Where are equals and hashCode contracts in Java used?

Where are equals and hashCode contracts in Java used?

Unless the data stored in the object is updated, the hashCode function should return the same integer value for the same object for each call. If two objects are equal (as determined by the equals function), the hashCode method should return the same integer result for both. This contract ensures that objects with identical contents will be hashed to the same address, which is essential for any hash-based collection.

The Object class provides two functions for doing this: the equals function and the hashCode function. These functions must be defined as public so that they can be used by collections without calling them themselves.

For example, suppose we want to create a hash table where the keys are strings. Before we can do this, we need to define an equals function that determines whether two strings are equal. We can use the equals function to compare any two objects, including strings, to see if they represent the same person or entity. It's important when using the equals function to remember that it compares object identity, not value. Two strings "John" and "Jack" are considered equal even though they represent two different people.

To make sure that strings are treated equally when using a hash table, we need to define a hashCode function for them. The default implementation of hashCode returns just the string's memory address.

What is hashCode in an object class?

HashCode It returns the object's hash code value. This approach takes advantage of hash tables, such as those given by HashMap. If two objects are equal according to the equals (Object) method, then using the hashCode method on both must have the same integer result. Thus, no iterative search is needed when using a hash table.

It is important to note that this method is not required to return distinct integers for each distinct object. That is, two objects may have the same hash code even if they are not equal. However, as long as this does not happen too often, it will not affect the performance of your program.

The default hash code implementation for objects generated by Java automatically calls this method. Therefore, it is not usually necessary to call this method directly. However, if you generate your own objects then you should probably call this method so that they will work properly with hash tables.

Example: Let's say we want to store the names of students in a hash table. We could use their full names as keys, but this would make searching very difficult because the string "John Smith" would be treated as the same key as the string "Smith John". To avoid this problem, we can convert both names to lowercase first, and then use them as keys in the table.

When to use equals and hashCode on the same object?

This number does not have to be consistent from one execution of an application to the next. This is an example of an equilibrium between equality and uniqueness.

Is it mandatory to override hashCode if you override the equals method?

You MUST override hashCode if you override equals. If two objects are unequal according to the equals method, then invoking the hashCode method on each of the two objects does not have to yield separate integer outputs. Instead, the program can rely on the fact that calling hashCode on an object will never change the value of this object, as long as its equals method remains unchanged.

What is the general contract of hashCode in Java?

The overall contract of hashCode is that if hashCode is executed more than once on the same object during application execution, it shall consistently return the same integer value, provided no information used in the equals (Object) comparison on the object is updated. This ensures that objects that are equal according to the equals(Object) method are also equal according to the hashCode method.

For example, consider the following code:

public class Test public static void main(String args) { System.out.println(new Test.hashCode); }

When this program is run, it produces the following output:


This shows that the two calls to hashCode returned 1, which is consistent with the requirement that objects that are "equal" must also have the same hash code.

What happens if we override only equals and not hashCode?

Overriding only the equals method without overriding hashCode produces unequal hash codes for the two equal instances, which violates the hashCode contract (mentioned in Javadoc), which clearly states that if two objects are equal according to the equals (Object) method, then calling the hashCode method on each of the two resulting objects produces unequal hash codes. This may not be a problem for some use cases but it is important to understand.

Why do you need to override the equals method in Java?

Nothing happens if you merely alter the hash-code function, because it always produces a fresh hash code for each object as an object class. If you merely override the equals method, if a.equals (b) is true, it implies that the hashCodes of a and b must be the same, which does not occur because you did not override the hashCode method.

What happens if we override only equals?

Only override HashCode, leave Equals alone: Only references to the same item will get a true result. In other words, applying the equals method on objects that you anticipated to be equal will result in them not being equal. Make use of the standard HashCode: The hashmap or hashset may include duplicates. If so, they will still produce different results when equated using object equality.

About Article Author

Jay Sitko

Jay Sitko is a man of many passions. He loves his family, works in tech and does special projects for the government. Jay has been an engineer by heart since he was a young boy, when he would take apart electronic devices just to see what made them work.

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