Overriding equal() method

Hi all,
I am new to this forum.This is my first post.
Please see below code...
class One
private int a,b;        //we override equal() method in this class
public One(int x,int y)
   a=x;
   b=y;
public boolean equals(Object o)
     if((o instanceof One )&&(((One)o).getAValue()==this.a)&&(((One)o).getBValue()==this.b))+
        return true;
     else
        return false;
public class ToCheckEqalityOfObjects4
public static void main(String[] args)
    One ob1=new One(2,4);
    One ob2=new One(2,4);
    One ob3=new One(6,12);
    One ob4=ob3;
    if(ob1==ob2)
      System.out.println("ob1==ob2 TRUE");
    else
      System.out.println("ob1==ob2 FALSE");
    if(ob1.equals(ob2))
      System.out.println("ob1.equals(ob2) TRUE");
    else
      System.out.println("ob1.equals(ob2) FALSE");
    if(ob4==ob3)
      System.out.println("ob4==ob3 TRUE");
    else
      System.out.println("ob4==ob3 FALSE");
    if(ob1.equals(ob3))
      System.out.println("ob4.equals(ob3) TRUE");
    else
      System.out.println("ob4.equals(ob3) FALSE");
}While compiling this code i got a error like..
E:\WINDOWS\java\IOStreams\ToCheckEqalityOfObjects4.java:23: cannot find symbol
symbol : method getAValue()
location: class IOStreams.One
     if((o instanceof One )&&(((One)o).getAValue()==this.a)&&(((One)o).getBValue()==this.b))
E:\WINDOWS\java\IOStreams\ToCheckEqalityOfObjects4.java:23: cannot find symbol
symbol : method getBValue()
location: class IOStreams.One
     if((o instanceof One )&&(((One)o).getAValue()==this.a)&&(((One)o).getBValue()==this.b))
2 errorsWhat's wrong with my code ?
Please tell me..
Thankq very much for reading this.

The error message tells you exactly what's wrong. You're calling methods that you didn't actually define.

Similar Messages

How to override equals method

The equals method accepts Object as the parameter. When overriding this method we need to downcast the parameter object to the specific. The downcast is not the good idea as it is not the OO. So can anyone please tell me how can we override the equals method without downcasting the parameter object. Thank you.

For comparing the objects by value overriding the equals method what I did is like this
public boolean equals(Object o){
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Hi I have an assignment where I have to override the equals() method, the thing that's bugging me though is my instructor says the parameter has to be Object. I understand you can pass anything through Object but if I want to get a method from the object I passed through how would I go about that? What I'm basically trying to do is compare two arrays of numbers and see if both array sizes are the same and both have the same values at each corresponding index.
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The issue here is that the signature of this equals method must match that of that which it is overriding, or in other words, it must match the equals method of Object. So it must take an object parameter. Here I would use the instanceof(...) method to see if the object matches what I want, if not, then return false, if so, cast the object as your specific object and call methods to my heart's content.

When we override Hashcode and Equal Methods

Hi....I have doubt regading Hashcode and equal methods
why we override this two methods....
why we override hashcode method when we are overriding equal method,
i would very thankful to give answser
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hash code is computed to check the equality of two
objects ,
that is why if u change the default equal method
implementation , u need to change the hashcode method
as well.That's an incomplete answer at best.
The hashcode method is used by hashing algorithms and data strcutures such as HashMap. This value is used to determine what 'bucket' the reference will go into. It cannot, by itself determine equality.
If you are still unsure, I suggest looking up 'hash table' on google. I'm sure there's a decent explanation on wikipedia.

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Can u pls tell me friends why should definetly i override the hashcode method if i override equals method of the Object class

Because the contracts of equals and hashCode are strongly bound to each other.
For each objects x and y where x.equals(y) returns true x.hashCode() must be equal to y.hashCode().
If you don't follow this convention you'll get hard-to-find bugs as soon as you use your object as a key in some kind of HashMap/HashTable/...

Why do we need to override Hascode and Equals method?

