Please see the below code
class TestToString
{
public static void main(String args[])
{
CheckToString cs = new CheckToString (2);
CheckToString c = new CheckToString (2);
if( cs.equals(c))
System.out.println(" Both objects are equal");
else
System.out.println(" Unequal objects ");
}
}
class CheckToString
{
int i;
CheckToString ()
{
i=10;
}
CheckToString (int a)
{
this.i=a;
}
}
Output:
Unequal objects
But I was expecting the output will be
Both objects are equal
I understood that both the objects have different refferences,
System.out.println(cs); //com.sample.personal.checkToString#19821f
System.out.println(c); //com.sample.personal.checkToString#addbf1
but I was asking, why do they have different referrences?
whereas in the below case, the objects have same memory locations.
Integer a = new Integer(2);
Integer b = new Integer(2);
System.out.println(a); //2
System.out.println(b); //2
I am comparing the object of user-defined class with the object of pre-defined class.
It seems the object of user-defined class behaves same as the object of Integer Class having value beyond -128 to 127. Why are the referrences different for both the cases?
(same for Integer class having value within -128 to 127 and different for user-defined class)
The default implementation of equals checks references. You are creating 2 different object, that don't refer the same thing in memory.
A decent implementation of equals would be something like:
public boolean equals(Object o) {
if (!(o instanceof CheckToString)) {
return false;
}
CheckToString other = (CheckToString) o;
return i == other.i;
}
When overridding equals, you need to override hashCode, too.
Whenever you say new CheckToString(), you are creating a new object in memory, so a totally different reference than another new CheckToString(). It doesn't matter what is inside the object definition.
The stuff you mention about Integer is true, but it applies to Integer, not to a custom object that you have created.
You need to override the equals method in your CheckToString class:
#Override
public boolean equals(Object o){
if(this==o){
return true;
}
if(o instanceof CheckString){
CheckString other = (CheckString) o;
return this.i == other.i;
}
return false;
}
It's recommended that when you override equals you also override hashCode, so that you can use your object in hashed collections (i.e. HasSet, LinkedHasSet, HashMap). In a case like this, since your class appears to be a wrapper over a primitive integer, I guess you could return the integer itself.
#Override
public int hashCode(){
return i;
}
And finally, it's always recommended to override toString, so that every time you look at instances of your class you see a friendly, human-readable representation of your object.
#Override
public String toString(){
StringBuilder sb = new StringBuilder();
sb.append(this.getClass().getSimpleName()).append("[");
sb.append("i: ").append(i).append("]");
return sb.toString();
}
the 2 objects are obviously not pointing in the same memory location (Notice that you are comparing Object References). For more info see JavaDoc of Object Class and the respective equals() methos
To compare their string representation you could do this modification:
class TestToString
{
public static void main(String args[])
{
CheckToString cs = new CheckToString(2);
CheckToString c = new CheckToString(2);
if( cs.toString().equalsIgnoreCase(c.toString()))
System.out.println(" Both objects are equal");
else
System.out.println(" Unequal objects ");
}
}
class CheckToString
{
int i;
CheckToString()
{
i=10;
}
CheckToString(int a)
{
this.i=a;
}
public String toString(){
return String.valueOf(i);
}
}
ps: Note also the change in the Object's Case (Java Conventions)
By default, equals method only checks the hashcodes of any two objects. So, if you need that equals method should return result depending on any underlying property in your object, you will have to override equals method accordingly.
equals method compare on the basis of reference by default in any user define class but if you override the equals method in the current class then it is compare on the basis of content...but in the java.lang.StringClass it is compare on the basis of content always
Related
The following code is a JUnit test function which fails upon execution.
