This question already has answers here:
You need to implement
default implementation of these methods simply checks for instance equality (in other words, two
Objects will only be equal if they are in fact the same object).
Effective Java - Methods common to all objects Can we use object as a key in hashmap in Java?, as long as it has properly defined hashCode and equals methods - those are absolutely crucial because the hashing mechanism depends on them. To use user defined objects as keys in Map you need to tell Java how to compare user defined objects by overriding the hashCode() and equals() methods. Most of the time we generally use Java PI provided classes (i.e., String) as keys for HashMap. So in situations, where you need to use custom object as a key in HashMap then you can use this
Your problem is likely that
Key did not implement
equals() correctly (or at all). In order to be used as a
HashMap key the class has to implement these two methods to reflect "equality" of two objects.
If you create two different instances with
Key a = new Key("xyz");
Key b = new Key("xyz");
and expect them to be equal and work in a
HashMap, you have to override
hashCode() so that it returns the same value in both instances, and
equals() returns true when comparing them.
If the object identity is based on the string value, then
public int hashCode()
public boolean equals(Object o)
HashMap Questions & Answers, What will happen if we put a key object in a HashMap which is already there? How to use User defined Object as key in HashMap with an example Siva Reddy. With Animation!! whats new in java8 tutorial HashMap example using User Defined Objects - Duration:
When a class does not override the
hashCode() methods, the default implementations found on the
Object class are used instead. In particular,
equals() simply does a check for reference equality.
That immediately explains why your approach isn't working: the new
Key object clearly isn't referring to the old
If you'd like to be able to specify a new instance with the same property, then you should override the
equals() method with an implementation that meets your criteria for key equality. You should override
hashCode() as well, to have full control over the key comparison process.
Using Custom Classes as Keys in HashMaps, class's equals and hashcode methods, both the employee1 and employee2 hashcode and equals methods will return the same thing. Hi friends, in this tutorial, we are going to see what happens when try to insert a custom object as key in HashMap. So, lets check with an example Output: Size: 4 Chennai:Tamil Nadu – Mohan Bengaluru:Karnataka – Ram Chennai:Tamil Nadu – Krishna Hyderabad:Andhra Pradesh – Rohan As you can notice the output which contains the keys getting repeated for Chennai:Tamil Nadu. In the normal
Which two method you need to implement for key Object in HashMap, Which two methods you need to implement to use an object as key in HashMap? A simple thumb rule is to use immutable objects as keys in a HashMap. because: If it were mutable, then the hashcode() value or equals() condition might change, and you would never be able to retrieve the key from your HashMap. More precisely, class fields that are used to compute equals() and hashcode() should be immutable! java, Using an instance of an object as a key in hashmap, and then access it with exactly new object? [duplicate] · Ask Question. Asked 7 years, 11 HashMap<Key, Object> test;. and make new Key("the same") as key.. so its like..: So, next time you call object’s hashCode() method, JVM recalculate the hash code for that object. 2. Make HashMap key object immutable. For above basic reasoning, key objects are suggested to be IMMUTABLE. Immutability allows you to get same hash code every time, for a key object. So it actually solves most of the problems in one go.
How to design good custom key object for HashMap, If you want to make a mutable object as key in hashmap, then you have to make sure that state change for key object does not change the hash code of object. This can be done by overriding the hashCode() method. But, you must make sure you are honoring the contract with equals() also. Effectively, you have HashMap< Object,Object >. That said, you still have some options, and some advice I'd suggest you take. Among them: Check to see if it's an instance of 'Map' rather than 'HashMap'. It will make your API much more flexible since you most likely only care that you have rapid access by key rather than any particular impementation
Comments It's hard to know what your question is asking. You might want to consider rewriting it... If you just want all the keys in your hashmap you can use the method
You need to design key class in such a way that it rerturns same hash code for similar field values when objects are equal using equals() method. A good hashmap key design Not duplicate : the other question does not directly deal with maps, but directly with
equals which may seem unrelated without the accepted answer. The accepted answer is just enough to understand the issue.
If you are working in Eclipse, just right click and go to Source>Generate hashCode() and equals()… choose the attributes you want to include in these methods and the code will be auto-generated. A time-saver!