Java MCQ Questions – Page 155 of 200

Q: Which Java primitive data type is used to store whole numbers with the largest possible range?

D) long. The `long` data type is a 64-bit integer, providing the largest range among Java's primitive integer types.

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

A) Runtime error: NullPointerException.. The `numbers` array reference is explicitly set to `null`. Attempting to access any member of a null reference, such as `.length`, results in a runtime `NullPointerException`.

Q: What is the key difference between the `trim()` and `strip()` String methods regarding the types of whitespace characters they remove?

B) `strip()` removes all Unicode whitespace, while `trim()` only removes whitespace characters less than or equal to U+0020.. The `trim()` method removes leading and trailing whitespace characters whose ASCII value is less than or equal to U+0020. The `strip()` method (Java 11+) removes all Unicode whitespace characters, which includes characters beyond the ASCII range.

Q: Which interface must a class implement to be eligible for Java's default serialization mechanism?

A) java.io.Serializable. The `Serializable` interface is a marker interface that signals to the Java Virtual Machine that instances of the class can be serialized.

Q: What does this Java code print?

A) 15.0. An int can be implicitly (widening) cast to a float. The integer value becomes a floating-point number, represented with a decimal point.

Q: Multiple threads concurrently access and modify a `TreeMap` instance without any external synchronization. Which of the following is the most likely outcome?

C) Data corruption and inconsistent state, potentially leading to infinite loops during iteration or incorrect size.. `TreeMap` is not thread-safe. Concurrent modifications can lead to race conditions, causing internal data structure corruption, which manifests as inconsistent states, incorrect sizes, or even infinite loops during operations like iteration, rather than guaranteed exceptions.

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

B) Compilation Error: `start()` in `Car` cannot override `start()` in `Vehicle`; attempting to assign weaker access privileges.. When overriding a method, the access modifier of the overriding method cannot be more restrictive (weaker) than the overridden method. Changing `public` to `protected` is a reduction in visibility, causing a compile-time error.

Q: What is the compilation error in the provided Java code?

A) CastingProblem.java:7: error: incompatible types: Alpha cannot be converted to Beta. Java enforces type safety at compile time. Since `Alpha` and `Beta` are completely unrelated classes (neither is a supertype/subtype of the other, and they don't implement common interfaces that could allow a cast), the compiler identifies this as an impossible cast.

Q: What is the output of this code?

A) 2,-1,0. Since `CustomObject` does not override `equals()` or `hashCode()`, `indexOf` and `lastIndexOf` use `Object.equals()`, which checks for reference equality. `obj2` is at index 2. `new CustomObject(2)` is a new instance, not present by reference, so `indexOf` returns -1. `obj1` is first found at index 0, so `lastIndexOf` also returns 0 as it finds the same object reference.

Q: What is the output of this code?

A) [2, 8]. The remove() method removes the specified element from the set. Since 5 is removed, the remaining sorted elements are 2 and 8.

Q3081 easy

Which Java primitive data type is used to store whole numbers with the largest possible range?

Q3082 easy code error

What is the error in the following Java code snippet?

java

public class ArrayError {
    public static void main(String[] args) {
        int[] numbers = null;
        System.out.println(numbers.length);
    }
}

Q3083 medium

What is the key difference between the `trim()` and `strip()` String methods regarding the types of whitespace characters they remove?

Q3084 easy

Which interface must a class implement to be eligible for Java's default serialization mechanism?

Q3085 easy code output

What does this Java code print?

java

public class TypeCast {
    public static void main(String[] args) {
        int count = 15;
        float price = count;
        System.out.println(price);
    }
}

Q3086 hard

Multiple threads concurrently access and modify a `TreeMap` instance without any external synchronization. Which of the following is the most likely outcome?