List<KGramPostingsEntry> a = new ArrayList<KGramPostingsEntry>();
List<KGramPostingsEntry> b = new ArrayList<KGramPostingsEntry>();
KGramPostingsEntry entry = new KGramPostingsEntry(1);
a.add(entry);
entry = new KGramPostingsEntry(1);
b.add(entry);
assertTrue(a.containsAll(b));
It uses the KGramPostingsEntry class:
package ir;
public class KGramPostingsEntry {
int tokenID;
public KGramPostingsEntry(int tokenID) {
this.tokenID = tokenID;
}
public KGramPostingsEntry(KGramPostingsEntry other) {
this.tokenID = other.tokenID;
}
public String toString() {
return tokenID + "";
}
public boolean equals(KGramPostingsEntry other) {
if(other.tokenID == this.tokenID) {
return true;
}
return false;
}
}
As you can see, there is an equals() function in the class that compares the tokenID of the different KGramPostingsEntry objects. It seems to me that this function is not used when calling containsAll() in the test. Further experimentation seems to verify this to be true:
List<KGramPostingsEntry> a = new ArrayList<KGramPostingsEntry>();
List<KGramPostingsEntry> b = new ArrayList<KGramPostingsEntry>();
KGramPostingsEntry entry = new KGramPostingsEntry(1);
a.add(entry);
b.add(entry);
assertTrue(a.containsAll(b));
Here, I'm inserting the same object in both lists. This test does not fail. As far as I've gathered, ArrayList makes a copy object of the object sent to add(), before storing a reference to that object. This means that the objects in the two Lists are not the same (even though they have the same tokenID), and that containsAll() does not check for object reference equality. But if it does not check for object reference equality and does not check the equals() function defined in my code, what does it check? The only plausible option to me is that it checks for object value equality, and that the two objects stored in the first test example are somehow different (even though their only property is tokenID, which is the same in both objects).
What is going on here? How can I make this test succeed the way I want it to?
Here the equals declaration of Object:
public boolean equals(Object obj)
(documentation). You're trying to override this method, but instead you overloaded it:
public boolean equals(KGramPostingsEntry other)
Notice how the argument type in your method is KGramPostingsEntry, which differs from the argument type in Object.equals, namely Object. When a method has the same name but different argument types, it is overloaded, not overridden.
When the ArrayList tries to compare its contents with equals, it'll use the most applicable overridden version of Object.equals. That unfortunately doesn't include your method.
Luckily the fix is easy: you need to implement your equals method with an Object argument:
public boolean equals(Object obj) {
if(obj == null || !(obj instanceof KGramPostingsEntry)) {
return false;
}
KGramPostingsEntry other = (KGramPostingsEntry) obj;
if(other.tokenID == this.tokenID) {
return true;
}
return false;
}
The equals method doesn't have the correct signature. It should be
public boolean equals(Object that) {
// ..
}
I constructed a class with one String field. Then I created two objects and I have to compare them using == operator and .equals() too. Here's what I've done:
public class MyClass {
String a;
public MyClass(String ab) {
a = ab;
}
public boolean equals(Object object2) {
if(a == object2) {
return true;
}
else return false;
}
public boolean equals2(Object object2) {
if(a.equals(object2)) {
return true;
}
else return false;
}
public static void main(String[] args) {
MyClass object1 = new MyClass("test");
MyClass object2 = new MyClass("test");
object1.equals(object2);
System.out.println(object1.equals(object2));
object1.equals2(object2);
System.out.println(object1.equals2(object2));
}
}
After compile it shows two times false as a result. Why is it false if the two objects have the same fields - "test"?
== compares object references, it checks to see if the two operands point to the same object (not equivalent objects, the same object).
If you want to compare strings (to see if they contain the same characters), you need to compare the strings using equals.
In your case, if two instances of MyClass really are considered equal if the strings match, then:
public boolean equals(Object object2) {
return object2 instanceof MyClass && a.equals(((MyClass)object2).a);
}
...but usually if you are defining a class, there's more to equivalency than the equivalency of a single field (a in this case).
