Java Constructors (With Examples) - Programiz

A constructor in Java is similar to a method that is invoked when an object of the class is created. ... Here, Test() is a constructor. It has the same name as ... 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WhatisaConstructor? AconstructorinJavaissimilartoamethodthatisinvokedwhenanobjectoftheclassiscreated. UnlikeJavamethods,aconstructorhasthesamenameasthatoftheclassanddoesnothaveanyreturntype.Forexample, classTest{ Test(){ //constructorbody } } Here,Test()isaconstructor.Ithasthesamenameasthatoftheclassanddoesn'thaveareturntype. RecommendedReading:Whydoconstructorsnotreturnvalues Example1:JavaConstructor classMain{ privateStringname; //constructor Main(){ System.out.println("ConstructorCalled:"); name="Programiz"; } publicstaticvoidmain(String[]args){ //constructorisinvokedwhile //creatinganobjectoftheMainclass Mainobj=newMain(); System.out.println("Thenameis"+obj.name); } } Output: ConstructorCalled: ThenameisProgramiz Intheaboveexample,wehavecreatedaconstructornamedMain().Insidetheconstructor,weareinitializingthevalueofthenamevariable. NoticethestatementofcreatinganobjectoftheMainclass. Mainobj=newMain(); Here,whentheobjectiscreated,theMain()constructoriscalled.And,thevalueofthenamevariableisinitialized. Hence,theprogramprintsthevalueofthenamevariablesasProgramiz. TypesofConstructor InJava,constructorscanbedividedinto3types: No-ArgConstructor ParameterizedConstructor DefaultConstructor 1.JavaNo-ArgConstructors Similartomethods,aJavaconstructormayormaynothaveanyparameters(arguments). Ifaconstructordoesnotacceptanyparameters,itisknownasano-argumentconstructor.Forexample, privateConstructor(){ //bodyoftheconstructor } Example2:Javaprivateno-argconstructor classMain{ inti; //constructorwithnoparameter privateMain(){ i=5; System.out.println("Constructoriscalled"); } publicstaticvoidmain(String[]args){ //callingtheconstructorwithoutanyparameter Mainobj=newMain(); System.out.println("Valueofi:"+obj.i); } } Output: Constructoriscalled Valueofi:5 Intheaboveexample,wehavecreatedaconstructorMain().Here,theconstructordoesnotacceptanyparameters.Hence,itisknownasano-argconstructor. Noticethatwehavedeclaredtheconstructorasprivate. Onceaconstructorisdeclaredprivate,itcannotbeaccessedfromoutsidetheclass.So,creatingobjectsfromoutsidetheclassisprohibitedusingtheprivateconstructor. Here,wearecreatingtheobjectinsidethesameclass.Hence,theprogramisabletoaccesstheconstructor.Tolearnmore,visitJavaImplementPrivateConstructor. However,ifwewanttocreateobjectsoutsidetheclass,thenweneedtodeclaretheconstructoraspublic. Example3:Javapublicno-argconstructors classCompany{ Stringname; //publicconstructor publicCompany(){ name="Programiz"; } } classMain{ publicstaticvoidmain(String[]args){ //objectiscreatedinanotherclass Companyobj=newCompany(); System.out.println("Companyname="+obj.name); } } Output: Companyname=Programiz RecommendedReading:JavaAccessModifier 2.JavaParameterizedConstructor AJavaconstructorcanalsoacceptoneormoreparameters.Suchconstructorsareknownasparameterizedconstructors(constructorwithparameters). Example4:Parameterizedconstructor classMain{ Stringlanguages; //constructoracceptingsinglevalue Main(Stringlang){ languages=lang; System.out.println(languages+"ProgrammingLanguage"); } publicstaticvoidmain(String[]args){ //callconstructorbypassingasinglevalue Mainobj1=newMain("Java"); Mainobj2=newMain("Python"); Mainobj3=newMain("C"); } } Output: JavaProgrammingLanguage PythonProgrammingLanguage CProgrammingLanguage Intheaboveexample,wehavecreatedaconstructornamedMain().Here,theconstructortakesasingleparameter.Noticetheexpression, Mainobj1=newMain("Java"); Here,wearepassingthesinglevaluetotheconstructor.Basedontheargumentpassed,thelanguagevariableisinitializedinsidetheconstructor. 