Q3087 hard code error

What is the error in the following Java code?

java

class Vehicle {
    public void start() {
        System.out.println("Vehicle starting");
    }
}

class Car extends Vehicle {
    @Override
    protected void start() { // Attempting to reduce access modifier
        System.out.println("Car starting");
    }
}

Q3088 medium code error

What is the compilation error in the provided Java code?

java

class Alpha {}
class Beta {}

public class CastingProblem {
    public static void main(String[] args) {
        Alpha a = new Alpha();
        Beta b = (Beta) a; // ERROR: incompatible types
        System.out.println("Casting successful!");
    }
}

Q3089 hard code output

What is the output of this code?

java

import java.util.ArrayList;

class CustomObject {
    int id;
    public CustomObject(int id) { this.id = id; }
    @Override
    public String toString() { return "Custom:" + id; }
}

public class Test {
    public static void main(String[] args) {
        ArrayList<CustomObject> list = new ArrayList<>();
        CustomObject obj1 = new CustomObject(1);
        CustomObject obj2 = new CustomObject(2);
        list.add(obj1);
        list.add(new CustomObject(2)); 
        list.add(obj2);
        list.add(new CustomObject(1)); 

        int index1 = list.indexOf(obj2);
        int index2 = list.indexOf(new CustomObject(2)); 
        int index3 = list.lastIndexOf(obj1);

        System.out.println(index1 + "," + index2 + "," + index3);
    }
}

Q3090 easy code output

What is the output of this code?

java

import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        TreeSet<Integer> ts = new TreeSet<>();
        ts.add(5);
        ts.add(2);
        ts.add(8);
        ts.remove(5);
        System.out.println(ts);
    }
}

Q3091 medium code output

What is the length of the StringBuilder object after execution?

java

public class StringBuilderTest {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Hello");
        sb.append("World");
        sb.insert(5, " ");
        System.out.println(sb.length());
    }
}

Q3092 medium code output

What is the output of this code?

java

import java.util.ArrayDeque;
import java.util.Queue;

public class ArrayDequeMethods {
    public static void main(String[] args) {
        Queue<Integer> q = new ArrayDeque<>();
        q.add(100);
        q.offer(200);
        q.remove();
        q.add(300);
        System.out.println(q.element());
    }
}

Q3093 hard

Which of the following statements about accessing local variables from within a lambda expression is TRUE?

Q3094 easy

How do you initialize a 2D integer array named `matrix` with 3 rows and 4 columns?

Q3095 medium code output

What does this code print?

java

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

public class Main {
    public static void checkValue(int value) throws CustomError {
        if (value < 10) {
            throw new CustomError("Value too low!");
        }
        System.out.println("Value is acceptable.");
    }

    public static void main(String[] args) {
        try {
            checkValue(5);
        } catch (CustomError e) {
            System.out.println("Error details: " + e.toString());
        } finally {
            System.out.println("Finally block executed.");
        }
    }
}

Q3096 easy

What is a primary difference between a Java array and a `java.util.ArrayList` in terms of size?

Q3097 easy code output

What does this Java code snippet print?

java

public class WhileLoopDemo {
    public static void main(String[] args) {
        int a = 0;
        while (a++ < 3) {
            System.out.print(a);
        }
    }
}

Q3098 hard code output

What does this code print, illustrating how serialization interacts with encapsulation of mutable fields?

java

import java.io.*; import java.util.HashMap; import java.util.Map;
class Config implements Serializable {
    private Map<String, String> settings;
    public Config(Map<String, String> initialSettings) { this.settings = initialSettings; }
    public Map<String, String> getSettings() { return settings; }
}
public class Main {
    public static void main(String[] args) throws Exception {
        Config originalConfig = new Config(new HashMap<>(Map.of("theme", "dark")));
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        new ObjectOutputStream(bos).writeObject(originalConfig);
        Config deserializedConfig = (Config) new ObjectInputStream(new ByteArrayInputStream(bos.toByteArray())).readObject();
        deserializedConfig.getSettings().put("language", "en");
        System.out.println("Original: " + originalConfig.getSettings());
        System.out.println("Deserialized: " + deserializedConfig.getSettings());
    }
}

Q3099 hard code error

What is the compilation error, if any, for the provided Java code snippet?

java

class Overloader {
    public void setup(int a, long b) {}
    public void setup(long a, int b) {}

    public static void main(String[] args) {
        Overloader o = new Overloader();
        o.setup(10, 20); // Line X
    }
}

Q3100 medium code output

What is the output of the following Java program?

java

public class StringTest {
    public static void main(String[] args) {
        String sentence = "The quick brown fox jumps over the lazy dog.";
        boolean check1 = sentence.startsWith("The");
        boolean check2 = sentence.endsWith("dog");
        boolean check3 = sentence.contains("fox");
        System.out.println(check1 + "," + check2 + "," + check3);
    }
}

☕ Java MCQ Questions – Page 155