Git-Leon Lecture Template

<h1 id="fundamental-programming-structures-in-java-arrays">Fundamental Programming Structures in Java: Arrays</h1>

<h2 id="what-well-cover">What we’ll cover</h2>

Arrays
ArrayLists
Wrapper Classes

<h2 id="arrays">Arrays</h2>
<ul>
 <li>An array is a data structure that stores a collection of values of the same type.</li>
 <li>each individual value is accessible by using specifting its index.</li>
</ul>

Declare an array variable by specifying the array type - which is the element type followed by [] - and the array variable name

Use the new operator to create the array.

To find the number of elements of an array, use array.length

Once you create an array, you cannot change its size

The enhanced for loop

<h3 id="array-initializers-and-anonymous-arrays">Array Initializers and Anonymous Arrays</h3>

shortcut for creating an array object and supplying initial values at the same time

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>int[] smallPrimes = { 2, 3, 5, 7, 11, 13 };
</code></pre></div></div>

anonymous array:
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>new int[] { 17, 19, 23, 29, 31, 37 }
</code></pre></div></div>

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>smallPrimes = new int[] { 17, 19, 23, 29, 31, 37 };
//is shorthand for
int[] anonymous = { 17, 19, 23, 29, 31, 37 };

smallPrimes = anonymous;
</code></pre></div></div>

<h3 id="array-copying">Array Copying</h3>

You can copy one array variable into another, but then both variables refer to the same array:

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>int[] luckyNumbers = smallPrimes;
luckyNumbers[5] = 12; // now smallPrimes[5] is also 12
</code></pre></div></div>

Figure 3.14 shows the result. If you actually want to copy all values of one array into a new array, you use the copyOf method in the Arrays class

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>int[] copiedLuckyNumbers = Arrays.copyOf(luckyNumbers, luckyNumbers.length);
</code></pre></div></div>

<h3 id="command-line-parameters">Command-Line Parameters</h3>

<code class="language-plaintext highlighter-rouge">java Example foo bar baz 1 2 3</code>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>public class Example {
 public static void main(String[] args) {
 for(String clArg : args) {
 System.out.println(clArg);
 }
 }
}
</code></pre></div></div>

<h3 id="array-sorting">Array Sorting</h3>

To sort an array of numbers, you can use one of the sort methods in the Arrays class:

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>int[] a = new int[10000];
...
Arrays.sort(a)
</code></pre></div></div>

This method uses a tuned version of the QuickSort algorithm that is claimed to be very efficient on most data sets.

<a href="https://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html">Arrays API</a>

<h3 id="multidimensional-arrays">Multidimensional Arrays</h3>

Multidimensional arrays use more than one index to access array elements

Declaring a two-dimensional array in Java

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>double[][] balances;
</code></pre></div></div>

You cannot use the array until you initialize it.

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>balances = new double[NYEARS][NRATES];
</code></pre></div></div>

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>int[][] magicSquare =
{
 {16, 3, 2, 13},
 {5, 10, 11, 8},
 {9, 6, 7, 12},
 {4, 15, 14, 1}
};

balances[i][j]
</code></pre></div></div>

<h2 id="arraylists">ArrayLists</h2>
An ArrayList is a generic class that stores data like an array, but with dynamic sizing and methods.

<blockquote>
 <code class="language-plaintext highlighter-rouge">ArrayList<Integer> intArrList = new ArrayList<Integer>();</code>
</blockquote>

or

<blockquote>
 <code class="language-plaintext highlighter-rouge">ArrayList<Integer> intArrList = new ArrayList<>();</code>
</blockquote>

<h3 id="wrapper-classes">Wrapper Classes</h3>
Generic classes (like ArrayList) cannot take primitive types as it’s type parameter. 
Luckily, Java has “wrapper classes” to fix this, and will “autobox” and “unbox” the values.
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>ArrayList<Integer> intArrList = new ArrayList<Integer>();
for(int i = 0; i < 5; i++) {
 intArrList.add(i);
}
</code></pre></div></div>

