If I override either method on a class, it must make sure that if A.equals(B) = true then (A.hashCode() == B.hashCode) must also be true.
Can someone show me a simple example ... |
For Every Equal Object their Hashcode must be equal.
Java returns a unique hashcode if we do not override the hashCode() method
/* A program to check hashcode values for object
@Author Myth17
*/
class HashValue ...
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why java Object class has two methods hashcode() and equals()? One of them looks redundant and its percolated to the bottom most derived class?
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Given this:
String s1= new String("abc");
String s2= new String("abc");
String s3 ="abc";
System.out.println(s1==s3);
System.out.println(s1==s2);
System.out.println(s1.equals(s2));
System.out.println(s1.equals(s3));
System.out.println(s1.hashCode());
System.out.println(s2.hashCode());
System.out.println(s3.hashCode());
Output is:
false
false
true
true
96354
96354
96354
Here == is giving false for each object but hashcode for each String object is same. Why ... |
I have a class with some non primitive members.
class Relation {
String name;
Role roleFrom;
Role roleTo;
}
class Role {
RoleType roleType;
String details;
}
class RoleType {
String ...
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I am running into a question about equals and hashCode contracts:
here it is
Given:
class SortOf {
String name;
int bal;
String code;
...
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I was learning hashcode in more depth and figured that:
1. If you override equals(), you must override hashcode() too.
2. To find if 2 objects are same object, use == operator
Given those ... |
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Implementing equals() and hashCode() for simple data POJOs is cluttering my code and maintaining is tedious.
What are the libraries handling this automatically?
I prefer bytecode instrumentation over AOP approach due to performance ... |
Consider the following test case, is it a bad practice to use the hashCode() method inside of equals as a convenient shortcut?
public class Test
{
...
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I need to implement equals() and hashCode() for an Address class.
I believe,the non null fields are taken to determine hashCode() and equals().In my application,Any of the fields except addressLine1 and country ... |
Hi I have a class Item which has the following information. public class Item { protected String mName;//name of the item, e.g. "Effective Java" protected double mPrice;//price e.g. "40.4" protected String mType;//type e.g. "Book" } I need to store Item in Map, hence I am overriding hashCode() and equals() methods. I know how to write both of them. My question is ... |
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Hi, I'm not sure what forum this belongs in, becasue this is only my second post; please move if inappropriate. I have been looking at the collections classes and have a few questions: 1) equals() does not *have* to be overridden in your class, but if you don't the test is for absolute equivalence of the same object in memory, and ... |
I have been using the following code. Is overridding of hashCode() method is just for the sake of definition (if two objcts are equal, then their hashCodes should be equal)even if we don't override hashCode(), the output will be true. It doesn't make any difference. Then why should we override hashCode()? Just for the sake of definition??? Any help on this ... |
Any Collection which has an internal implementation using hashes will need a decent implementation of hashCode() to be efficient. Fortunately most of them have 'Hash' in their classnames - HashMap, Hashtable, HashSet, LinkedHashSet etc. There's not a huge amount of application for it outside the realm of storing objects inside hashing data structures though. |
Could there be practical use cases where you may want to override only hashCode() & not equals() ? I do not think so. The Java 5 API states for hashCode: If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. Consider: class ... |
Hi Siddharth, the equals() method is used to check for semantic equality of objects of the same class type. The programmer is free to reasonably define what "equal" means for some specific kind of class. It depends on the class and the requirements so it's not easy to give a good default implementation for all cases. For Strings for example equal ... |
Its recommended that IF you override equals() you over ride hashcode(). Why? If code is not going to use hashtables, is it sufficient to do public int hashcode(){return 1;} If not, or if hashtables are required, what are some approaches that will meet all the requirements of equals? Or where would be a good place to read about this. I can ... |
I have written equals and hashCode methods to a class. It is suppose to detect and reject duplicate grades entered into the JPanel. When you press the View button it should show up only once but it shows up twice. I am bewildered. I have included the code I wrote. Please advise. public boolean equals(Object o){ if (this == o){ return ... |
Which of the following statements are true about correctly overriden hashCode() and equals() methods? 1) the equals() must return the same value as using the == operator 2) equals() and hashCode() code must return the same value. 3) If two objects are different according to equals() they will return different hashCode() values. 4) If two objects are equal according to equals() ... |
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Hi , I stumbled on this question- Given that the objects denoted by the parameters override the equals() and the hashCode() methods appropriately, which return values are possible from the following method? String func(Object x, Object y) { return (x == y) + " " + x.equals(y) + " " + (x.hashCode() == y.hashCode()); } Select the two correct answers. a ... |
HashCode and equals are entirely 2 diff methods on Object class in JAVA . hasgcode returns a unique indentifier for the object while equals is used for comparison of 2 objects . equals method can be overridden by all custom objects for there equality comparison . By default equals method just compares the references. Hope it helps. |
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Hi Preethi, unfortunately the full concept of equals() and hashCode() and how they can be implemented can't be said in a few words. In short equals() is used to compare to objects for equality. The default implementation just compares the objects for identity which is the same you'd get if you used the == operator. If this isn't satisfying you could ... |
Hi guys, I have a question that i would like some feedback on. Take a look at the following code fragments: Example A: public boolean equals(Object obj) { if(this == obj) return true; // 1st conditional check if(obj == null) || (obj.getClass() != this.getClass())) return false; Example B: public boolean equals(Object obj) { if(this == obj) return true; // 2nd conditional ... |
Manoj, The first step is to define what you want object equality to mean. (just id or all fields, what level of granularity for the date) Note that nobody here will write your code for you. We will help you so do feel free to post your first attempt here for comments/direction. |
