What issues should be considered when overriding equals and hashCode in Java

Question

What issues   pitfalls must be considered when overriding equals and hashCode

User · Answer

There are some issues worth noticing if you're dealing with classes that are persisted using an Object-Relationship Mapper (ORM) like Hibernate, if you didn't think this was unreasonably complicated already!

Lazy loaded objects are subclasses

If your objects are persisted using an ORM, in many cases you will be dealing with dynamic proxies to avoid loading object too early from the data store. These proxies are implemented as subclasses of your own class. This means thatthis.getClass() == o.getClass() will return false. For example:

Person saved = new Person("John Doe");
Long key = dao.save(saved);
dao.flush();
Person retrieved = dao.retrieve(key);
saved.getClass().equals(retrieved.getClass()); // Will return false if Person is loaded lazy

If you're dealing with an ORM, using o instanceof Person is the only thing that will behave correctly.

Lazy loaded objects have null-fields

ORMs usually use the getters to force loading of lazy loaded objects. This means that person.name will be null if person is lazy loaded, even if person.getName() forces loading and returns "John Doe". In my experience, this crops up more often in hashCode() and equals().

If you're dealing with an ORM, make sure to always use getters, and never field references in hashCode() and equals().

Saving an object will change its state

Persistent objects often use a id field to hold the key of the object. This field will be automatically updated when an object is first saved. Don't use an id field in hashCode(). But you can use it in equals().

A pattern I often use is

if (this.getId() == null) {
    return this == other;
}
else {
    return this.getId().equals(other.getId());
}

But: you cannot include getId() in hashCode(). If you do, when an object is persisted, its hashCode changes. If the object is in a HashSet, you'll "never" find it again.

In my Person example, I probably would use getName() for hashCode and getId() plus getName() (just for paranoia) for equals(). It's okay if there are some risk of "collisions" for hashCode(), but never okay for equals().

hashCode() should use the non-changing subset of properties from equals()

User · Answer

Still amazed that none recommended the guava library for this       Sample taken from a current working project of mine just to illustrate the idea       Override     public int hashCode            return Objects hashCode this getDate    this datePattern               Override     public boolean equals Object obj           if     obj instanceof DateAndPattern                 return false                    return Objects equal   DateAndPattern obj  getDate    this getDate                     amp  amp  Objects equal   DateAndPattern obj  getDate    this getDatePattern

User · Answer

For an inheritance-friendly implementation  check out Tal Cohen s solution  How Do I Correctly Implement the equals   Method   Summary   In his book Effective Java Programming Language Guide  Addison-Wesley  2001   Joshua Bloch claims that  There is simply no way to extend an instantiable class and add an aspect while preserving the equals contract    Tal disagrees   His solution is to implement equals   by calling another nonsymmetric blindlyEquals   both ways   blindlyEquals   is overridden by subclasses  equals   is inherited  and never overridden   Example   class Point       private int x      private int y      protected boolean blindlyEquals Object o            if    o instanceof Point               return false          Point p    Point o          return  p x    this x  amp  amp  p y    this y             public boolean equals Object o            return  this blindlyEquals o   amp  amp  o blindlyEquals this             class ColorPoint extends Point       private Color c      protected boolean blindlyEquals Object o            if    o instanceof ColorPoint               return false          ColorPoint cp    ColorPoint o          return  super blindlyEquals cp   amp  amp           cp color    this color             Note that equals   must work across inheritance hierarchies if the Liskov Substitution Principle is to be satisfied

User · Answer

equals   method is used to determine the equality of two objects   as int value of 10 is always equal to 10  But this equals   method is about equality of two objects  When we say object  it will have properties  To decide about equality those properties are considered  It is not necessary that all properties must be taken into account to determine the equality and with respect to the class definition and context it can be decided  Then the equals   method can be overridden   we should always override hashCode   method whenever we override equals   method  If not  what will happen  If we use hashtables in our application  it will not behave as expected  As the hashCode is used in determining the equality of values stored  it will not return the right corresponding value for a key   Default implementation given is hashCode   method in Object class uses the internal address of the object and converts it into integer and returns it   public class Tiger     private String color    private String stripePattern    private int height      Override   public boolean equals Object object        boolean result   false      if  object    null    object getClass      getClass            result   false        else         Tiger tiger    Tiger  object        if  this color    tiger getColor              amp  amp  this stripePattern    tiger getStripePattern              result   true                    return result            just omitted null checks    Override   public int hashCode         int hash   3      hash   7   hash   this color hashCode        hash   7   hash   this stripePattern hashCode        return hash         public static void main String args          Tiger bengalTiger1   new Tiger  Yellow    Dense   3       Tiger bengalTiger2   new Tiger  Yellow    Dense   2       Tiger siberianTiger   new Tiger  White    Sparse   4       System out println  bengalTiger1 and bengalTiger2              bengalTiger1 equals bengalTiger2        System out println  bengalTiger1 and siberianTiger              bengalTiger1 equals siberianTiger         System out println  bengalTiger1 hashCode      bengalTiger1 hashCode         System out println  bengalTiger2 hashCode      bengalTiger2 hashCode         System out println  siberianTiger hashCode              siberianTiger hashCode            public String getColor         return color         public String getStripePattern         return stripePattern         public Tiger String color  String stripePattern  int height        this color   color      this stripePattern   stripePattern      this height   height           Example Code Output   bengalTiger1 and bengalTiger2  true  bengalTiger1 and siberianTiger  false  bengalTiger1 hashCode  1398212510  bengalTiger2 hashCode  1398212510  siberianTiger hashCode     1227465966