3.JavaDefaultConstructor Ifwedonotcreateanyconstructor,theJavacompilerautomaticallycreateano-argconstructorduringtheexecutionoftheprogram.Thisconstructoriscalleddefaultconstructor. Example5:DefaultConstructor classMain{ inta; booleanb; publicstaticvoidmain(String[]args){ //Adefaultconstructoriscalled Mainobj=newMain(); System.out.println("DefaultValue:"); System.out.println("a="+obj.a); System.out.println("b="+obj.b); } } Output: a=0 b=false Here,wehaven'tcreatedanyconstructors.Hence,theJavacompilerautomaticallycreatesthedefaultconstructor. Thedefaultconstructorinitializesanyuninitializedinstancevariableswithdefaultvalues. Type DefaultValue boolean false byte 0 short 0 int 0 long 0L char \u0000 float 0.0f double 0.0d object Referencenull Intheaboveprogram,thevariablesaandbareinitializedwithdefaultvalue0andfalserespectively. Theaboveprogramisequivalentto: classMain{ inta; booleanb; //aprivateconstructor privateMain(){ a=0; b=false; } publicstaticvoidmain(String[]args){ //calltheconstructor Mainobj=newMain(); System.out.println("DefaultValue:"); System.out.println("a="+obj.a); System.out.println("b="+obj.b); } } TheoutputoftheprogramisthesameasExample5. ImportantNotesonJavaConstructors Constructorsareinvokedimplicitlywhenyouinstantiateobjects. Thetworulesforcreatingaconstructorare: Thenameoftheconstructorshouldbethesameastheclass. AJavaconstructormustnothaveareturntype. Ifaclassdoesn'thaveaconstructor,theJavacompilerautomaticallycreatesadefaultconstructorduringrun-time.Thedefaultconstructorinitializesinstancevariableswithdefaultvalues.Forexample,theintvariablewillbeinitializedto0 Constructortypes:No-ArgConstructor-aconstructorthatdoesnotacceptanyargumentsParameterizedconstructor-aconstructorthatacceptsargumentsDefaultConstructor-aconstructorthatisautomaticallycreatedbytheJavacompilerifitisnotexplicitlydefined. Aconstructorcannotbeabstractorstaticorfinal. Aconstructorcanbeoverloadedbutcannotbeoverridden. ConstructorsOverloadinginJava SimilartoJavamethodoverloading,wecanalsocreatetwoormoreconstructorswithdifferentparameters.Thisiscalledconstructorsoverloading. Example6:JavaConstructorOverloading classMain{ Stringlanguage; //constructorwithnoparameter Main(){ this.language="Java"; } //constructorwithasingleparameter Main(Stringlanguage){ this.language=language; } publicvoidgetName(){ System.out.println("ProgrammingLangauage:"+this.language); } publicstaticvoidmain(String[]args){ //callconstructorwithnoparameter Mainobj1=newMain(); //callconstructorwithasingleparameter Mainobj2=newMain("Python"); obj1.getName(); obj2.getName(); } } Output: ProgrammingLanguage:Java ProgrammingLanguage:Python Intheaboveexample,wehavetwoconstructors:Main()andMain(Stringlanguage).Here,boththeconstructorinitializethevalueofthevariablelanguagewithdifferentvalues. Basedontheparameterpassedduringobjectcreation,differentconstructorsarecalledanddifferentvaluesareassigned. Itisalsopossibletocalloneconstructorfromanotherconstructor.Tolearnmore,visitJavaCallOneConstructorfromAnother. Note:Wehaveusedthiskeywordtospecifythevariableoftheclass.Toknowmoreaboutthiskeyword,visitJavathiskeyword. TableofContents WhatisaConstructor? Example:JavaConstructor JavaNo-ArgConstructors JavaParameterizedConstructor JavaDefaultConstructor ImportantNotesonJavaConstructors ConstructorsOverloadinginJava PreviousTutorial: JavaMethods NextTutorial: JavaStrings Shareon: Didyoufindthisarticlehelpful? Sorryaboutthat. Howcanweimproveit? Feedback* Leavethisfieldblank RelatedTutorialsJavaTutorialJavaenumConstructorJavaTutorialJavathisKeywordJavaTutorialJavasuperJavaTutorialJavaReflection GetApp GetJavaApp