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import java.util.HashMap; import java.util.HashSet; public class Testing { 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; Testing test = (Testing)obj; return num == test.num && (data == test.data || (data != null && data.equals(test.data))); } /* public int hashCode() { int hash = 7; ... |
Hi Ranchers , Can you please tell me how Hashcode & Equals are related to each other. I didnt get how equals & Hashcode methods are called when you are calling get method on HashMap. In what are all scenarios these Hashcode & Equals are called internally. your help is Appreciated Thanks in advance Siva |
Overriding equals and hashcode is useful in the cases when you want to check the equality of objects based on some field of the object. for ex. class Book{ int bookId ; Book(int id){bookId = id;} } Book b1 = new Book(1); Book b2 = new Book(1); now if you try b1 == b2 you will get false because b1 and ... |
Hi guys, Please take a look at the code below. I am confident in this implementation, but a colleague argues that the variable (myData) must be marked as final. The problem is that this variable needs to be set dynamically (i.e., via the constructor or an extended class). Please advised. public class MyClass { ... protected Object myData; // Constructor public ... |
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when do we override equals and hashcode nmethods ? What happens if we dont override them in our class ? Can we overload them ? Whats the relationship between the two ie. equals and hashcode. Program : Test10 t1= new Test10("abc"); Test10 t2= new Test10("abc boolean bl = t1.equals(t2); System.out.println(bl); System.out.println(t1.hashCode()); System.out.println(t2.hashCode()); Output : false 2025617596 (some random value) 2036496738 (some ... |
There can be many objects which are not equal() but which return the same hashcode() but you can never have two objects which are equal() with different hashcode() For example consider a name object, where two names are equal() if they have the same first name and last name. The hashcode() could be simply to take the first letter of the ... |
class MyClass { } public class Test { public static void main(String[] args) { MyClass a = new MyClass(); MyClass b = new MyClass(); MyClass c = a; System.out.println("a equals b ? " + a.equals(b)); // false System.out.println("b equals c ? " + b.equals(c)); // false System.out.println("a equals c ? " + a.equals(c)); // true } } |
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I do not understand why the behavior is different. import java.util.HashMap; /** * Beispiel Klasse, die inkonsistentes Verhalten aufzeigt, wenn hashCode nicht berschrieben wurde. * Was erwarten Sie beim Aufruf der main-Methode? * Entspricht das Ergebnis Ihren Erwartungen? * Lesen Sie die Dokumentation zu den Methoden equals und hashCode in der Klasse Object. * Weshalb kommt es hier zu diesem Verhalten? ... |
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If we override equals, then we must override hashcode 1. Do I really need to do it if I choose to use a separate function defined in the class say compareID(int id) for equality? When are we forced to invoke obj1.equals(obj2) 2. If my custom hashcode always returns same value for instance public int hashCode() { return 20; } Does that ... |
hi friends, i have a doubt about collections. i learned that collections use equals() method to check whether an object is meaningfully similar to other, and hashCode() is to arrange them in different pools. so is equals() is enough for avoiding duplications in a collections like HashSet? when i tried to make HashSet of my own class it allow duplication even ... |
Here's an exercise from a book. Given: class SortOf { String name; int bal; String code; short rate; public int hashCode() { return (code.length()*bal); } public boolean equals(Object o) { //insert code here } } Which of the following will fulfill the equals() and hashCode() contracts for this class? (Choose all that apply) A return ((SortOf)o).bal == this.bal; B return ((SortOf)o).code.length() ... |
Two custom objects of the same class can have equal attributes, but therefor are not the same in the eyes of the JVM. In order to force Java to see if these objects (e.g. two citizens) are the same, you should compare their attributes (e.g. Social Security Number) than the methods from its superclasse Object. Same goes for hashcode. |
Both if blocks work perfectly fine in either case. In the first, they evaluate the expression to be false and do nothing, which means it works like it should. Uncommenting the first commented line makes ab hold the same exact object as aa. So of course it evaluates both if statements to be true. They are the same object. |
Have googled a lot but still unable to understand this. In case equals() returns true for 2 objects, then the hashcode for the 2 objects must also be the same. But what im not able to understand is why this requirement? Why is equals() returning true alone not enough to determine the equality of 2 objects, or why is equality of ... |
Because you would have to overwrite it in every class,... ? hashCode() always returns the same int value for the same object, equals() checks for reference. If you want to be more specific, e.g. check values of an object, or you receive an object by remote, you can overwrite these methods, but it's not always necessary,... regards slowfly |
vinothkp wrote: The two instances of the following class have different hashcodes returned (Even though they are not extremely efficient). Those two instances are not equal right? It seems that you think that if hashCode returns two different values, then equals will automatically return false. It doesn't work that way. You the programmer are required to ensure that equal objects have ... |
are quite incomplete and stupid since you do not know anything about x and y. If you know only that if (x.equals(y)) you know nothing. You can not infere the equality of the hashcodes. Edited by: Sorin_Ciolofan on May 16, 2008 4:00 AM Edited by: Sorin_Ciolofan on May 16, 2008 4:03 AM |
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... and the implementation of removeAddress looks like? Also, why are you only setting found and breaking out of the loop if that method returns false? Seems like you "found" it due to the code above that. Maybe you're removing an entry, then continuing on and trying to remove another one inappropriately because you didn't break out of the loop and ... |
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Hello all, 1. Why always override hashcode() if overriding equals()? - If we don't override hashcode, what problem will happen seriously? 2. It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. - can you show me examples about this ... |
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package TestCollection; import java.util.Collections; import java.util.Comparator; import java.util.HashSet; import java.util.SortedSet; import java.util.TreeSet; class Student { private int id; private String fname; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getFname() { return fname; } public void setFname(String fname) { this.fname = fname; } |
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