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Overriding the java equals() method - not working?

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Can we override the equals method in Java?

You can override the equals method on a record, if you want a behavior other than the default. But if you do override equals , be sure to override hashCode for consistent logic, as you would for a conventional Java class.

What if equals method is not overridden?

You must override hashCode in every class that overrides equals. Failure to do so will result in a violation of the general contract for Object. hashCode, which will prevent your class from functioning properly in conjunction with all hash-based collections, including HashMap, HashSet, and Hashtable.

What is the reason for overriding equals () method?

We can override the equals method in our class to check whether two objects have same data or not.

What are equals () and hashCode () overriding rules?

if a class overrides equals, it must override hashCode. when they are both overridden, equals and hashCode must use the same set of fields. if two objects are equal, then their hashCode values must be equal as well. if the object is immutable, then hashCode is a candidate for caching and lazy initialization.


In Java, the equals() method that is inherited from Object is:

public boolean equals(Object other);

In other words, the parameter must be of type Object. This is called overriding; your method public boolean equals(Book other) does what is called overloading to the equals() method.

The ArrayList uses overridden equals() methods to compare contents (e.g. for its contains() and equals() methods), not overloaded ones. In most of your code, calling the one that didn't properly override Object's equals was fine, but not compatible with ArrayList.

So, not overriding the method correctly can cause problems.

I override equals the following everytime:

@Override
public boolean equals(Object other){
    if (other == null) return false;
    if (other == this) return true;
    if (!(other instanceof MyClass)) return false;
    MyClass otherMyClass = (MyClass)other;
    ...test other properties here...
}

The use of the @Override annotation can help a ton with silly mistakes.

Use it whenever you think you are overriding a super class' or interface's method. That way, if you do it the wrong way, you will get a compile error.


If you use eclipse just go to the top menu

Source --> Generate equals() and hashCode()


Slightly off-topic to your question, but it's probably worth mentioning anyway:

Commons Lang has got some excellent methods you can use in overriding equals and hashcode. Check out EqualsBuilder.reflectionEquals(...) and HashCodeBuilder.reflectionHashCode(...). Saved me plenty of headache in the past - although of course if you just want to do "equals" on ID it may not fit your circumstances.

I also agree that you should use the @Override annotation whenever you're overriding equals (or any other method).


Another fast solution that saves boilerplate code is Lombok EqualsAndHashCode annotation. It is easy, elegant and customizable. And does not depends on the IDE. For example;

import lombok.EqualsAndHashCode;

@EqualsAndHashCode(of={"errorNumber","messageCode"}) // Will only use this fields to generate equals.
public class ErrorMessage{

    private long        errorNumber;
    private int         numberOfParameters;
    private Level       loggingLevel;
    private String      messageCode;

See the options avaliable to customize which fields to use in the equals. Lombok is avalaible in maven. Just add it with provided scope:

<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.14.8</version>
    <scope>provided</scope>
</dependency>

in Android Studio is alt + insert ---> equals and hashCode

Example:

    @Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    Proveedor proveedor = (Proveedor) o;

    return getId() == proveedor.getId();

}

@Override
public int hashCode() {
    return getId();
}

Consider:

Object obj = new Book();
obj.equals("hi");
// Oh noes! What happens now? Can't call it with a String that isn't a Book...