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Avoid Using @Nonnull in equals Method Parameters

High
reliabilitybug

What is it?​

This practice advises against using @Nonnull in the parameter of the overridden equals method, as the contract of equals requires it to properly handle null parameters.

Why apply it?​

The equals method should be able to process a null argument by returning false, as specified in the java.lang.Object documentation. Using @Nonnull contradicts this requirement and can lead to unexpected behavior.

How to fix it?​

Remove the @Nonnull annotation from the equals method parameter and handle null appropriately within the method's implementation.

Examples​

Example 1:​

Negative​

The negative example shows a noncompliant equals method with a @Nonnull parameter, violating the method's contract.

import javax.annotation.Nonnull;

public class Car {
  private String model;

  public Car(String model) {
    this.model = model;
  }

  @Override
  public boolean equals(@Nonnull Object obj) {  // Noncompliant
    if (this == obj) {
      return true;
    }
    if (getClass() != obj.getClass()) {
      return false;
    }
    Car car = (Car) obj;
    return model != null ? model.equals(car.model) : car.model == null;
  }
  
  @Override
  public int hashCode() {
    // Simplified hash code implementation
    return model != null ? model.hashCode() : 0;
  }
}

Example 2:​

Positive​

The positive example shows a compliant equals method that checks for null without using @Nonnull.

public class Car {
  private String model;

  public Car(String model) {
    this.model = model;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null || getClass() != obj.getClass()) {
      return false;
    }
    Car car = (Car) obj;
    return model != null ? model.equals(car.model) : car.model == null;
  }
  
  @Override
  public int hashCode() {
    // Simplified hash code implementation
    return model != null ? model.hashCode() : 0;
  }
}

Negative​

This negative example incorrectly uses @Nonnull in the equals method parameter, which should accept null.

import javax.annotation.Nonnull;

public class Book {
  private String title;

  public Book(String title) {
    this.title = title;
  }

  @Override
  public boolean equals(@Nonnull Object obj) {  // Noncompliant
    if (this == obj) {
      return true;
    }
    if (getClass() != obj.getClass()) {
      return false;
    }
    Book book = (Book) obj;
    return title != null ? title.equals(book.title) : book.title == null;
  }

  @Override
  public int hashCode() {
    return title != null ? title.hashCode() : 0;
  }
}

Example 3:​

Positive​

This positive example correctly handles null by checking for it at the beginning of the equals method.

public class Book {
  private String title;

  public Book(String title) {
    this.title = title;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null || getClass() != obj.getClass()) {
      return false;
    }
    Book book = (Book) obj;
    return title != null ? title.equals(book.title) : book.title == null;
  }

  @Override
  public int hashCode() {
    return title != null ? title.hashCode() : 0;
  }
}