Lab 4 - Mutability

“Immutable objects are simple. They can only be in one state, which is carefully controlled by the constructor. One of the most difficult elements of program design is reasoning about the possible states of complex objects. Reasoning about the state of immutable objects, on the other hand, is trivial.

Immutable objects are also safer. Passing a mutable object to untrusted code, or otherwise publishing it where untrusted code could find it, is dangerous — the untrusted code might modify its state, or, worse, retain a reference to it and modify its state later from another thread. On the other hand, immutable objects cannot be subverted in this manner by malicious or buggy code, so they are safe to share and publish freely without the need to make defensive copies.” 

—Brian Goetz, Java Concurrency in Practice

Introduction

An immutable object is one whose externally visible state cannot change after it is instantiated. The String, Integer, and BigDecimal classes in the Java class library are examples of immutable objects -- they represent a single value that cannot change over the lifetime of the object.

Lab Instructions:

Consider the following code:

import java.util.Date;

public final class Planet {

  String name;

  privatefinalDate discoveryDate;

  public Planet (String name, Date discoveryDate) {

     this.name = name;

this.discoveryDate = new Date(discoveryDate.getTime());

  }

  public String getName() {

    return name;

  }

  public Date getDiscoveryDate() {

    return new Date(discoveryDate.getTime());

  }

}

1.      Is this program immutable? If you think it is not, what changes would need to be implemented and why? If you think it is, explain why? To explain you need to create a tester class to run the code and see if you can change any values.

2.      Did you notice anything about the Date object? The Date object is no longer used in the latest versions of Java. Research why this is so and how it relates to Mutable Code.

3.      Using what you have learned about Mutability. Detail a strategy that could be used for defining immutable objects.

Remember to write your conclusions on your work.

Submission and Grading Details

This lab will form part of your complete Lab book, which must be submitted at the end of the module. You must reference your work.  You should follow the layout of a typical lab book, and add extra headings as necessary. Sample layout below:

·         Lab3 Mutability

o   Description

o   Aims

o   Method  

o   Results

§  Section 1 What is Mutability and Immutability

§  Section 2 Immutable Code or Mutable Code?

§  Section 3 Deprecated Object ‘Date’

§  Section 4 A strategy for defining immutable objects

o   Conclusions

o   Appendix

o   References 

	Marks Available
Description	5
Aims	5
Method	5
Results/Testing/Evaluation	10
Is code Mutability or Not, explain	15
Deprecated Object ‘Date’	10
Outline a Strategy that could be used	10
Conclusion	40
TOTAL	100