Though modern IDE similar Eclipse, IntelliJ or Netbeans allows you lot to generate equals, hashCode too compareTo methods for your value classes, it's as important, you lot know how to exercise that yesteryear hand. By overriding equals too hashcode method yesteryear yourself, you lot know how they work, what form of errors you lot tin get, too most importantly, it's expected shape you, as a Java programmer inwards whatever core Java interview. More often, you lot would run into a coding query inwards Java, which enquire you lot to override equals(), hashcode(), compare() too compareTo() methods for a value class. Since I convey already shared about tips on How to override compareTo method inwards Java, too pair of instance of writing your ain Comparator inwards Java, hither I am sharing about other uncomplicated instance of overriding equals too hashCode methods. If you lot know rules of overriding equals too hashCode, you lot mightiness know that, whenever you lot override equals, you lot must convey to override hashCode, otherwise your object volition non bear properly on diverse collection classes e.g. Map or Set, which uses equals, compareTo, hashCode to implement at that topographic point invariants e.g. Set implementations should non allow whatever duplicates.
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Core Java Interview Questions for 2 to iv years experienced Programmers
Overriding Equals, HashCode too CompareTo inwards Java
NullPointerException.
See this post service for detailed tips on overriding equals method inwards Java. For overriding hashCode, you lot postulate to select a prime, commonly 31, but you lot tin besides select other prime numbers e.g. 37, 17 etc. The argue for choosing these prime numbers are to generate a uniquely distributed hashcode, which eventually helps to avoid collision, when used inwards hash based collections similar Hashtable too HashMap.
Another worth noting affair is using all variables, used inwards equals, inwards hashCode method to proceed equals too hashCode consistent, too adhere to rules of overriding hashCode inwards Java. I convey already discussed almost Comparable too compareTo method, piece sorting listing of objects inwards Java.
A uncomplicated implementation of compareTo must render negative let on if electrical flow object is lesser than provided object, positive if electrical flow object is greater inwards gild than provided too null if both objects are equal. Another worth noting indicate is that, compareTo() too equals() must last consistent amongst each other, otherwise your object volition non bear properly on Collection classes, which uses compareTo() for sorting e.g. TreeSet or TreeMap. Anyway, hither is a uncomplicated instance of overriding equals, hashcode too compareTo inwards Java.
See this post service for detailed tips on overriding equals method inwards Java. For overriding hashCode, you lot postulate to select a prime, commonly 31, but you lot tin besides select other prime numbers e.g. 37, 17 etc. The argue for choosing these prime numbers are to generate a uniquely distributed hashcode, which eventually helps to avoid collision, when used inwards hash based collections similar Hashtable too HashMap.
Another worth noting affair is using all variables, used inwards equals, inwards hashCode method to proceed equals too hashCode consistent, too adhere to rules of overriding hashCode inwards Java. I convey already discussed almost Comparable too compareTo method, piece sorting listing of objects inwards Java.
A uncomplicated implementation of compareTo must render negative let on if electrical flow object is lesser than provided object, positive if electrical flow object is greater inwards gild than provided too null if both objects are equal. Another worth noting indicate is that, compareTo() too equals() must last consistent amongst each other, otherwise your object volition non bear properly on Collection classes, which uses compareTo() for sorting e.g. TreeSet or TreeMap. Anyway, hither is a uncomplicated instance of overriding equals, hashcode too compareTo inwards Java.
Sample implementation of Equals, HashCode too CompareTo inwards Java
In this sample instance of overriding equals, hashcode too compareTo method, nosotros volition locomote a shape named Person which has 3 properties String name, int id too Date to stand upward for appointment of birth. We volition besides locomote Generics along amongst Comparable to supply a type rubber implementation. Remember nosotros convey used getClass() method instead of instanceof operator, which agency a Person shape cannot last equal to its subclass, this could exercise occupation if you lot are using this shape inwards EJB or whatever application server, where at that topographic point is a adventure that same shape is loaded yesteryear 2 split upward shape loader. On those cases it's ameliorate to locomote instanceof operator because it volition allow a Class to last equal to its subclass if residue of properties matched. This is besides truthful for framework similar Hibernate, which provides proxy implementation, which is essentially sub shape of entity classes. In short, locomote instanceof if your shape tin last loaded yesteryear multiple shape loader or it tin last used yesteryear framework to exercise proxies.
/** * Simple Java Class to stand upward for Person amongst name, id too appointment of birth. * * @author Javin Paul */ public class Person implements Comparable<Person>{ private String name; private int id; private Date dob; public Person(String name, int id, Date dob) { this.name = name; this.id = id; this.dob = dob; } @Override public boolean equals(Object other){ if(this == other) return true; if(other == null || (this.getClass() != other.getClass())){ return false; } Person invitee = (Person) other; return (this.id == guest.id) && (this.name != null && name.equals(guest.name)) && (this.dob != null && dob.equals(guest.dob)); } @Override public int hashCode(){ int effect = 0; effect = 31*result + id; effect = 31*result + (name !=null ? name.hashCode() : 0); effect = 31*result + (dob !=null ? dob.hashCode() : 0); return result; } @Override public int compareTo(Person o) { return this.id - o.id; } }
Sample JUnit examine instance for testing Equals too HashCode
Here is simplest of uncomplicated examine instance to verify equals too hashCode inwards Java. Remember this unit of measurement examine is non enough, if you lot desire to verify all properties of equals too hashCode. If you lot should write examine cases to banking concern check if it verify cardinal properties of equals too hashcode method e.g. if a equal b too thence b should besides last equal to a, or if a a equals b too b equals c too thence a too c should besides last equal to each other. You must besides verify whether compareTo() render 0 for equal object too render non null value for non equal objects.
import java.util.Date; import org.junit.Test; import static org.junit.Assert.*; /** * Simple instance of using equals too hashCode method * @author Javin Paul */ public class EqualsAndHashCodeTest { @Test public void testEquals(){ Person james = new Person("James", 21, new Date(1980,12, 1)); Person same = new Person("James", 21, new Date(1980,12, 1)); Person similar = new Person("Harry", 21, new Date(1981,12,1)); assertTrue(james.equals(same)); assertTrue(james.hashCode() == same.hashCode()); assertFalse(james.equals(null)); assertFalse(james.equals(similar)); assertTrue(james.hashCode() != similar.hashCode()); } }
That's all on this simple instance of overriding equals, hashcode too compareTo method inwards Java. As I said, these examine are merely to verify that equals too hashCode methods are working, you lot tin add together to a greater extent than examine to verify compareTo, too other behaviors of all 3 methods.
Further Learning
Complete Java Masterclass
Core Java Interview Questions for 2 to iv years experienced Programmers
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