Interviewer's Favorite Pitfall: Why must I rewrite hashCode when rewriting equals?

Tech 2023-10-08 12:48:31 Source: Network

#Autumn Life Clock In Season#Hello, I'm Xiaomi. Welcome to my WeChat official account! Today, we will discuss a frequently asked question in Java development: "Why is rewritingEqualsMethod also needs to be rewrittenHashCodeMethod This question may appear frequently in your interviews, but it is not just an interview question, it also involves very important concepts in Java, Namely object equality and hash codes

#Autumn Life Clock In Season#

Hello, I'm Xiaomi. Welcome to my WeChat official account! Today, we will discuss a frequently asked question in Java development: "Why is rewritingEqualsMethod also needs to be rewrittenHashCodeMethod This question may appear frequently in your interviews, but it is not just an interview question, it also involves very important concepts in Java, Namely object equality and hash codes. Let's delve deeper into this issue and understand why it is so important.

equalandHashCode

In Java, each object has a defaultEqualsMethod, which compares whether the references of objects are equal, that is, comparing whether two objects are the same instance. However, in actual development, we usually need to compare the contents of objects for equality, rather than just comparing their references. That's why we need to rewriteEqualsThe reason for the method.

HashCode..Java...

Equals

By default, in Java, theEqualsThe method compares references to objects. If we don't rewriteEqualsMethod, then for two different objects, even if their contents are the same, callEqualsThe method will also returnFalseBecause their references are different.

Consider the following example:

In this example, althoughPerson1andPerson2The content of is the same, but they are different objects, soEqualsMethod returnFalse. This is obviously not the behavior we want.

To solve this problem, we need to rewriteEqualsMethod to compare the content of an object instead of a reference. Usually, we override in custom classesEqualsMethod to implement our own equality logic and compare whether the properties of an object are equal.

HashCode

Okay, now we know why we need to rewrite itEqualsMethod, but why does it still need to be rewrittenHashCodeWhat about the method? This is because when using hash data structures, such as hash tables, we want equal objects to have equal hash codes.

In Java, hash tables use hash codes to determine the location of stored objects. If two equal objects have different hash codes, they will be stored in different positions in the hash table, resulting in incorrect lookup of these objects.

Consider the following example:

In this example, althoughPerson3andPerson4The content of is the same, but due to their different hash codes,set.contains(Person4)returnFalse. This is because the hash table cannot locate correctlyPerson4.

To solve this problem, we need to ensure that the rewriteEqualsThe object of the method must also be overriddenHashCode ..

HashCode

So, how to rewrite correctlyHashCodeWhat about the method? Java forHashCode .

Here are some rewritesHashCodeRules for methods:

If two objects pass throughEquals .
HashCode .
HashCode ..
.

EqualsandHashCode

Let's take a look at an example of how to rewriteEqualsandHashCodeMethod:

In this example, we have rewritten theEqualsMethod, comparedNameandAgeWhether the attributes are equal and then overriddenHashCodeMethod, usingObjects. hash .

END

To ensure the equality of objects and the correctness of the hash table, we need to rewriteEqualsMethod is also overriddenHashCode ..

When you encounter this issue in an interview, don't forget to emphasize itEqualsandHashCodeConsistency and performance of methods, as well as adherence to rewritingHashCode ..

I hope this article can help you better understand why rewriting is necessaryEqualsMethod also needs to be rewrittenHashCode ..

Knowing what it is and why it is

Disclaimer: The content of this article is sourced from the internet. The copyright of the text, images, and other materials belongs to the original author. The platform reprints the materials for the purpose of conveying more information. The content of the article is for reference and learning only, and should not be used for commercial purposes. If it infringes on your legitimate rights and interests, please contact us promptly and we will handle it as soon as possible! We respect copyright and are committed to protecting it. Thank you for sharing.(Email:[email protected])

Mobile advertising space rental

Tag: Interviewer Favorite Pitfall Why must rewrite hashCode when rewriting