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Comparators Should Be "Serializable"

High
maintainabilityreliabilitydefensive_programming

What is it?​

A Comparator is an interface used for sorting or ordering collections. By default, it is not serializable and can prevent an otherwise-Serializable ordered collection from being serialized.

Why apply it?​

Making a Comparator serializable ensures that collections using these comparators remain serializable. The change allows for good defensive programming and often requires minimal overhead.

How to fix it?​

To fix this issue, ensure that Comparator implementations also implement Serializable.

Examples​

Example 1:​

Negative​

In this negative example, the comparator is not serializable, which can cause serialization issues with collections using it.

import java.util.Comparator;

public class FruitComparator implements Comparator<Fruit> {  // Noncompliant
  @Override
  public int compare(Fruit f1, Fruit f2) {
    return f1.getName().compareTo(f2.getName());
  }

  @Override
  public boolean equals(Object obj) {
    return this == obj || obj instanceof FruitComparator;
  }
}

Example 2:​

Positive​

This positive example shows how to declare a comparator that is serializable, maintaining serializable integrity for collections using it.

import java.io.Serializable;
import java.util.Comparator;

public class FruitComparator implements Comparator<Fruit>, Serializable {
  private static final long serialVersionUID = 1L;

  @Override
  public int compare(Fruit f1, Fruit f2) {
    return f1.getName().compareTo(f2.getName()); // Compliant
  }

  @Override
  public boolean equals(Object obj) {
    return this == obj || obj instanceof FruitComparator; // Compliant
  }
}

Negative​

This negative example does not implement Serializable, which may prevent serialization when used within a serializable collection.

import java.util.Comparator;
import java.util.Locale;

public class LexicalFruitComparator implements Comparator<Fruit> { // Noncompliant
  @Override
  public int compare(Fruit f1, Fruit f2) {
    return f1.getName().toLowerCase(Locale.ROOT).compareTo(f2.getName().toLowerCase(Locale.ROOT));
  }

  @Override
  public boolean equals(Object obj) {
    return this == obj || obj instanceof LexicalFruitComparator;
  }
}

Example 3:​

Positive​

Another positive example demonstrating a lexical comparator that implements Serializable, ensuring compatibility with serializable collections.

import java.io.Serializable;
import java.util.Comparator;
import java.util.Locale;

public class LexicalFruitComparator implements Comparator<Fruit>, Serializable {
  private static final long serialVersionUID = 2L;

  @Override
  public int compare(Fruit f1, Fruit f2) {
    return f1.getName().toLowerCase(Locale.ROOT).compareTo(f2.getName().toLowerCase(Locale.ROOT)); // Compliant
  }

  @Override
  public boolean equals(Object obj) {
    return this == obj || obj instanceof LexicalFruitComparator; // Compliant
  }
}