How Hash. Set works in java. Lets first see introduction of Hashset then we will go through internals of it. Hash. Set implements Set interface which does not allow duplicate value. Fc73%2Fc736711d-b47b-4c6b-9576-b88b1a236dbe%2FphpBXlYq4.png' alt='Serialize Hashset Java' title='Serialize Hashset Java' />It is not synchronized and is not thread safe. Definition of duplicate can be quite tricky sometimes. Lets consider two cases here. In case of primitive typessuch as interger, StringIn case of custom defined objects. In case of primitive types In case of primitives type, it is very straight forward. Lets see with help of example Lets create a java program 1. Hash. Set publicclass. Hash. Set. MainpublicstaticvoidmainStringargs  Hash. Set name. Setnew. Hash. Set  name. Set. Arpit  name. Set. Arpit  name. Set. System. Setname. Set. System. Set When you run above program, you will get following output size of name. Set2Arpit,johnSo we tried to add String Arpit twice, but as Hash. Set does not allow duplicate value, it will add Arpit once in Hash. Set. In case of Custom Objects For understanding how Hash. Set will work in case of custom objects, you need to understand hashcode and equals method in java. Lets create a class called Country and implement only equals method in it. CountryStringname longpopulation public. Stringget. Name  returnname  publicvoidset. NameStringname  this. Population  returnpopulation publicvoidset. Populationlongpopulation  this. Stringto. String  returnname OverridepublicbooleanequalsObjectobj  ifthisobj   returntrue  ifobjnull   returnfalse  ifget. Classobj. get. SpringJavaJavaSpring. The important points about Java HashSet class are HashSet stores the elements by using a mechanism called hashing. HashSet contains unique elements only. Difference between List and Set. List can contain duplicate elements whereas Set contains unique elements only. Hierarchy of HashSet class. The HashSet class extends AbstractSet class which implements Set interface. Id like to take a HashSetlt. Serialize a HashSetltString with LinQ. How to convert a String to an int in Java 2598. Class   returnfalse  Country otherCountryobj  ifnamenull   ifother. Hash. Set publicclass. Hash. Set. Country. MainpublicstaticvoidmainStringargs  Hash. Set country. Setnew. Dilwale Video Songs Free Download Mp4 Tukur Tukur more. Hash. Set  Country india. Country  india. NameIndia  Country india. Country  india. NameIndia  country. Set. addindia. 1  country. Set. addindia. 2  System. Oracle Technology Network is the ultimate, complete, and authoritative source of technical information and learning about Java. ImportNew Java 20121111 1111 ImportNew Java import. I would like to serialize an HashSet of IXmlSerializable objects, is HashSet xml serializable In my situation, I have the class User and a HashSet called Users. Can. HashSet implements the empty Serializable interface HashSet В Collections В Java Tutorial. HashSet add HashMap put override. The difference is that in C your object is on the stack, whereas in Java, your object is in the heap. If A and B are Objects, anytime in java you do. Serialize Hashset Java' title='Serialize Hashset Java' />Setcountry. Set. System. out. Set When you run above program, you will get following output size of name. Set2India,IndiaNow you must be wondering even through two objects are equal why Hash. Set contains two values instead of one. This is because First Hash. Set calculates hashcode for that key object, if hashcodes are same then only it checks for equals method and because hashcode for above two country objects uses default hashcode method,Both will have different memory address hence different hashcode. Now lets add hashcode method in above Country classOverridepublicinthash. Code  finalintprime3. Code  returnresult Run above main program again, you will get following output So now we have good understanding of Hash. Set, lets see its internal representation Internal working of Hash. Set When you add any duplicate element to Hash. Set, add method returns false and do not add duplicate element to Hash. Set. How add method return false For this, we need to see Hash. Sets add method in Java. API1. 23. 45. 67. Hash. Set    extends. Abstract. Set    implements. Set,Cloneable,java. Serializable    privatetransient Hash. Maplt E,Object map     PRESENT is dummy value which will be used as value in map    privatestaticfinal. Object. PRESENTnew. Object        Constructs a empty map. Hash. Set     mapnew. Hash. Maplt E,Object          return false if e is already present in Hash. Set    publicbooleanaddEe     returnmap. PRESENTnull         other Hash. Set methodsSo from above code, It is clear that Hash. Set uses Hash. Map for checking duplicate elements. As we know that in Hash. Map, key should be unique. So Hash. Set uses this concept, When element is added to Hash. Set, it is added to internal Hash. Map as Key. This Hash. Map required some value so a dummy ObjectPRESENT is used as value in this Hash. Map. PRESENT is dummy value which is used value for internal map. Lets see add method return false if e is already present in Hash. Set    publicbooleanaddEe     returnmap. PRESENTnull    So here there will be two casesmap. PRESENT will return null, if element not present in that map. So map. pute, PRESENT null will return true ,hence add method will return true and element will be added in Hash. Set. map. pute,PRESENT will return old value ,if element is already present in that map. So  map. pute, PRESENT null will return false, hence add method will return false and element will not be added in Hash.