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Designing Data-Intensive Applications
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Part 1. Foundations of Data Systems
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Part 1. Foundations of Data Systems
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Chapter 1. Reliable, Scalable and Maintainable Applications
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Chapter 2. Data Models and Query Languages
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Java Collections
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UML Diagrams
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<h1 id="java-collections">Java Collections<a class="headerlink" href="#java-collections" title="Permanent link">¶</a></h1>
|
|
<p>Week 3 (Oct 5<sup>th</sup>)</p>
|
|
<p><strong>Part 1</strong></p>
|
|
<p>In java a <em>collection</em> is an object that represents a group of objects.
|
|
The collections API is a unified framework for representing and manipulating collections independently of their implementation.</p>
|
|
<p>An <em>API</em> (application programming interface) is an interface protocol between a client and a server, intended to simplify the client side software.</p>
|
|
<p>A <em>library</em> contains re-usable chunks of code.</p>
|
|
<p><strong>Java Collections framework</strong>
|
|
- We have container objects that contain objects
|
|
- All containers are either "collections" or "maps"
|
|
- All containers provide a common set of method signatures, in addition of their unique set of method signatures</p>
|
|
<p><em>Collection</em> - Something that holds a dynamic collection of objects
|
|
<em>Map</em> - Defines mapping between keys and objects (two collections)
|
|
<em>Iterable</em> - Collections are able to return an iterator objects that can scan over the contents of a collection one object at a time</p>
|
|
<p>NOTE: Vector is a legacy structure in Java replaced with <em>ArrayList</em>
|
|
Stack is now <em>ArrayDeque</em></p>
|
|
<p><code>LinkedList(Collection<? extends E> c)</code> - means some type that either is E or a subtype of E. The <code>?</code> is a wildcard.</p>
|
|
<div class="highlight"><pre><span></span><code><span class="kd">public</span><span class="w"> </span><span class="kd">static</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">main</span><span class="p">(</span><span class="n">String</span><span class="o">[]</span><span class="w"> </span><span class="n">args</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
|
|
<span class="w"> </span><span class="n">LinkedList</span><span class="w"> </span><span class="n">list</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">LinkedList</span><span class="p">();</span>
|
|
<span class="w"> </span><span class="n">list</span><span class="p">.</span><span class="na">add</span><span class="p">(</span><span class="s">"string"</span><span class="p">);</span>
|
|
<span class="w"> </span><span class="n">String</span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="n">String</span><span class="p">)</span><span class="n">list</span><span class="p">.</span><span class="na">getFirst</span><span class="p">();</span>
|
|
<span class="w"> </span><span class="n">System</span><span class="p">.</span><span class="na">out</span><span class="p">.</span><span class="na">println</span><span class="p">(</span><span class="n">s</span><span class="p">);</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p>This is bad coding practice, the collection constructor are not able to specify the type of objects the collection is intended to contain. A <code>ClassCastException</code> will be thrown if we attempt to cast the wrong type.</p>
|
|
<div class="highlight"><pre><span></span><code><span class="kd">public</span><span class="w"> </span><span class="kd">static</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">main</span><span class="p">(</span><span class="n">String</span><span class="o">[]</span><span class="w"> </span><span class="n">args</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
|
|
<span class="w"> </span><span class="n">LinkedList</span><span class="o"><</span><span class="n">String</span><span class="o">></span><span class="w"> </span><span class="n">list</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">LinkedList</span><span class="o"><></span><span class="p">();</span>
|
|
<span class="w"> </span><span class="n">list</span><span class="p">.</span><span class="na">add</span><span class="p">(</span><span class="s">"string"</span><span class="p">);</span>
|
|
<span class="w"> </span><span class="n">String</span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">list</span><span class="p">.</span><span class="na">getFirst</span><span class="p">();</span>
|
|
<span class="w"> </span><span class="n">System</span><span class="p">.</span><span class="na">out</span><span class="p">.</span><span class="na">println</span><span class="p">(</span><span class="n">s</span><span class="p">);</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p>This is a type safe collection using generics.
|
|
- Classes support generics by allowing a type variable to be included in their declaration.
