<?xml version="1.0"?>

<chapter xml:id="common-mistakes-id1">

<title>Common Mistakes</title>

<introduction>

</introduction>

<section xml:id="forgetting-to-declare-your-variables">

<title>Forgetting to declare your variables</title>

<p>

The 'cannot find symbol' error for the variable <c>count</c> on line 7 indicates that <c>count</c> was used before it was declared within the <c>Histo</c> class. In Java, all variables must be explicitly declared with a data type (e.g., <c>int</c>, <c>String</c>, <c>ArrayList&lt;Integer&gt;</c>) before they can be assigned a value or referenced in any way. The arrow in the error message points to where the undeclared variable <c>count</c> was first encountered. To resolve this, <c>count</c> needs to be declared with its appropriate type (e.g., <c>ArrayList&lt;Integer&gt; count;</c>) before any attempt to initialize or use it.

</p>

<pre>

Main.java:7: error: cannot find symbol

count = new ArrayList&lt;Integer&gt;(10);

^

symbol: variable count

location: class Main

</pre>

</section>

<section xml:id="not-importing-a-class">

<title>Not importing a class</title>

<p>

You may notice that this error message looks similar to the previous one, however, it has an entirely different cause. In Java, classes like <c>Scanner</c> that are part of external packages (like <c>java.util</c>) must be explicitly imported into your source file. Java does not automatically recognize these classes. To resolve this error, you need to add an import statement for the <c>Scanner</c> class at the beginning of your <c>Histo.java</c> file, typically import <c>java.util.Scanner;</c>.

</p>

<pre>

Histo.java:3: error: cannot find symbol

Scanner data = null;

^

symbol: class Scanner

location: class Histo

</pre>

</section>

<section xml:id="forgetting-to-use-the-new-keyword-to-create-an-object">

<title>Forgetting to use the new keyword to create an object</title>

<p>

This error message occurs when you forget to use the <c>new</c> keyword to instantiate an object.

Specifically, on line 8 of <c>Histo.java</c>, <c>data = Scanner(new File("test.dat"));</c> leads to a 'cannot find symbol' error.

While the message states 'symbol: method Scanner(File)', this can be misleading.

Java incorrectly interprets <c>Scanner()</c> as an attempt to call a static method named <c>Scanner</c> within the <c>Histo</c> class

(or an inherited one). However, <c>Scanner</c> is a constructor of the <c>Scanner</c> class, not a standalone method,

and therefore requires the new keyword (e.g., <c>data = new Scanner(new File("test.dat"));</c>) to create a new <c>Scanner</c> object.

</p>

<pre>

Histo.java:8: error: cannot find symbol

data = Scanner(new File("test.dat"));

^

symbol: method Scanner(File)

location: class Histo

</pre>

</section>

<section xml:id="forgetting-a-semicolon">

<title>Forgetting a Semicolon</title>

<p>

The error "';' expected" on line 7 of <c>Histo.java</c> means that a semicolon is missing at the end of the statement <c>Scanner data = null</c>. In Java, every statement must be terminated with a semicolon (<c>;</c>) to indicate its completion. The arrow points to <c>null</c> because that's where the compiler expected to find the semicolon.

</p>

<pre>

Histo.java:7: error: ';' expected

Scanner data = null

^

</pre>

</section>

<section xml:id="forgetting-to-declare-the-kind-of-object-in-a-container">

<title>Forgetting to declare the kind of object in a container</title>

<p>

This is a compiler warning, not an error, indicating a potential type safety issue. It occurs because you are calling the <c>add()</c> method on <c>rawList</c>, which is an <c>ArrayList</c> used as a raw type (i.e., without specifying a generic type like <c>&lt;String&gt;</c> or <c>&lt;Integer&gt;</c>).

</p>

<p>

When <c>ArrayList</c> is used as a raw type, the compiler cannot guarantee the type of elements being added or retrieved, leading to "unchecked" operations. The <c>E extends Object</c> part refers to the generic type parameter <c>E</c> that <c>ArrayList</c> should have, which defaults to <c>Object</c> when the type is not specified, making the <c>add()</c> call unchecked. This can lead to <c>ClassCastExceptions</c> at runtime if incompatible types are later retrieved and cast.

</p>

<pre>

UncheckedWarningDemo.java:8: warning: [unchecked] unchecked call to add(E) as a member of the raw type ArrayList

rawList.add("Hello");

^

where E is a type-variable:

E extends Object declared in class ArrayList

</pre>

</section>

</chapter>