Hi,
== checks if the two references are equal and .equlas will check if the value is same, if we want .equals to take care of both reference and value are correct. why cant I just override equals with an extra check that references are equal(==).
Please someone elaborate on this and also tell me what role hashcode plays in this.
thanks
Anirudh

anirudh1983 wrote:
if we want .equals to take care of both reference and value are correct. why cant I just override equals with an extra check that references are equal(==).Many equals() methods do run an '==' check first for efficiency (and I would recommend it if you're writing one yourself).
Please someone elaborate on this and also tell me what role hashcode plays in this.The reason that it is good practise to override equals() and hashCode() together is to maintain consistency.
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Note that the reverse is NOT true: objects that are not equal() do not have to have different hashcodes, but it is usually better if they do.
There is quite a lot to know about hashcodes, and what makes a good one, so I suggest you follow dcminter's advice if you want to be a happy and prosperous Java programmer.
Winston

Overriding equals(Object o) method

hi,
I have a problem in equals method ..returns true always...
can anyone suggest whats wrong ?
thanks for the help
public boolean equals(Object NumberC)
            if (NumberC == null) return false;
             if ( NumberC instanceof Counter)
                  Counter myCr = ( Counter) NumberC;
                 if(this.number == (myCr)
                     return true;
                 }//end if
             } //end if
             return false;
         }// end equals


sorry..yes it always returns true
here's my code again
public boolean equals(Object NumberCounter)
            if (NumberCounter == null) return false;
             if ( NumberCounter instanceof Counter )
                 Counter myCr = ( Counter ) NumberCounter;
                 if(this.number == (myCr.number))
                     return true;
                 }//end if
             } //end if
             return false;
         }// end equals

...is not abstract and does not override abstract method compare

Why am I getting the above compile error when I am very clearly overriding abstract method compare (ditto abstract method compareTo)? Here is my code -- which was presented 1.5 code and I'm trying to retrofit to 1.4 -- followed by the complete compile time error. Thanks in advance for your help (even though I'm sure this is an easy question for you experts):
import java.util.*;
   This program sorts a set of item by comparing
   their descriptions.
public class TreeSetTest
   public static void main(String[] args)
      SortedSet parts = new TreeSet();
      parts.add(new Item("Toaster", 1234));
      parts.add(new Item("Widget", 4562));
      parts.add(new Item("Modem", 9912));
      System.out.println(parts);
      SortedSet sortByDescription = new TreeSet(new
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               String descrA = a.getDescription();
               String descrB = b.getDescription();
               return descrA.compareTo(descrB);
      sortByDescription.addAll(parts);
      System.out.println(sortByDescription);
   An item with a description and a part number.
class Item implements Comparable
      Constructs an item.
      @param aDescription the item's description
      @param aPartNumber the item's part number
   public Item(String aDescription, int aPartNumber)
      description = aDescription;
      partNumber = aPartNumber;
      Gets the description of this item.
      @return the description
   public String getDescription()
      return description;
   public String toString()
      return "[descripion=" + description
         + ", partNumber=" + partNumber + "]";
   public boolean equals(Object otherObject)
      if (this == otherObject) return true;
      if (otherObject == null) return false;
      if (getClass() != otherObject.getClass()) return false;
      Item other = (Item) otherObject;
      return description.equals(other.description)
         && partNumber == other.partNumber;
   public int hashCode()
      return 13 * description.hashCode() + 17 * partNumber;
   public int compareTo(Item other)   // OTHER LINE CAUSING THE ERROR
      return partNumber - other.partNumber;
   private String description;
   private int partNumber;
}Compiler error:
TreeSetTest.java:25: <anonymous TreeSetTest$1> is not abstract and does not over
ride abstract method compare(java.lang.Object,java.lang.Object) in java.util.Com
parator
            public int compare(Item a, Item b)
                       ^
TreeSetTest.java:41: Item is not abstract and does not override abstract method
compareTo(java.lang.Object) in java.lang.Comparable
class Item implements Comparable
^
2 errors

According to the book I'm reading, if you merely take
out the generic from the code, it should compile and
run in v1.4 (assuming, of course, that the class
exists in 1.4). I don't know what book you are reading but that's certainly incorrect or incomplete at least. I've manually retrofitted code to 1.4, and you'll be inserting casts as well as replacing type references with Object (or the erased type, to be more precise).
These interfaces do exist in 1.4, and
without the generics.Exactly. Which means compareTo takes an Object, and you should change your overriding method accordingly.
But this raises a new question: how does my 1.4
compiler know anything about generics? It doesn't and it can't. As the compiler is telling you, those interfaces expect Object. Think about it, you want to implement one interface which declares a method argument type of Object, in several classes, each with a different type. Obviously all of those are not valid overrides.

Use of hashCode and equals method in java(Object class)

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You could find that out easily with google, that is a standard junior developer interview question.

