Skip to main content

Why do I need to override the equals and hashCode methods in Java?

 Imagine you have this

MyClass first = new MyClass("a","first");
MyClass second = new MyClass("a","second");

Override only equals

If only equals is overriden, then when you call myMap.put(first,someValue) first will hash to some bucket and when you call myMap.put(second,someOtherValue) it will hash to some other bucket (as they have a different hashCode). So, although they are equal, as they don't hash to the same bucket, the map can't realize it and both of them stay in the map.

Although it is not necessary to override equals() if we override hashCode(), let's see what would happen in this particular case where we know that two objects of MyClass are equal if their importantField is equal but we do not override equals().

Override only hashCode

If you only override hashCode then when you call myMap.put(first,someValue) it takes first, calculates its hashCode and stores it in a given bucket. Then when you call myMap.put(second,someOtherValue) it should replace first with second as per the Map Documentation because they are equal (according to the business requirement).

But the problem is that equals was not redefined, so when the map hashes second and iterates through the bucket looking if there is an object k such that second.equals(k) is true it won't find any as second.equals(first) will be false.

Comments

Post a Comment

Popular posts from this blog

Momo suicide game

Microsoft clamps down on sick 'Momo suicide game' in 'Minecraft' Microsoft is clamping down on the sick “Momo suicide challenge,” which recently infiltrated the wildly popular online game “Minecraft.”The tech giant owns “Minecraft” developer Mojang. The vile “Momo suicide game” has been garnering attention after spreading on WhatsApp, prompting police warnings. "Momo" is a viral challenge that asks people to add a contact via WhatsApp - they are then   urged   to commit self-harm or suicide. The "game" has fueled comparisons to the sinister " Blue Whale challenge " that led to reports of suicides in Russia and the U.S, as well as the online fictional character of "Slender Man." In 2014 two 12-year-old girls in Wisconsin  attempted to kill   a classmate in an attempt to please the horror character. The Buenos Aires Times recently  reported  that police in Argentina are investigating whether “Momo” is linked to the suicide of a 12-y...
Post a Comment
Read more

Java Functional Interfaces

  The term   Java functional interface   was introduced in Java 8. A   functional interface   in Java is an interface that contains only a single abstract (unimplemented) method. A functional interface can contain default and static methods which do have an implementation, in addition to the single unimplemented method. Here is a Java functional interface example: public interface MyFunctionalInterface { public void execute(); } The above counts as a functional interface in Java because it only contains a single method, and that method has no implementation. Normally a Java interface does not contain implementations of the methods it declares, but it can contain implementations in default methods, or in static methods. Below is another example of a Java functional interface, with implementations of some of the methods: public interface MyFunctionalInterface2{ public void execute(); public default void print(String text) { System.out.println(t...
Post a Comment
Read more

The Java 8 Stream API

1 Overview In this comprehensive tutorial, we'll go through the practical uses of Java 8 Streams from creation to parallel execution. To understand this material, readers need to have a basic knowledge of Java 8 (lambda expressions,   Optional,  method references) and of the Stream API. 2 Stream Creation There are many ways to create a stream instance of different sources. Once created, the instance  will not modify its source,  therefore allowing the creation of multiple instances from a single source. Empty Stream We should use the  empty()  method in case of the creation of an empty stream: Stream<String> streamEmpty = Stream.empty(); We often use the  empty()  method upon creation to avoid returning  null  for streams with no element: public Stream<String> streamOf (List<String> list) { return list == null || list.isEmpty() ? Stream.empty() : list.stream(); } 2.2. Stream of  Collection We can also creat...
Post a Comment
Read more