User · Answer

For equals  look into Secrets of Equals by Angelika Langer  I love it very much  She s also a great FAQ about Generics in Java  View her other articles here  scroll down to  Core Java    where she also goes on with Part-2 and  mixed type comparison   Have fun reading them

User · Answer

There are a couple of ways to do your check for class equality before checking member equality  and I think both are useful in the right circumstances    Use the instanceof operator  Use this getClass   equals that getClass       I use  1 in a final equals implementation  or when implementing an interface that prescribes an algorithm for equals  like the java util collection interfaces mdash the right way to check with with  obj instanceof Set  or whatever interface you re implementing   It s generally a bad choice when equals can be overridden because that breaks the symmetry property   Option  2 allows the class to be safely extended without overriding equals or breaking symmetry   If your class is also Comparable  the equals and compareTo methods should be consistent too  Here s a template for the equals method in a Comparable class   final class MyClass implements Comparable lt MyClass gt               Override   public boolean equals Object obj             If compareTo and equals aren t final  we should check with getClass instead         if    obj instanceof MyClass          return false      return compareTo  MyClass  obj     0

User · Answer

The theory  for the language lawyers and the mathematically inclined    equals    javadoc  must define an equivalence relation  it must be reflexive  symmetric  and transitive   In addition  it must be consistent  if the objects are not modified  then it must keep returning the same value   Furthermore  o equals null  must always return false   hashCode    javadoc  must also be consistent  if the object is not modified in terms of equals    it must keep returning the same value    The relation between the two methods is      Whenever a equals b   then a hashCode   must be same as b hashCode      In practice   If you override one  then you should override the other   Use the same set of fields that you use to compute equals   to compute hashCode     Use the excellent helper classes EqualsBuilder and HashCodeBuilder from the Apache Commons Lang library  An example   public class Person       private String name      private int age                   Override     public int hashCode             return new HashCodeBuilder 17  31      two randomly chosen prime numbers                if deriving  appendSuper super hashCode                 append name               append age               toHashCode                Override     public boolean equals Object obj           if    obj instanceof Person               return false          if  obj    this              return true           Person rhs    Person  obj          return new EqualsBuilder                   if deriving  appendSuper super equals obj                append name  rhs name               append age  rhs age               isEquals              Also remember   When using a hash-based Collection or Map such as HashSet  LinkedHashSet  HashMap  Hashtable  or WeakHashMap  make sure that the hashCode   of the key objects that you put into the collection never changes while the object is in the collection  The bulletproof way to ensure this is to make your keys immutable  which has also other benefits

User · Answer

One gotcha I have found is where two objects contain references to each other  one example being a parent child relationship with a convenience method on the parent to get all children   These sorts of things are fairly common when doing Hibernate mappings for example   If you include both ends of the relationship in your hashCode or equals tests it s possible to get into a recursive loop which ends in a StackOverflowException  The simplest solution is to not include the getChildren collection in the methods

User · Answer

Logically we have   a getClass   equals b getClass     amp  amp  a equals b    a hashCode      b hashCode    But not vice-versa

User · Answer

A clarification about the obj getClass      getClass     This statement is the result of equals   being inheritance unfriendly  The JLS  Java language specification  specifies that if A equals B     true then B equals A  must also return true  If you omit that statement inheriting classes that override equals    and change its behavior  will break this specification   Consider the following example of what happens when the statement is omitted       class A         int field1         A int field1            this field1   field1                 public boolean equals Object other            return  other    null  amp  amp  other instanceof A  amp  amp    A  other  field1    field1                      class B extends A           int field2           B int field1  int field2                super field1               this field2   field2                     public boolean equals Object other                return  other    null  amp  amp  other instanceof B  amp  amp    B other  field2    field2  amp  amp  super equals other                          Doing new A 1  equals new A 1   Also  new B 1 1  equals new B 1 1   result give out true  as it should   This looks all very good  but look what happens if we try to use both classes   A a   new A 1   B b   new B 1 1   a equals b     true  b equals a     false    Obviously  this is wrong   If you want to ensure the symmetric condition  a b if b a and the Liskov substitution principle call super equals other  not only in the case of B instance  but check after for A instance   if  other instanceof B      return  other    null  amp  amp    B other  field2    field2  amp  amp  super equals other     if  other instanceof A  return super equals other       else return false    Which will output   a equals b     true  b equals a     true    Where  if a is not a reference of B  then it might be a be a reference of class A  because you extend it   in this case you call super equals   too

User · Answer

There are two methods in super class as java lang Object  We need to override them to custom object   public boolean equals Object obj  public int hashCode     Equal objects must produce the same hash code as long as they are equal  however unequal objects need not produce distinct hash codes   public class Test       private int num      private String data      public boolean equals Object obj                if this    obj              return true          if  obj    null      obj getClass      this getClass                 return false             object must be Test at this point         Test test    Test obj          return num    test num  amp  amp           data    test data     data    null  amp  amp  data equals test data                public int hashCode                 int hash   7          hash   31   hash   num          hash   31   hash    null    data   0   data hashCode             return hash                other methods     If you want get more  please check this link as http   www javaranch com journal 2002 10 equalhash html  This is another example  http   java67 blogspot com 2013 04 example-of-overriding-equals-hashcode-compareTo-java-method html  Have Fun

[java] What issues should be considered when overriding equals and hashCode in Java?

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