Java Notes
HubArray IntroductionArray OperationsArray Programs — FundamentalsArray Interview QuestionsJagged Array in JavaMulti Dimensional ArrayOne Dimensional Arraybreak and continue in JavaDo-While Loop in JavaFor Loop in JavaIf-Else Statements in JavaNested Loops in JavaSwitch-Case in JavaWhile Loop in JavaJava Compilation ProcessFeatures of JavaYour First Java ProgramHistory of JavaJava EditionsJava Program StructureJDK, JRE, and JVMWhat is Java?Immutable Strings in JavaString Class in JavaString Interview QuestionsString MethodsString Programs — FundamentalsStringBuffer in JavaStringBuilder in JavaComments in JavaData Types in JavaIdentifiers in JavaInput and Output in JavaKeywords in JavaOperators in JavaType Casting in JavaVariables in JavaAbstract Class in JavaAbstraction in JavaAnonymous Class in JavaClass and Object in JavaConstructor in JavaEncapsulation in JavaInheritance in JavaInner Class in JavaInterface in JavaMethod Overloading in JavaMethod Overriding in JavaObject Class in JavaObject Cloning in JavaOOPs IntroductionPolymorphism in Javastatic Keyword in Javasuper Keyword in Javathis Keyword in JavaWrapper Class in JavaCollections Framework OverviewCollections Practice ProgramsComparable vs Comparator in JavaGenerics in JavaIterator in JavaArrayList in JavaLinkedList in JavaStack in Java — Collections FrameworkVector in JavaHashMap in JavaHashtable in JavaLinkedHashMap in JavaTreeMap in JavaArrayDeque in JavaDeque Interface in JavaPriorityQueue in JavaHashSet in JavaLinkedHashSet in JavaTreeSet in JavaException Handling Best PracticesCustom Exceptions in JavaException Hierarchy in JavaFinally Block in JavaException Handling Introductionthrow and throws Keywords in JavaTry-Catch Block in JavaBuffered Streams in JavaByte Streams in JavaCharacter Streams in JavaDeserialization in JavaFile Class in JavaJava NIO PackageSerialization in JavaDate and Time API in Java 8Default and Static Methods in InterfacesFunctional Interface in Java 8Lambda Expression in Java 8Method Reference in Java 8Optional Class in Java 8Stream API in Java 8Creating Threads in JavaDaemon Threads in JavaExecutor Framework in JavaInter-Thread Communication in JavaMultithreading Programs in JavaRunnable Interface in JavaSynchronization in JavaThread Class in JavaThread IntroductionThread Life Cycle in JavaThread Priority in JavaBatch Processing in JDBCCallableStatement in JDBCJDBC ArchitectureJDBC IntroductionJDBC ProjectsMySQL Connection with JDBCPreparedStatement in JDBCResultSet in JDBCStatement Interface in JDBCTransaction Management in JDBCCookies in ServletsExpression Language (EL)GenericServletHttpServletJSP IntroductionJSP Tags and JSTLMVC ArchitectureRequest and Response ObjectsServlet IntroductionServlet Life CycleSession TrackingDependency InjectionAPI GatewayConfig ServerDocker DeploymentEureka Server - Service DiscoverySpring BeansCRUD ApplicationException Handling in Spring BootJWT Authentication — Java ProgrammingREST API with Spring BootSpring SecuritySpring Boot IntroductionSpring Data JPAValidation in Spring BootSpring Core ContainerSpring Framework IntroductionDynamic Programming in JavaGraph Data Structure in JavaHashing in JavaHeap (DSA)Linked List in JavaQueue in JavaRecursion in JavaSearching Algorithms in JavaSorting Algorithms in JavaStack in Java — DSA in JavaTime ComplexityTree Data Structure in JavaAdapter Design PatternBuilder Design PatternFactory Design PatternMVC Pattern — Java ProgrammingObserver Design PatternSingleton Design PatternArray Programs — PracticeBasic Java ProgramsCollection ProgramsJDBC ProgramsMultithreading ProgramsOOP ProgramsPlacement Coding QuestionsString Programs — PracticeBanking System — Java ProgrammingChat ApplicationE-Commerce BackendEmployee Management SystemLibrary Management System — Java ProgrammingSpring Boot Fullstack ProjectStudent Management System — Java ProgrammingCoding Interview QuestionsCollections Interview QuestionsCore Java Interview QuestionsJDBC Interview QuestionsMultithreading Interview QuestionsOOPs Interview QuestionsSpring Framework Interview QuestionsJava Cheat SheetImportant Formulas & ConceptsImportant Java ProgramsQuick Revision Notes — Java ProgrammingJava Master Course - Learning Roadmap
Complete guide to Java do-while loop — guaranteed first execution, syntax, practical examples, menu-driven programs, and comparison with while loop.
The do-while loop is an exit-controlled loop — it executes the body at least once before checking the condition. This makes it perfect for scenarios where you need to perform an action first and then decide whether to repeat it.
Syntax and Behavior
Example
public class DoWhileBasics {
public static void main(String[] args) {
// do { body } while (condition);
// NOTE: semicolon after while is REQUIRED!
int count = 1;
do {
System.out.print(count + " ");
count++;
} while (count <= 5);
System.out.println();
// Key difference from while: executes at least once
int x = 100;
do {
System.out.println("This executes even though x > 10!");
} while (x < 10); // false immediately, but body already ran
}
}
Output
1 2 3 4 5 This executes even though x > 10!