|
|
- The <code><></code> show the same type as stated (in this case string)
|
|
- You cannot type a collection with a primitive data type eg int</p>
|
|
<p><strong>HashMap Class</strong>
|
|
- A HashMap is a hash table based implementation of the map interface. This implementation provides all if the optional map operations, and permits null values and the null key.</p>
|
|
<div class="highlight"><pre><span></span><code><span class="kd">public</span><span class="w"> </span><span class="kd">static</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">main</span><span class="p">(</span><span class="n">String</span><span class="o">[]</span><span class="w"> </span><span class="n">args</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
|
|
<span class="w"> </span><span class="n">HashMap</span><span class="o"><</span><span class="n">String</span><span class="p">,</span><span class="w"> </span><span class="n">Integer</span><span class="o">></span><span class="w"> </span><span class="n">userData</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">HashMap</span><span class="o"><></span><span class="p">();</span>
|
|
<span class="w"> </span><span class="n">userData</span><span class="p">.</span><span class="na">put</span><span class="p">(</span><span class="s">"Emma"</span><span class="p">,</span><span class="w"> </span><span class="mi">30</span><span class="p">);</span>
|
|
<span class="w"> </span><span class="n">userData</span><span class="p">.</span><span class="na">put</span><span class="p">(</span><span class="s">"John"</span><span class="p">,</span><span class="w"> </span><span class="kc">null</span><span class="p">);</span>
|
|
<span class="w"> </span><span class="n">userData</span><span class="p">.</span><span class="na">put</span><span class="p">(</span><span class="s">"Millie"</span><span class="p">,</span><span class="w"> </span><span class="mi">17</span><span class="p">);</span>
|
|
|
|
<span class="w"> </span><span class="n">Set</span><span class="o"><</span><span class="n">String</span><span class="o">></span><span class="w"> </span><span class="n">keys</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">userData</span><span class="p">.</span><span class="na">keySet</span><span class="p">();</span>
|
|
|
|
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">String</span><span class="w"> </span><span class="n">key</span><span class="p">:</span><span class="n">keys</span><span class="p">){</span>
|
|
<span class="w"> </span><span class="n">System</span><span class="p">.</span><span class="na">out</span><span class="p">.</span><span class="na">println</span><span class="p">(</span><span class="n">key</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="s">"="</span><span class="w"> </span><span class="n">userData</span><span class="p">.</span><span class="na">get</span><span class="p">(</span><span class="n">key</span><span class="p">));</span>
|
|
<span class="w"> </span><span class="p">}</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<div class="highlight"><pre><span></span><code>Emma = 30
|
|
John = null
|
|
Millie = 17
|
|
</code></pre></div>
|
|
<p><strong>Part 2</strong></p>
|
|
<p><strong>Relationships between objects</strong></p>
|
|
<p><em>Aggregation</em> - The object exists outside the other. It is created outside so it is passed as an argument.
|
|
An animal object <em>is part of</em> a compound object (semantically) but the animal object can be shared and if the compound object is deleted, the animal object isn't deleted.</p>
|
|
<div class="highlight"><pre><span></span><code><span class="kd">public</span><span class="w"> </span><span class="kd">class</span> <span class="nc">Compound</span><span class="w"> </span><span class="p">{</span>
|
|
<span class="w"> </span><span class="kd">private</span><span class="w"> </span><span class="n">Animal</span><span class="w"> </span><span class="n">dog</span><span class="p">;</span>
|
|
|
|
<span class="w"> </span><span class="kd">public</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">setAnimal</span><span class="p">(</span><span class="n">Animal</span><span class="w"> </span><span class="n">dog</span><span class="p">){</span>
|
|
<span class="w"> </span><span class="k">this</span><span class="p">.</span><span class="na">dog</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">dog</span><span class="p">;</span>
|
|
<span class="w"> </span><span class="p">}</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p><img alt="Image" src="../assets/1.png" /></p>
|
|
<p><em>Composition</em> - The object only exists if the parent object exists, if the parent object is deleted then so is the child object.
|
|
The zoo object owns the compound object. If the zoo object is deleted then the compound object is also deleted.</p>
|
|
<div class="highlight"><pre><span></span><code><span class="kd">public</span><span class="w"> </span><span class="kd">class</span> <span class="nc">Zoo</span><span class="w"> </span><span class="p">{</span>
|
|
<span class="w"> </span><span class="kd">private</span><span class="w"> </span><span class="n">Compound</span><span class="w"> </span><span class="n">dogArea</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">Compound</span><span class="p">();</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p><img alt="Image" src="../assets/2.png" /></p>
|
|
<p><strong>Inheritance</strong>
|
|
A way of forming new classes based on existing classes. Has a "is-a" relationship.</p>
|
|
<p><em>Polymorphism</em> - A concept in object oriented programming. Method overloading and method overriding are two types of polymorphism.
|
|
- <em>Method Overloading</em> - Methods with the same name co-exist in the same class but they must have different method signatures. Resolved during compile time (static binding).
|
|
- <em>Method Overriding</em> - Methods with the same name is declared in parent and child class. Resolved during runtime (dynamic binding).</p>
|
|
<div class="highlight"><pre><span></span><code><span class="kd">public</span><span class="w"> </span><span class="kd">class</span> <span class="nc">Child</span><span class="w"> </span><span class="kd">extends</span><span class="w"> </span><span class="n">Parent</span><span class="w"> </span><span class="p">{</span>
|
|
<span class="w"> </span><span class="kd">public</span><span class="w"> </span><span class="nf">Child</span><span class="p">(</span><span class="n">String</span><span class="w"> </span><span class="n">name</span><span class="p">){</span>
|
|
<span class="w"> </span><span class="kd">super</span><span class="p">(</span><span class="n">name</span><span class="p">);</span>
|
|
<span class="w"> </span><span class="p">}</span>
|
|
|
|
<span class="w"> </span><span class="nd">@Override</span>
|
|
<span class="w"> </span><span class="n">Public</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">eat</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
|
|
<span class="w"> </span><span class="n">chew</span><span class="p">();</span>
|
|
<span class="w"> </span><span class="p">}</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p>The super keyword called the parent class' constructor.</p>
|
|
<p><img alt="Image" src="../assets/3.png" /></p>
|
|
<p><strong>What is the difference between an abstract class and an interface</strong>
|
|
- Java abstract class can have instance methods that implement a default behaviour. May contain non-final variables.
|
|
- Java interfaces have methods that are implicitly abstract and cannot have implementations. Variables are declared final by default.</p>
|
|
<p>Interfaces are less restrictive when it comes to inheritance, interfaces can have many levels of inheritance where as a class can only have one level.</p>
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</article>
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