How does a ArrayList s contains method evaluate objects

Question

Say I create one object and add it to my ArrayList  If I then create another object with exactly the same constructor input  will the contains   method evaluate the two objects to be the same  Assume the constructor doesn t do anything funny with the input  and the variables stored in both objects are identical   ArrayList lt Thing gt  basket   new ArrayList lt Thing gt       Thing thing   new Thing 100     basket add thing     Thing another   new Thing 100     basket contains another      true or false      class Thing         public int value         public Thing  int x            value   x             equals  Thing x            if  x value    value  return true          return false            Is this how the class should be implemented to have contains   return true

User · Answer

It uses the equals method on the objects  So unless Thing overrides equals and uses the variables stored in the objects for comparison  it will not return true on the contains   method

User · Answer

I think that right implementations should be  public class Thing       public int value         public Thing  int x                this value   x              Override     public boolean equals Object object                boolean sameSame   false           if  object    null  amp  amp  object instanceof Thing                        sameSame   this value      Thing  object  value                     return sameSame

User · Answer

ArrayList implements the List Interface   If you look at the Javadoc for List at the contains method you will see that it uses the equals   method to evaluate if two objects are the same

User · Answer

The ArrayList uses the equals method implemented in the class  your case Thing class  to do the equals comparison

User · Answer

Generally you should also override hashCode   each time you override equals    even if just for the performance boost  HashCode   decides which  bucket  your object gets sorted into when doing a comparison  so any two objects which equal   evaluates to true should return the same hashCode value    I cannot remember the default behavior of hashCode    if it returns 0 then your code should work but slowly  but if it returns the address then your code will fail   I do remember a bunch of times when my code failed because I forgot to override hashCode   though

User · Answer

Shortcut from JavaDoc   boolean contains Object o   Returns true if this list contains the specified element  More formally  returns true if and only if this list contains at least one element e such that  o  null   e  null   o equals e

User · Answer

Other posters have addressed the question about how contains   works   An equally important aspect of your question is how to properly implement equals     And the answer to this is really dependent on what constitutes object equality for this particular class   In the example you provided  if you have two different objects that both have x 5  are they equal   It really depends on what you are trying to do   If you are only interested in object equality  then the default implementation of  equals    the one provided by Object  uses identity only  i e  this    other    If that s what you want  then just don t implement equals   on your class  let it inherit from Object    The code you wrote  while kind of correct if you are going for identity  would never appear in a real class b c it provides no benefit over using the default Object equals   implementation   If you are just getting started with this stuff  I strongly recommend the Effective Java book by Joshua Bloch   It s a great read  and covers this sort of thing  plus how to correctly implement equals   when you are trying to do more than identity based comparisons

User · Answer

class Thing         public int value         public Thing  int x            value   x             equals  Thing x            if  x value    value  return true          return false            You must write   class Thing         public int value         public Thing  int x            value   x             public boolean equals  Object o        Thing x    Thing  o          if  x value    value  return true          return false            Now it works

User · Answer

Just wanted to note that the following implementation is wrong when value is not a primitive type   public class Thing       public Object value         public Thing  Object x                this value   x              Override     public boolean equals Object object                boolean sameSame   false           if  object    null  amp  amp  object instanceof Thing                        sameSame   this value      Thing  object  value                     return sameSame            In that case I propose the following   public class Thing       public Object value         public Thing  Object x            value   x              Override     public boolean equals Object object             if  object    null  amp  amp  object instanceof Thing                Thing thing    Thing  object              if  value    null                    return  thing value    null                             else                   return value equals thing value                                    return false

[java] How does a ArrayList's contains() method evaluate objects?

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