Do-While vs While
Example
public class DoWhileVsWhile {
public static void main(String[] args) {
int value = 10;
// While: condition checked FIRST
System.out.print("while loop: ");
while (value < 5) {
System.out.print(value + " ");
value++;
}
System.out.println("(never executed)");
// Do-while: body executed FIRST
value = 10;
System.out.print("do-while: ");
do {
System.out.print(value + " ");
value++;
} while (value < 5);
System.out.println("(executed once)");
}
}
Output
while loop: (never executed) do-while: 10 (executed once)
Practical Examples
Menu-Driven Program
Example
import java.util.Scanner;
public class MenuProgram {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int choice;
do {
System.out.println("\n╔═══════════════════╗");
System.out.println("║ CALCULATOR ║");
System.out.println("╠═══════════════════╣");
System.out.println("║ 1. Add ║");
System.out.println("║ 2. Subtract ║");
System.out.println("║ 3. Multiply ║");
System.out.println("║ 4. Exit ║");
System.out.println("╚═══════════════════╝");
System.out.print("Choose (1-4): ");
choice = sc.nextInt();
if (choice >= 1 && choice <= 3) {
System.out.print("Enter two numbers: ");
double a = sc.nextDouble(), b = sc.nextDouble();
double result = switch (choice) {
case 1 -> a + b;
case 2 -> a - b;
case 3 -> a * b;
default -> 0;
};
System.out.printf("Result: %.2f%n", result);
} else if (choice != 4) {
System.out.println("Invalid choice!");
}
} while (choice != 4);
System.out.println("Goodbye!");
sc.close();
}
}
Output
╔═══════════════════╗ ║ CALCULATOR ║ ╠═══════════════════╣ ║ 1. Add ║ ║ 2. Subtract ║ ║ 3. Multiply ║ ║ 4. Exit ║ ╚═══════════════════╝ Choose (1-4): 1 Enter two numbers: 15 7 Result: 22.00 Choose (1-4): 4 Goodbye!
Number Guessing Game
Example
import java.util.Scanner;
import java.util.Random;
public class GuessingGame {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
Random rand = new Random();
int secret = rand.nextInt(100) + 1;
int guess;
int attempts = 0;
System.out.println("Guess the number (1-100)!");
do {
System.out.print("Your guess: ");
guess = sc.nextInt();
attempts++;
if (guess < secret) System.out.println("Too low!");
else if (guess > secret) System.out.println("Too high!");
else System.out.println("Correct! You got it in " + attempts + " attempts!");
} while (guess != secret);
sc.close();
}
}
Output
Guess the number (1-100)! Your guess: 50 Too low! Your guess: 75 Too high! Your guess: 62 Correct! You got it in 3 attempts!
Common Mistakes
- Forgetting the semicolon —
do { ... } while (cond)requires;at the end. Without it, you get a compile error. - Using do-while when while is more appropriate — if you don't need guaranteed first execution, use while. Do-while adds unnecessary complexity.
- Forgetting to update the condition — same risk as while loop: ensure progress toward termination.
Interview Questions
Q1: When should you use do-while instead of while?
Answer: Use do-while when the loop body must execute at least once regardless of the condition — common in menu-driven programs, input validation (prompt then check), and game loops (do one round then ask to continue).
Q2: What is the key difference between while and do-while?
Answer: while is entry-controlled (condition checked before body — may execute 0 times). do-while is exit-controlled (body executes first, then condition is checked — always executes at least once).
Q3: Is the semicolon after do-while mandatory?
Answer: Yes. do { } while (condition); — the semicolon after the closing parenthesis is required. It terminates the do-while statement.
Summary
The do-while loop guarantees at least one execution of the loop body before checking the condition. Use it for menu-driven programs, game loops, and any scenario where you need to perform an action before deciding whether to repeat. Remember the mandatory semicolon and use it only when the "at least once" guarantee is genuinely needed.
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Do-While Loop in Java.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.
Search Terms
java-master-course, java master course, java, master, course, fundamentals, control, statements
Related Java Master Course Topics
Continue learning this concept
FundamentalsWhile Loop in JavaComplete guide to Java while loop — syntax, execution flow, sentinel-controlled loops, input validation, infinite loops, and comparison with for loop.Fundamentalsbreak and continue in JavaComplete guide to break and continue statements in Java covering labeled/unlabeled break, continue in loops, break in switch, use cases, flow diagrams, and best practices with detailed examples.FundamentalsFor Loop in JavaComplete guide to Java for loops — traditional for loop, enhanced for-each loop, infinite loops, multiple variables, nested loops, and practical patterns with examples.FundamentalsIf-Else Statements in JavaComplete guide to Java if-else statements — simple if, if-else, if-else-if ladder, nested if, and best practices with practical examples and common patterns.FundamentalsNested Loops in JavaComplete guide to nested loops in Java covering nested for, while, do-while loops, pattern printing, matrix operations, multiplication tables, time complexity analysis, and optimization techniques.FundamentalsSwitch-Case in JavaComplete guide to Java switch statement — traditional switch, enhanced switch expressions (Java 14+), fall-through behavior, pattern matching, and when to use switch vs if-else.