Side note: If you override equals, you almost always need to override hashCode. As it says in the equals JavaDoc:
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
You should override equals
public boolean equals (Object obj) {
if (this==obj) return true;
if (this == null) return false;
if (this.getClass() != obj.getClass()) return false;
// Class name is Employ & have lastname
Employe emp = (Employee) obj ;
return this.lastname.equals(emp.getlastname());
}
The best way to compare 2 objects is by converting them into json strings and compare the strings, its the easiest solution when dealing with complicated nested objects, fields and/or objects that contain arrays.
sample:
import com.google.gson.Gson;
Object a = // ...;
Object b = //...;
String objectString1 = new Gson().toJson(a);
String objectString2 = new Gson().toJson(b);
if(objectString1.equals(objectString2)){
//do this
}
The overwrite function equals() is wrong.
The object "a" is an instance of the String class and "object2" is an instance of the MyClass class. They are different classes, so the answer is "false".
It looks like equals2 is just calling equals, so it will give the same results.
Your equals2() method always will return the same as equals() !!
Your code with my comments:
public boolean equals2(Object object2) { // equals2 method
if(a.equals(object2)) { // if equals() method returns true
return true; // return true
}
else return false; // if equals() method returns false, also return false
}
The "==" operator returns true only if the two references pointing to the same object in memory. The equals() method on the other hand returns true based on the contents of the object.
Example:
String personalLoan = new String("cheap personal loans");
String homeLoan = new String("cheap personal loans");
//since two strings are different object result should be false
boolean result = personalLoan == homeLoan;
System.out.println("Comparing two strings with == operator: " + result);
//since strings contains same content , equals() should return true
result = personalLoan.equals(homeLoan);
System.out.println("Comparing two Strings with same content using equals method: " + result);
homeLoan = personalLoan;
//since both homeLoan and personalLoan reference variable are pointing to same object
//"==" should return true
result = (personalLoan == homeLoan);
System.out.println("Comparing two reference pointing to same String with == operator: " + result);
Output:
Comparing two strings with == operator: false
Comparing two Strings with same content using equals method: true
Comparing two references pointing to same String with == operator: true
You can also get more details from the link: http://javarevisited.blogspot.in/2012/12/difference-between-equals-method-and-equality-operator-java.html?m=1
Statements a == object2 and a.equals(object2) both will always return false because a is a string while object2 is an instance of MyClass
Your implementation must like:
public boolean equals2(Object object2) {
if(a.equals(object2.a)) {
return true;
}
else return false;
}
With this implementation your both methods would work.
If you dont need to customize the default toString() function, another way is to override toString() method, which returns all attributes to be compared. then compare toString() output of two objects. I generated toString() method using IntelliJ IDEA IDE, which includes class name in the string.
public class Greeting {
private String greeting;
#Override
public boolean equals(Object obj) {
if (this == obj) return true;
return this.toString().equals(obj.toString());
}
#Override
public String toString() {
return "Greeting{" +
"greeting='" + greeting + '\'' +
'}';
}
}
Your class might implement the Comparable interface to achieve the same functionality. Your class should implement the compareTo() method declared in the interface.
public class MyClass implements Comparable<MyClass>{
String a;
public MyClass(String ab){
a = ab;
}
// returns an int not a boolean
public int compareTo(MyClass someMyClass){
/* The String class implements a compareTo method, returning a 0
if the two strings are identical, instead of a boolean.
Since 'a' is a string, it has the compareTo method which we call
in MyClass's compareTo method.
*/
return this.a.compareTo(someMyClass.a);
}
public static void main(String[] args){
MyClass object1 = new MyClass("test");
MyClass object2 = new MyClass("test");
if(object1.compareTo(object2) == 0){
System.out.println("true");
}
else{
System.out.println("false");
}
}
}
the return type of object.equals is already boolean.
there's no need to wrap it in a method with branches. so if you want to compare 2 objects simply compare them:
boolean b = objectA.equals(objectB);
b is already either true or false.
When we use == , the Reference of object is compared not the actual objects. We need to override equals method to compare Java Objects.
Some additional information C++ has operator over loading & Java does not provide operator over loading.
Also other possibilities in java are implement Compare Interface .which defines a compareTo method.
Comparator interface is also used compare two objects
Here the output will be false , false beacuse in first sopln statement you are trying to compare a string type varible of Myclass type to the other MyClass type and it will allow because of both are Object type and you have used "==" oprerator which will check the reference variable value holding the actual memory not the actual contnets inside the memory .
