Computer Knowledge

Java Core Classes and Threads

1,473 Questions

Java core classes and threads form the foundation of object oriented programming and are crucial for IT officer and programming exams. This includes concepts like string mutability, collections, and multithreading. Solve these questions to test your practical coding and theoretical knowledge.

String and StringBuffer classesThread execution methodsJava collections frameworkCharacter streams input outputVariable serialization rules

Java Core Classes and Threads Questions

Multiple choice

Which of the following statements is/are correct about volatile variables?

  1. These variable are used to save an object into a file.

  2. The thread will read the most recent updated value by using volatile variable.

  3. It is used to create variables and methods that will exist independently of any instances created for the class.

  4. Volatile variables are used in asynchronous medium.

  5. Both (2) and (4)
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

.

Multiple choice

What is the output? class sample4 extends Thread { public static void main(String[] args) { new sample4().start(); } public void run() { try { System.out.println("Starting to wait"); wait(1000); System.out.println("Done waiting, returning back"); } catch(InterruptedException e) { System.out.println("InterruptedException Found"); } catch(Exception e) { System.out.println("Exception found"); } } }

  1. Starting to waitGoes an infinite wait and deadlock

  2. Starting to waitException found

  3. Starting to waitDone waiting, returning back

  4. Starting to waitInterruptedException Found

  5. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Correct choice The wait() method calls without lock so it causes IllegalMonitorStateException, which catch block finds Exception.

Multiple choice

What is the output of the following program? import java.io.*; class sample3 { int a; int b; sample3() { a=0; b=0; } sample3(int b1,int c1) { a=b1; b=c1; } final void print() { System.out.print(a); System.out.print(b); } } class sample31 extends sample3 { int c; sample31() { c=0; } sample31(int c) { this.c=c; super(4,5); } public static void main(String args[]) { sample3 ob1=new sample3(4,5); sample31 ob=new sample31(); System.out.print(ob.a); System.out.print(ob.b); System.out.print(ob.c); sample31 ob2=new sample31(5); System.out.print(ob2.a); System.out.print(ob2.b); System.out.print(ob2.c); }

  1. Cannot call super class constructor Call to the super must be first in the constructor.

  2. 000455

  3. 000005

  4. 000450

  5. 000000
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

super(*) keyword should be the first line constructor, in which it is defined.