Java Notes
HubWhat Is Java?History of JavaFeatures of JavaJava EditionsJDK, JRE, and JVMYour First Java ProgramJava Program StructureJava Compilation ProcessIdentifiers in JavaKeywords in JavaVariables in JavaData Types in JavaType Casting in JavaOperators in JavaInput and Output in JavaComments in JavaIf-Else Statements in JavaSwitch-Case in JavaFor Loop in JavaWhile Loop in JavaDo-While Loop in JavaNested Loops in Javabreak and continue in JavaArray IntroductionOne Dimensional ArrayMulti Dimensional ArrayJagged Array in JavaArray OperationsArray Programs — FundamentalsArray Interview QuestionsString Class in JavaImmutable Strings in JavaString MethodsStringBuilder in JavaStringBuffer in JavaString Programs — FundamentalsString Interview QuestionsOOPs IntroductionClass and Object in JavaConstructor in Javathis Keyword in Javastatic Keyword in JavaEncapsulation in JavaInheritance in Javasuper Keyword in JavaPolymorphism in JavaMethod Overloading in JavaMethod Overriding in JavaAbstraction in JavaAbstract Class in JavaInterface in JavaObject Class in JavaWrapper Class in JavaInner Class in JavaAnonymous Class in JavaObject Cloning in JavaJava Collections Framework OverviewGenerics in JavaArrayList in JavaLinkedList in JavaVector in JavaStack in JavaHashSet in JavaLinkedHashSet in JavaTreeSet in JavaPriorityQueue in JavaArrayDeque in JavaDeque Interface in JavaHashMap in JavaLinkedHashMap in JavaTreeMap in JavaHashtable in JavaIterator in JavaComparable vs Comparator in JavaJava Collections Practice ProgramsException Handling IntroductionException Hierarchy in JavaTry-Catch Block in JavaFinally Block in Javathrow and throws Keywords in JavaCustom Exceptions in JavaException Handling Best PracticesFile Class in JavaByte Streams in JavaCharacter Streams in JavaBuffered Streams in JavaSerialization in JavaDeserialization in JavaJava NIO PackageLambda Expression in Java 8Functional Interface in Java 8Method Reference in Java 8Default and Static Methods in InterfacesStream API in Java 8Optional Class in Java 8Date and Time API in Java 8Thread IntroductionThread Life Cycle in JavaThread Class in JavaRunnable Interface in JavaCreating Threads in JavaThread Priority in JavaDaemon Threads in JavaSynchronization in JavaInter-Thread Communication in JavaExecutor Framework in JavaMultithreading Programs in JavaJDBC IntroductionJDBC ArchitectureMySQL Connection with JDBCStatement Interface in JDBCPreparedStatement in JDBCResultSet in JDBCCallableStatement in JDBCTransaction Management in JDBCBatch Processing in JDBCJDBC ProjectsServlet IntroductionGenericServletHttpServletServlet Life CycleRequest and Response ObjectsCookies in ServletsSession TrackingJSP IntroductionJSP Tags and JSTLExpression Language (EL)MVC ArchitectureSpring Framework IntroductionSpring Core ContainerSpring BeansDependency InjectionSpring Boot IntroductionREST API with Spring BootValidation in Spring BootCRUD ApplicationSpring Data JPAException Handling in Spring BootSpring SecurityJWT Authentication — Java ProgrammingConfig ServerEureka Server - Service DiscoveryAPI GatewayDocker DeploymentTime ComplexityRecursion in JavaLinked List in JavaStack in Java — DSA in JavaQueue in JavaHashing in JavaSorting Algorithms in JavaSearching Algorithms in JavaTree Data Structure in JavaHeap (DSA)Graph Data Structure in JavaDynamic Programming in JavaSingleton Design PatternFactory Design PatternBuilder Design PatternAdapter Design PatternObserver Design PatternMVC Pattern — Java ProgrammingBasic Java ProgramsArray Programs — PracticeString Programs — PracticeOOP ProgramsCollection ProgramsMultithreading ProgramsJDBC ProgramsPlacement Coding QuestionsStudent Management System — Java ProgrammingEmployee Management SystemLibrary Management System — Java ProgrammingBanking System — Java ProgrammingChat ApplicationE-Commerce BackendSpring Boot Fullstack ProjectCore Java Interview QuestionsOOPs Interview QuestionsCollections Interview QuestionsMultithreading Interview QuestionsJDBC Interview QuestionsSpring Framework Interview QuestionsCoding Interview QuestionsJava Cheat SheetQuick Revision Notes — Java ProgrammingImportant Java ProgramsImportant Formulas & ConceptsJava Master Course - Learning Roadmap
StringBuilder is a mutable sequence of characters for efficiently building or editing text, especially inside loops in a single-threaded program.
StringBuilder holds editable text. Use it when a program repeatedly adds, removes, or changes text. Unlike String, its methods change the same object and then return that object for further work.
Build text in a loop
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Every append() adds to the existing builder. With String, repeated + inside this loop would repeatedly create temporary values. Use StringBuilder for generated reports, CSV lines, HTML fragments, or any text assembled in many steps.
Edit the characters already present
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insert(position, value) adds characters. replace(start, end, value) replaces the range from start up to but not including end. delete(start, end) removes that range. Check indexes carefully: an invalid index throws StringIndexOutOfBoundsException.
Turn it back into String
Use toString() when you have finished constructing the text and need a normal immutable String.
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Use StringBuilder in ordinary single-threaded code. If one mutable buffer is deliberately shared by multiple threads, use StringBuffer, whose operations are synchronised.
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Study next in this module
01FundamentalsString MethodsJava String methods read, search, compare, extract, and transform text. Because String is immutable, methods that change text return a new String.02FundamentalsStringBuffer in JavaStringBuffer is a mutable, synchronized character sequence for text that must be changed safely by more than one thread.03FundamentalsImmutable Strings in JavaA Java String cannot be changed after it is created. Concatenation, replacement, and case conversion produce a new String and leave the old value intact.04FundamentalsString Programs — FundamentalsPractice essential Java String programs by learning the logic for reversing text, checking a palindrome, and counting characters rather than memorising output.