<h3 id="wrapper-classes-1">Wrapper Classes</h3>
A wrapper class is, essentially, the Object representation of the primitive type. 
They also, sometimes, have helpful methods and class constants within them that the primitives don’t. 
For example, Integer has a

<blockquote>
 <code class="language-plaintext highlighter-rouge">public static String toBinaryString(int i)</code>
</blockquote>

method to return the bit string of a number. 
Double has the <code class="language-plaintext highlighter-rouge">NEGATIVE_INFINITY</code>, <code class="language-plaintext highlighter-rouge">POSITIVE_INFINITY</code>, and <code class="language-plaintext highlighter-rouge">NaN</code> constants.

<table>
 <thead>
 <tr>
 <th>primitive</th>
 <th>Wrapper</th>
 </tr>
 </thead>
 <tbody>
 <tr>
 <td>int</td>
 <td>Integer</td>
 </tr>
 <tr>
 <td>byte</td>
 <td>Byte</td>
 </tr>
 <tr>
 <td>short</td>
 <td>Short</td>
 </tr>
 <tr>
 <td>long</td>
 <td>Long</td>
 </tr>
 <tr>
 <td>char</td>
 <td>Character</td>
 </tr>
 <tr>
 <td>float</td>
 <td>Float</td>
 </tr>
 <tr>
 <td>double</td>
 <td>Double</td>
 </tr>
 <tr>
 <td>boolean</td>
 <td>Boolean</td>
 </tr>
 </tbody>
</table>

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>String[] arr = {"Foo", "Bar", "Baz"};
ArrayList<String> arrList = new ArrayList<>();
for(String word : arr) {
 arrList.add(word);
}
arrList.get(0);
arrList.set(2, "new one");
arrList.remove(1);
arrList.size();
</code></pre></div></div>

<h3 id="copying-arrays-and-arraylists">Copying Arrays and ArrayLists</h3>

This is just one array with two things referencing it.

You need <code class="language-plaintext highlighter-rouge">Arrays.copyOf()</code> to quickly do a copy of an array.

For arrayLists:
<blockquote>
 <code class="language-plaintext highlighter-rouge">ArrayList<String> copiedList = new ArrayList<>(originalList);</code>
</blockquote>

You can also turn ArrayLists into arrays and vice-versa.

Look into the ArrayList and Arrays APIs to see what methods they have available.

<h2 id="functional-decomposition">Functional Decomposition</h2>

public static void main(String[] args) {

int[] arr = {1, 2, 3, 4, 5};
 int[] doubledArr = new int[arr.length];

for(int i = 0; i < arr.length; i++) {
 doubledArr[i] = arr[i] * 2;
 }

int[] quadArr = new int[arr.length];

for(int i = 0; i < arr.length; i++) {
 quadArr[i] = doubledArr[i] * 2;
 }

for(int item : quadArr) {
 System.out.println(item);
 }
 }
}
</code></pre></div></div>

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>public class Example {
 public static void main(String[] args) {
 int[] arr = {1, 2, 3, 4, 5};
 int[] doubledArr = doubleArr(arr);
 int[] quadArr = doubleArr(doubledArr);

printArr(quadArr);
 }

public static int[] doubleArr(int[] arr) {
 int[] doubled = new int[arr.length];
 for(int i = 0; i < arr.length; i++) {
 doubled[i] = arr[i] * 2;
 }
 return doubled;
 }

public static void printArr(int[] arr) {
 for(int item : arr) {
 System.out.println(item);
 }
 }
}
</code></pre></div></div>

<h3 id="var-args">Var Args</h3>
Essentially just an array and must be the last parameter in the function declaration.

<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code>public static int avg(int... values) {
 int sum = 0;
 for(int value : values) {
 sum += value;
 }
 return sum / values.length;
}
</code></pre></div></div>

<img id="logo" src="/Curriculeon/resources/reveal/img/logo.png" style="max-width: 30%;max-height: 30%;">
						 
						<ul>
							<li>Finished!</li>
						</ul>