Problem in not overriding equals and Hashcode

I have a very small question. We know that if we do not override the equals and hascode of Object class, we can not use the object in the Hashed collection. Can anybody provide me a concrete example which indicates the problem if we do not override the equals and hascode.

back to the original question from DebadattaMishra
I have a very small question. We know that if we do
not override the equals and hascode of Object class,
we can not use the object in the Hashed collection.
Can anybody provide me a concrete example which
indicates the problem if we do not override the
equals and hascode.lets extend the example from jverd. Suppose we have a Person and want to store some Score in a HashMapclass Person {
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public class PersonTest {
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If we add an equals methodclass Person {
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            return name.equals(p.name);
        } else {
            return false;
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}we probably will get "Found: null" most if the time. The HashMap uses the hashcode() to create an index for saving the keys, so it mostly will not find the Person.((there is still a very small probability that the correct Person is found))
It will work fine if we add a hashcode like    public int hashCode() {
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Hi everyone,
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In fact you don't override the equals method. The signature defined in the Object class is
boolean equals(Object o)In your case it is
boolean equals(ExperimentalEvent e)So instead of overriding the method you're overloading it.
Regards

Help need to know hashcode() and Equals method

Hi ,
Now i'm working under hashtable functions. In that all are asking me to overwrite hashCode and equals method. Why?
Please let me know briefly...........

jverd wrote:
euph wrote:
jverd wrote:
euph wrote:
REFTY5_ wrote:
Hi ,
Now i'm working under hashtable functions. In that all are asking me to overwrite hashCode and equals method. Why?
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Need help!! How to use super in equals() method and makeCopy() method?

This is my Name class:
class Name {
    private String title;
    private String name;
    public Name(){
         title=" ";
         name=" ";
    public Name(String title){
         setTitle(title);
         name=" ";
    public Name(String title,String name){
         setTitle(title);
         setName(name);
    public void setTitle(String title){
         this.title=title;
    public String getTitle(){
         return title;
    public void setName(String name){
         this.name=name;
    public String getName(){
         return name;
    }   This is my Person Class
class Person extends Name{
    private String address;
    private String tel;
    private String email;
    public Person(){
         super();
         address=" ";
         tel=" ";
         email=" ";
    public Person(String name){
         super(name);
         address=" ";
         tel=" ";
         email=" ";
    public Person(String title,String name,String addressStr){
         super(title,name);
         setAddress(addressStr);
         tel=" ";
         email=" ";
    public Person(String title,String name,String addressStr,String phoneNum){
         super(title,name);
         setAddress(addressStr);
         setTel(phoneNum);
         email=" ";
    public Person(String title,String name,String addressStr,String phoneNum,String emailStr){
         super(title,name);
         setAddress(addressStr);
         setTel(phoneNum);
         setEmail(emailStr);
    public void setAddress(String add){
         address=add;
    public String getAddress(){
         return address;
    public void setTel(String telephone){
         tel=telephone;
    public String getTel(){
         return tel;
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         email=emailAdd;
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         return email;
     public boolean equals(Person inputRecords){
         boolean equalOrNot=false;
         if(inputRecords.super(getTitle()).equals(title))         //this is wrong
              if(inputRecords.super(getName()).equals(name))           //this is wrong
                   if(inputRecords.getAddress().equals(address))
                        if(inputRecords.getTel().equals(tel))
                             if(inputRecords.getEmail().equals(email))
                                  equalOrNot=true;
         return equalOrNot;
        public void makeCopy(Person copyInto){
             copyInto.super(setTitle(title));             //this is wrong
             copyInto.super(setName(name));           //this is wrong
             copyInto.setAddress(address);
             copyInto.setTel(tel);
             copyInto.setEmail(email);
}How can I correct the wrong part?
I don't understand how to use super in equals() method and makeCopy() method.

Try something like this ...
public boolean equals(Object other) {
if (other==null || !(other instanceof Person)) return false;
Person that = (Person) other;
return getTitle().equals(that.getTitle())
      && getName().equals(that.getName())
      && getAddress().equals(that.getAddress())
      && getTel().equals(that.getTel())
      && getEmail().equals(that.getEmail())
Notes:
* This overrides Object's public boolean equals(Object other), which is (probably) what you meant.
* You don't need to call "super.getWhatever" explicitly... just call "getWhatever" and let java workout where it's defined... this is better because if you then override getTitle() for example you don't need to change your equals method (which you would otherwise probably forget to do, which would probably have some interesting side effects.
Cheers. Keith.

Overriding equal() method

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