In the second sopln also it is the same as you are again calling a.equals(object2) where a is a varible inside object1 . Do let me know your findings on this .
In short, == compares two POINTERS.
If the two pointers are equal, then they both point to same object in memory (which, obviously has the same value as itself).
However, .equals will compare the VALUES of whatever is pointed to, returning true iff they both evaluate to the same value.
Thus, two separate strings (i.e., at different addresses in memory) are always != but are .equal iff they contain the same (null-terminated) sequence of chars.
IN the below code you are calling the overriden method .equals().
public boolean equals2(Object object2) {
if(a.equals(object2)) { // here you are calling the overriden method, that is why you getting false 2 times.
return true;
}
else return false;
}
Here is the code which produces hashset size 3 instead 2
package dump.test;
import java.util.*;
public class WrappedString {
private String s;
public WrappedString(String s) { this.s = s; }
public static void main(String[] args) {
HashSet<Object> hs = new HashSet<Object>();
WrappedString ws1 = new WrappedString("aardvark");
WrappedString ws2 = new WrappedString("aardvark");
String s1 = new String("aardvark");
String s2 = new String("aardvark");
hs.add(ws1); hs.add(ws2); hs.add(s1); hs.add(s2);
System.out.println(hs.size()+hs.toString());
}
public boolean equals(Object aSong) {
String s = aSong.toString();
System.out.println(s);
return s.equals(this.s);
}
public int hashCode() {
System.out.println(this.s + "-" + this.s.hashCode());
return this.s.hashCode();
}
/*public int compareTo(Object aSong) {
String s = aSong.toString();
return this.s.compareTo(s);
}*/
}
It always print below output if equals and hashCode are overridden
you can see both objects having same code in output but counted as different and produced count as 3
this is if we do not override equals and hashCode
Please assist me how this works.
The problem is that your case is not symmetric. If the implementation decides to invoke equals on the String instance with your class as an argument, it will definitely return false and thus your code will not always work.
From the JavaDoc for the Set interface:
A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.
So, HashSet checks the equals(Object) before adding the new element. As the second String is equal to the first, it's not added. On your WrappedString, you are not overriding the equals(Object), so, you are using the one inherited from Object, which simply checks the object IDs (659e0bfd and 2a139a55 in your case).
adding
public String toString() {
return this.s;
}
cleared my confusion. Previously it couldn't convert true value from casting object to string and needed to override toString to return underlying value.
I have a test class like so:
public class CompareObjects {
public static class Obj {
public int i;
public Obj(int i) {
this.i = i;
}
}
public static void main(String[] args) {
Obj o1 = new Obj(0);
Obj o2 = new Obj(0);
if(o1 == o2) {
System.out.println("Equal");
}else{
System.out.println("Not equal");
}
}
}
I though the test would return "Equal", but it didn't. Why doesn't Java consider two objects with equal components not the same? Am I doing something wrong here? I have a feeling I completely overlooked something when I started learning Java.
Also, I tested the two against each other with the equals() method, and I get the same result. My reason for this question is that I would like to be able to test an ArrayList with the contains() method to see if one object has the same components as another and therefore equal. Any ideas?
== compares the references to the object. For example:
Obj a = new Obj(0);
Obj b = a;
//a == b
Try implementing equals():
public static class Obj {
public int i;
public Obj(int i) {
this.i = i;
}
#Override
public boolean equals(Object other) {
if (other == this) return true;
if (!(other instanceof Obj) || other == null) return false;
return i == ((Obj)other).i;
}
#Override
public int hashCode() {
return i;
}
}
Then, you can use if(o1.equals(o2)) {. However, this is not really a good example, read this (link) for more information.
== returns true only if you are comparing the same object [i.e. the same memory location].
If you want to compare objects by their fields, you have to overload the equals() method, in order to induce an equivalence relation over them.
public boolean equals(Object other){
return this.i == other.i;
}
Be sure that the equals() method respects reflexivity, symmmetry, transitivity.
