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.
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- 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 04, 2014 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.
Both hashCode() and equals() method are defined in Java.lang.Object class.
Dec 11, 2018 But I never had the need to override hashCode and equals methods. Most of the time we use HashMap when it comes to hash based collections. With in HashMap, we mostly use primitive wrapper class like Integer or String class, as key.These classes are immutable and provide their own proper implementation of hashCode and equals methods, thus these. I understand that A.hashCode B.hashCode but A.equals(B) false. However, if you put A into a HashMap as a key. Then calling hashMap.contains(B) returns false although they have the same hashCode. Here's how Java implements some functions in HashMap.
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; }*/ |
Hashcode Generate Key For Hashmap Mac
Output:
What HashMap’s get method does here?
In short, it first creates the hashcode from the key object (new Employee(“krishna”,1002)).
Then it finds the hash map value object based on this hashcode. It will look for the hash map value object based on hashcode. Since we passed object as new Employee(“krishna”,1002), it creates new hashcode which is not there in the Entry table.
If you pass the key object as e3, it finds the hashcode and try to see in the table to find the value of a map. Since we stored the map value as map.put(e3, “employee3”);, so it found the result in the Entry table and it returned employee3.
But if you pass new Employee(“krishna”,1002) as a key, then it will return null. So we should override the hashCode method.
Java Generate Hashcode
Uncomment the equals and hashcode methods code in above program and execute it. We will get output as below.
hashCode() – HashCode is generated based on the Employee name and id. So it will gives the same hash code for the below object keys.
Hashcode Generate Key For Hashmap In C
new Employee(“krishna”,1002) object
e3 object
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Hashcode Generate Key For Hashmap Online
equals() – equality of objects has been done based on Employee name and id. So it will gives the true for the below object keys.
new Employee(“krishna”,1002) object
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.