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Ensure Serializable Classes Have a serialVersionUID

High
correctnessmaintainabilityreliability

What is it?​

This practice requires all classes that implement Serializable to explicitly declare a serialVersionUID with the modifiers static final long.

Why apply it?​

Failing to declare a serialVersionUID can result in a new, incompatible value being generated when the class is compiled. This can cause deserialization failures if the class evolves by missing its original intent of compatibility during object serialization.

How to fix it?​

Add a static final long serialVersionUID field with a fixed value in all serializable classes.

Examples​

Example 1:​

Negative​

In this negative example, the class Mango implements Serializable but does not declare a serialVersionUID, leading to potential compatibility issues.

import java.io.Serializable;

public class Mango implements Serializable {  // Noncompliant; no serialVersionUID
  private String color;
  private double weight;

  public Mango(String color, double weight) {
    this.color = color;
    this.weight = weight;
  }

  public String getColor() {
    return color;
  }

  public double getWeight() {
    return weight;
  }
}

Example 2:​

Positive​

In this positive example, the class Mango properly implements the Serializable interface and includes a static final long serialVersionUID.

import java.io.Serializable;

public class Mango implements Serializable {
  private static final long serialVersionUID = 123456789L;
  private String color;
  private double weight;

  public Mango(String color, double weight) {
    this.color = color;
    this.weight = weight;
  }

  public String getColor() {
    return color;
  }

  public double getWeight() {
    return weight;
  }
}

Negative​

In this negative example, the Apple class declares serialVersionUID, but incorrectly uses an int type instead of long, and the modifiers are inappropriate.

import java.io.Serializable;

public class Apple implements Serializable {
  private final int serialVersionUID = 12345; // Noncompliant; not static & int rather than long
  private String type;
  private boolean isRipe;

  public Apple(String type, boolean isRipe) {
    this.type = type;
    this.isRipe = isRipe;
  }

  public String getType() {
    return type;
  }

  public boolean getIsRipe() {
    return isRipe;
  }
}

Example 3:​

Positive​

In this positive example, the Apple class declares serialVersionUID correctly with all the necessary modifiers: static, final, and long.

import java.io.Serializable;

public class Apple implements Serializable {
  private static final long serialVersionUID = 987654321L;
  private String type;
  private boolean isRipe;

  public Apple(String type, boolean isRipe) {
    this.type = type;
    this.isRipe = isRipe;
  }

  public String getType() {
    return type;
  }

  public boolean getIsRipe() {
    return isRipe;
  }
}