Hashmap works on hashing method. So if you insert a object with a key which is already in keyset. It will replace exsisting object and return you previous object. Internal process. First it will generate hash of key. Then it will got to key loc. HashCode The hashCode method of objects is used when you insert them into a HashTable, HashMap or HashSet. If you do not know the theory of how a hashtable works internally, you can read about hastables on Wikipedia.org. When inserting an object into a hastable you use a key. Nov 30, 2017 When you create your own key pair object, you should face a few thing. First, you should be aware of implementing hashCode and equals.You will need to do this. Second, when implementing hashCode, make sure you understand how it works. You must override hashCode in every class that overrides equals. Failure to do so will result in a violation of the general contract for Object.hashCode, which will prevent your class from functioning properly in conjunction with all hash-based collections, including HashMap, HashSet, and Hashtable.
Do I have to override equals due to using an object as a hashmap key? Faber Siagian. Ranch Hand Posts: 52. Posted 11 years ago. K&B tells that HashMap key has to override the equals properly so that it can be found. So you must override hashCode method in.
In this tutorial, we will see the importance of hashCode and equals method while writing code by using HashMap. We will see first what the default behaviour of these methods and later will see how to override these methods.
Both hashCode() and equals() method are defined in Java.lang.Object class.
hashCode () :public navtive int hashCode();
It returns the hash code value for the object. This method returns the unique integer value of a given object. That returned integer value is used to find the bucket location, when you are trying to insert that object in Hash Map/Hash Table.
equals ():
publicboolean equals(Object obj) {
return (this obj);
}
Equals method returns Boolean value. This method is used to compare the 2 objects. By default it compares the object reference of two objects to check their equality.
Example on HashMap with equals and hashCode methods.
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 | importjava.util.Map; Employee e2=newEmployee('sai',1001); Map<Employee,String>map=newHashMap<Employee,String>(); map.put(e1,'employee1'); map.put(e3,'employee3'); { System.out.println(entry.getKey().getName()+'-'+entry.getKey().getid()+'-'+entry.getValue()); System.out.println(map.get(newEmployee('krishna',1002))); } classEmployee{ Stringname; publicStringgetName(){ } this.name=name; publicEmployee(Stringname,intid){ this.name=name; } returnid; publicvoidsetid(intid){ } public int hashCode() { int result = 1; result = prime * result + ((name null) ? 0 : name.hashCode()); } public boolean equals(Object obj) { return true; return false; return false; if (id != other.id) if (name null) { return false; return false; }*/ |
Output:
2 4 6 8 10 12 |
hashCode() – HashCode is generated based on the Employee name and id. So it will gives the same hash code for the below object keys.
new Employee(“krishna”,1002) object
e3 object
equals() – equality of objects has been done based on Employee name and id. So it will gives the true for the below object keys.
Hashcode Generate Key For Hashmap Mac
new Employee(“krishna”,1002) object 128 bit wep key generator.
e3 object
You can override these methods easily from Eclipse. Right click on code, Click on Souce, and Click on Generate HashCode and equals.
Please post comments, if you have any doubts. I will explain.