56