== compares the reference equality, i.e if they refer to the same object in the memory.
You need to override equals() method, and use it whenever you want to compare their values. Also, override hashCode() which is used by HashMap for example.
The == operator does not check for equality in class data; rather, it checks to see if they are the same location in memory. If you did o2 = o1 instead of initializing them the same way, they would be the same location in memory, so o2==o1 would return true. However, since they were initialized some separately, it returns false. Instead, you should define an equals method and implement that.
I'm having a problem with getting an ArrayList to correctly use an overriden equals. the problem is that I'm trying to use the equals to only test for a single key field, and using ArrayList.contains() to test for the existence of an object with the correct field. Here is an example
public class TestClass {
private static class InnerClass{
private final String testKey;
//data and such
InnerClass(String testKey, int dataStuff) {
this.testKey =testKey;
//etc
}
#Override
public boolean equals (Object in) {
System.out.println("reached here");
if(in == null) {
return false;
}else if( in instanceof String) {
String inString = (String) in;
return testKey == null ? false : testKey.equals(inString);
}else {
return false;
}
}
}
public static void main(String[] args) {
ArrayList<InnerClass> objectList = new ArrayList<InnerClass>();
//add some entries
objectList.add(new InnerClass("UNIQUE ID1", 42));
System.out.println( objectList.contains("UNIQUE ID1"));
}
}
What worries me is that not only am I getting false on the output, but I'm also not getting the "reached here" output.
Does anyone have any ideas why this override is being completely ignored? Is there some subtlety with overrides and inner classes I don't know of?
Edit:
Having problems with the site so I cant seem to mark the answered.
Thanks for the quick response: yes an oversight on my part that it is the String .equals thta is called, not my custom one. I guess it's old fashioned checks for now
If you check sources of ArrayList, you will see that it calls equals of other object. In your case it will call equals of String "UNIQUE ID1" which will check that other object is not of type String and just returns false:
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
public int indexOf(Object o) {
...
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
...
return -1;
}
For your case call contains with InnerClass that only contains id:
objectList.contains(new InnerClass("UNIQUE ID1"))
Don't forget to implement equals for InnerClass which compares id only.
According to the JavaDoc of List.contains(o), it is defined to return true
if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).
Note that this definition calls equals on o, which is the parameter and not the element that is in the List.
Therefore String.equals() will be called and not InnerClass.equals().
Also note that the contract for Object.equals() states that
It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
But you violate this constraint, since new TestClass("foo", 1).equals("foo") returns true but "foo".equals(new TestClass("foo", 1)) will always return false.
Unfortunately this means that your use case (a custom class that can be equal to another standard class) can not be implemented in a completely conforming way.
If you still want to do something like this, you'll have to read the specification (and sometimes the implementation) of all your collection classes very carefully and check for pitfalls such as this.
You're invoking contains with an argument that's a String and not an InnerClass:
System.out.println( objectList.contains("UNIQUE ID1"))
In my JDK:
public class ArrayList {
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
public int indexOf(Object o) {
if (o == null) {
// omitted for brevity - aix
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i])) // <<<<<<<<<<<<<<<<<<<<<<
return i;
}
return -1;
}
}
Note how indexOf calls o.equals(). In your case, o is a String, so your objectList.contains will be using String.equals and not InnerClass.equals.
Generally, you need to also override hashCode() but this is not the main problem here. You are having an asymmetric equals(..) method. The docs make it clear that it should be symmetric:
It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
And what you observe is an unexpected behaviour due to broken contract.
Create an utility method that iterates all items and verifies with equals(..) on the string:
public static boolean containsString(List<InnerClass> items, String str) {
for (InnerClass item : items) {
if (item.getTestKey().equals(str)) {
return true;
}
}
return false;
}
You can do a similar thing with guava's Iterables.any(..) method:
final String str = "Foo";
boolean contains = Iterables.any(items, new Predicate<InnerClass>() {
#Override
public boolean apply(InnerClass input){
return input.getTestKey().equals(str);
}
}
Your equals implementation is wrong. Your in parameter should not be a String. It should be an InnerClass.
