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223 lines
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HTML
223 lines
8 KiB
HTML
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<!DOCTYPE html>
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<html>
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<meta charset="utf-8" />
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<meta name="author" content="Fredrik Danielsson, http://lostkeys.se">
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Home
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</nav>
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</aside>
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<main class="main-content">
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<article class="article">
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<h1>Lecture for January 18</h1>
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<h2>Variables and Assignment</h2>
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<p>Think about variables as buckets that hold information. Once the bucket is created, only one type of item can go in the bucket.</p>
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<pre><code class="language-java">sand bucket1;</code></pre>
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<p>We can say that bucket1 is of type <code>sand</code>, that means the only thing that can go in the bucket is sand.</p>
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<pre><code class="language-java">int bucket1;
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double bucket2;</code></pre>
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<p>From the two lines above, we have <em>declared</em> the variable.</p>
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<p>Variables store state, they are a name for a location in memory. </p>
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<p>Always remember to initialize your variables. Otherwise there's nothing in the bucket!</p>
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<pre><code class="language-java">bucket1 = 5;</code></pre>
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<p>You can combine both the declaration and initialization</p>
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<pre><code class="language-java">int count = 15;</code></pre>
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<p>Remember when dealing with variables to stay true with the type, don't mix a bucket of water with a bucket of sand.</p>
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<p>We can update <code>count</code> to contain a true value</p>
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<pre><code class="language-java">count = 55;</code></pre>
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<p><code>count</code> no longer has the value of <code>15</code> in it. There's no record of it! It has been overwritten with the value <code>55</code></p>
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<h3>Primitive Types</h3>
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<p>There are 8 primitive types in Java</p>
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<ul>
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<li>boolean</li>
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<li>char</li>
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<li>byte</li>
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<li>short</li>
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<li>int</li>
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<li>long</li>
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<li>float</li>
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<li>double</li>
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</ul>
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<p>byte through double are all <em>numeric</em> types</p>
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<h4>Boolean</h4>
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<p><code>boolean</code> can only be equal to <code>true</code> or <code>false</code></p>
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<pre><code class="language-java">boolean student = true;</code></pre>
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<h4>Char</h4>
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<p>Stores a single character from the Unicode set</p>
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<p>There are 16 bits per character which adds up to 65,536 characters</p>
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<p>ASCII is the US subset of the characters. You can look this up online when needing to deal with ASCII values</p>
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<pre><code class="language-java">char firstLetter = 'A';</code></pre>
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<h3>Numeric types</h3>
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<p>The different numeric types determine the precision of your number. Since numbers are not represented the same in the computer as they are in real life, there are some approximations.</p>
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<p>The default type you can use your code is <code>int</code> for integers and <code>double</code> for numbers with a decimal point</p>
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<p>There are certain types of operations you can perform on numeric type</p>
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<table>
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<thead>
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<tr>
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<th>Symbol</th>
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<th>Meaning</th>
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<th>Example</th>
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<th>Value</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>+</td>
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<td>addition</td>
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<td>43 + 8</td>
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<td>51</td>
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</tr>
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<tr>
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<td>-</td>
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<td>subtraction</td>
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<td>43.0-8.0</td>
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<td>35.0</td>
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</tr>
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<tr>
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<td>*</td>
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<td>multiplication</td>
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<td>43 * 8</td>
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<td>344</td>
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</tr>
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<tr>
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<td>/</td>
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<td>division</td>
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<td>43.0 / 8.0</td>
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<td>5.375</td>
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</tr>
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<tr>
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<td>%</td>
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<td>remainder / mod</td>
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<td>43 % 8</td>
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<td>3</td>
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</tr>
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<tr>
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<td>-</td>
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<td>unary minus</td>
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<td>-43</td>
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<td>-43</td>
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</tr>
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</tbody>
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</table>
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<h4>Increment/ Decrement</h4>
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<p>There are two types of in/decrementers postfix and prefix</p>
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<p>Postfix:</p>
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<pre><code class="language-java">int x = 0;
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int y = 7;
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x++; // Shortcut for x = x + 1
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y--; // Shortcut for y = y - 1</code></pre>
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<p>Prefix</p>
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<pre><code class="language-java">int x = 0, y = 7, z;
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z = y * x++; // Equivalent to (y * x) + 1 = 7 * 0
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z = y * ++x; // Equivalent to y * (x + 1) = 7 * 1</code></pre>
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<h3>Data Conversion</h3>
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<p>There are two types of data conversion, implicit and explicit</p>
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<p>The compiler can perform implicit data conversion automatically.</p>
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<p>Performing an explicit data conversion requires additional work on the programmer's part</p>
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<p>A conversion is implicit if you do <strong>not</strong> lose any information in it</p>
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<pre><code class="language-java">double price = 6.99;
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int sale = 3;
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double total = price - sale;</code></pre>
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<p>A <em>cast</em> is an explicit data conversion. This is requested by a programmer, this can lead to loss of information</p>
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<pre><code class="language-java">int nextChar = 'b';
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Character.isAlphabetic( (char) nextChar); // Let's you print the actual letter 'b' instead of the number corresponding to it
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float price = 6.99;
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int cost = (int) price; // cost is now 6</code></pre>
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<h3>Printing variables</h3>
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<p>You can print the values of variables using <code>System.out.println</code> and <code>System.out.print</code></p>
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<p>The difference is that <code>System.out.println</code> adds a new line at the end. Meaning the next print out will be on the next line.</p>
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<pre><code class="language-java">int cost = 5;
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double sale = .30;
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System.out.print(cost);
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System.out.print(sale);
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// Prints out '5.30`
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System.out.println(cost);
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System.out.println(sale);
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// Prints out '5'
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// Prints out '0.30'</code></pre>
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<p>To add a space between two variables in a print, add <code>" "</code> to the expression in between the two variables</p>
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<pre><code class="language-java">System.out.println("The total cost is " + 5 " dollars and" + " " + 93 + " cents");
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// The total cost is 5 dollars and 94 cents</code></pre>
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<h3>Input from User</h3>
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<p>You can get import from the user, we can do this using the <code>Scanner</code> class.</p>
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<p>First import it at the top of your file</p>
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<pre><code class="language-java">import java.util.Scanner;</code></pre>
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<p>All you can do with <code>Scanner</code> is outlined in the Java API at this link <a href="https://docs.oracle.com/javase/8/docs/api/index.html?java/util/Scanner.html">https://docs.oracle.com/javase/8/docs/api/index.html?java/util/Scanner.html</a></p>
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<p>Create a Scanner object</p>
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<pre><code class="language-java">Scanner input = new Scanner(System.in);
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System.out.print("Please enter an integer: ");
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price = input.nextInt(); // The integer that the user inputs is now stored in price
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System.out.println("Your input: " + price);</code></pre>
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</article>
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