Which two statements are true about the hashCode method? (Choose two.) A. The hashCode method for a given class can be used to test for object equality and object inequality for that class. B. The hashCode method is used by the java.util.SortedSet collection class to order the elements within that set. C. The hashCode method for a given class can be ... |
If you look at the source of HashMap public V get(Object key) { if (key == null) return getForNullKey(); int hash = hash(key.hashCode()); for (Entry e = table[indexFor(hash, table.length)]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) return e.value; } return null; }you can see that the entries ... |
11. public class Person { 12. private String name, comment; 13. private int age; 14. public Person(String n, int a, String c) { 15. name = n; age = a; comment = c; 16. } 17. public boolean equals(Object o) { 18. if (! (o instanceof Person)) return false; 19, Person p = (Person)o; 20. return age == p.age && name.equals(p.name); ... |
/** * Returns a hash code for this string. The hash code for a * String object is computed as * * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] // Line 1 *
* using int arithmetic, where s[i] is the * ith character of the string, n is the length of * the string, and ^ indicates exponentiation. ... |
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Hi, I was reading about the need to implement both equals() and hashCode() at Java theory and practice: Hashing it out and so I want to ask what y'all think of this hashCode() idea. I am looking at an object with 25 data members that I have coded an equals() for, and frankly, unique hashCodes in an int don't seem possible. ... |
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Not exactly. Object.hashCode() returns a value based on System.identityHashCode(). That may or may not be based on the memory location of the object, but that's implementation dependent. The only guaranteed thing is that an object will have the same idenityHashCode() during its entire lifetime. It's not even guaranteed to be unique (even though it usually is). Read the API documentation of ... |
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11. hashcode() forums.oracle.com |
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13. hashCode forums.oracle.comHi, I'm in confusion little bit. I have read the documentation for the hashCode() method but unable to understand two things: 1. if obj1.equals(obj2) is true means both will have the same hashcode. but how it can be possible because both objects are stored on the different locations in the memory so the hashcodes must be different. please clarify that thing ... |
Sorry friends.I was busy with my assignment. Actually I am trying to store a string object by encrypting it in some way as to store in a data type with least size possible say an integer and then when I search for that string I decode it back in the original form. This encrypted code needs to be unique and I ... |
Hi baftos, Thanks for the link, I read through item 8 briefly. I think this 31 is an "arbitrary multiplier". Why does every overridden equals() method must have a hashCode() method? It's because the Java rules say so! (okay~) "Always override hashCode when you override equals. Failure to do so will result in a violation of the general contract for Object.hashCode, ... |
The == operator and the hashCode() method are completely separate. == is a comparison operator that tells you whether two references have the same value--that is, whether both point to the same object (or both are null). You can think of it as comparing object addresses, but that's not specified in the JLS or JVM spec, so it's not technically accurate. ... |
17. hashCode forums.oracle.com |
Hi I have a set of 6 keys that are represented by a string but with long length such like 20 character each string . I want to save these 12 keys as one key in HashMap is it recommended to concatenate all the string and put it as a key in the map ,I think its not good Because there ... |
I understand that java.lang.Object's.hashCode() return type is an integer. I have a question around this : If I have to write an efficient hashCode() override for a class that has, say, two String variables, x, y public int hashCode() { return x.hashCode() + y.hashCode() } 1) Is this a good hashCode() implementation 2) What happens if the sum of the two ... |
20. HashCode forums.oracle.comNo, that number is related to the the instance's location in memory. Not in the Sun JDK it isn't. For one, garbage collection moves stuff around in memory (generational & compacting), and the hash code needs to stay constant. I suspect in some early versions of the JDK the garbage collector didn't compact, and the hash code was indeed the object's ... |
I have no idea what net.sf.ehcache.Ehcache is/does but it should not matter that the hash code is not unique. If it does then net.sf.ehcache.Ehcache is rubbish. P.S. java.util.HashMap and java.util.HashSet do not require the hashcode to be unique. In fact they will work when all the hash methods return the same value. It just makes them slower. |
When I were a lad I took a course in [Combinatorics |http://en.wikipedia.org/wiki/CombinaTorics], which is just the fancy name for "the art of counting". Our professor introduced the pigeon-hole principle this way: there are two men in this city with the same number of hairs on their head. Prove it! (There was a clarification where we ignored baldies.) I've heard two people ... |
rahulb1 wrote: means I want to go a bit deep into this... You find the answer in any textbook on algorithms & data structures. Every hash based data structure uses a hash function to map data elements onto a range from 0 to N-1. In principle this means that in order to know where to store a data element it transforms ... |
24. hashcode forums.oracle.com |
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Depends on the kind of fields involved, but assuming a and b are ints the last, return 31 * a + b, would get the job done. Personally I tend to multiply each fields value by successive primes (e.g. (5*a)^(7*b) and xor them together, but the only real requirement is that if a == o.a and b == o.b then hashCode ... |
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and it is written there that : . ....... As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.) |
And I thought as long as the hashCode method from Object is not overwritten it is. The API states: "This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the Java(TM) programming language." So first: it's the >>internal<< address (which I guess refers to the JVM, and not ... |
A hashcode is a number generated from any object. This is what allows objects to be stored/retrieved quickly in a Hashtable. Imagine the following simple example: On the table in front of you you have nine boxes, each marked with a number 1 to 9. You also have a pile of wildly different objects to store in these boxes, but once ... |
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hello, i just want to ask, if a certain string converted into a hash code can be retrieve/turn back to it's original form? ex. String n = "alice"; ---> int value = n.hashCode(); can "value" return to "n" as (alice)? can they provide example code in order to help me solve my problem? thanks in advance.. |
Hi All, How does the hashCode() of Object class compute the hashcode? I went through the documentation. All it says is : "(This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the Java programming language.)" And the method is a native method, so I couldn't even look ... |
34. hashcode forums.oracle.coma hash code is used by Map and other collection classes. It uses hashcodes to decide if two different objects might actually be equal. If you have redefined equal you will find that Maps and HashTables will not work correctly with your class unless you have also redefined hashcode. A hashcode is actually just an integer value obtained by grinding up ... |
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36. hashCode() forums.oracle.comGood day, Java Developers. May you help me, please: I have Java Object with different primitive types: boolean, short, int, char, byte, long, float, double and one link type java.lang.String. How it is better to compound hashCode() realization in this case? Or, may be you know links where I may find this information. Thnx! |
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38. hashcode() forums.oracle.com |
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Complements: Indeed they will end up in the same "bucket" (a list of key-value pairs whose keys have the same hashcode), which is suboptimal performance-wise but nothing so far proves that will be measurable in the real app (and certainly not in a PoC with two keys). Just to shed some more words on this subject, we can say that a ... |
I'm happy to fall back on reduction if I need to, but I'd really prefer only to do it as an absolute last resort. I'm also quite happy to have 1 calculation for small (? still not quite sure what that is) and for "not so small" values (the one I'm bangin' my head about). |
user575089 wrote: This could also ensure the hashCode returns same when the two objects are equal by the above rule. why such mathematical expression to return ? In addition to all the good advice from jverd and Kayaman, who are wise in the ways of hashCode() and equals() (as I can attest to; they convinced me that I was wrong on ... |