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof InnerClass) return false;
InnerClass that = (InnerClass)o;
// check for null keys if you need to
return this.testKey.equals(that.testKey);
}
(Note that instanceof null returns false, so you don't need to check for null first).
You would then test for existence of an equivalent object in your list using:
objectList.contains(new InnerClass("UNIQUE ID1"));
But if you really want to check for InnerClass by String key, why not use Map<String,InnerClass> instead?
Although not answering your question, many Collections use hashcode(). You should override that too to "agree" with equals().
Actually, you should always implement both equals and hashcode together, and they should always be consistent with each other. As the javadoc for Object.equals() states:
Note that it is generally necessary to
override the hashCode method whenever
this method is overridden, so as to
maintain the general contract for the
hashCode method, which states that
equal objects must have equal hash
codes.
Specifically, many Collections rely on this contract being upheld - behaviour is undefined otherwise.
There are a few issues with your code. My suggestion would be to avoid overriding the equals entirely if you are not familiar with it and extend it into a new implementation like so...
class MyCustomArrayList extends ArrayList<InnerClass>{
public boolean containsString(String value){
for(InnerClass item : this){
if (item.getString().equals(value){
return true;
}
}
return false;
}
}
Then you can do something like
List myList = new MyCustomArrayList()
myList.containsString("some string");
I suggest this because if you override the equals should also override the hashCode and it seems you are lacking a little knowledge in this area - so i would just avoid it.
Also, the contains method calls the equals method which is why you are seeing the "reached here". Again if you don't understand the call flow i would just avoid it.
in the other way, your equal method gets called if you change your code as follows. hope this clears the concept.
package com.test;
import java.util.ArrayList;
import java.util.List;
public class TestClass {
private static class InnerClass{
private final String testKey;
//data and such
InnerClass(String testKey, int dataStuff) {
this.testKey =testKey;
//etc
}
#Override
public boolean equals (Object in1) {
System.out.println("reached here");
if(in1 == null) {
return false;
}else if( in1 instanceof InnerClass) {
return ((InnerClass) this).testKey == null ? false : ((InnerClass) this).testKey.equals(((InnerClass) in1).testKey);
}else {
return false;
}
}
}
public static void main(String[] args) {
ArrayList<InnerClass> objectList = new ArrayList<InnerClass>();
InnerClass in1 = new InnerClass("UNIQUE ID1", 42);
InnerClass in2 = new InnerClass("UNIQUE ID1", 42);
//add some entries
objectList.add(in1);
System.out.println( objectList.contains(in2));
}
}
As many posts have said, the problem is that list.indexOf(obj) function calls "equals" of the obj, not the items on the list.
I had the same problem and "contains()" didn't satisfy me, as I need to know where is the element!. My aproach is to create an empty element with just the parameter to compare, and then call indexOf.
Implement a function like this,
public static InnerClass empty(String testKey) {
InnerClass in = new InnerClass();
in.testKey =testKey;
return in;
}
And then, call indexOf like this:
ind position = list.indexOf(InnerClass.empty(key));
There are two errors in your code.
First:
The "contains" method called on "objectList" object should pass a new InnerClass object as the parameter.
Second:
The equals method (should accept the parameter as Object, and is correct) should handle the code properly according to the received object.
Like this:
#Override
public boolean equals (Object in) {
System.out.println("reached here");
if(in == null) {
return false;
}else if( in instanceof InnerClass) {
String inString = ((InnerClass)in).testKey;
return testKey == null ? false : testKey.equals(inString);
}else {
return false;
}
}
This post was first written before Java 8 was available but now that it's 2017 instead of using the List.containts(...) method you can use the new Java 8 way like this:
System.out.println(objectList.stream().filter(obj -> obj.getTestKey().equals("UNIQUE ID1")).findAny().isPresent());
And give your TestClass a getter for your testKey field:
public String getTestKey() {
return testKey;
}
The benefit of this approach is that you don't have to modify the equals or hash method and you'll look like a boss to your peers!