<?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>

In Python, you can use a variable without declaring it first, but in Java, you must declare all variables before using them.

If you try to use a variable that has not been declared, the Java compiler will give you an error message like this:

</p>

<program language="java" interactive="activecode" line-numbers="yes">

<code>

import java.util.ArrayList; // Import necessary class

public class Main { // Class declaration

// Main method declaration

public static void main(String[] args) {

// Attempt to use 'count' without declaration

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

// This line won't be reached due to the error.

System.out.println("ArrayList created.");

} // End of main method

} // End of class

</code>

</program>

<p>

The 'cannot find symbol' error for the variable <c>count</c> on line 6 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>

</section>

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

<title>Not importing a class</title>

<p>

In Python, many classes are available by default. However, in Java, you must explicitly import most classes from external packages that you want to use.

</p>

<program language="java" interactive="activecode" line-numbers="yes">

<code>

// Histo.java

public class Histo { // Class declaration

public static void main(String[] args) { // Main method declaration

// This line will cause the "cannot find symbol" error

Scanner data = null;

} // End of main method

} // End of class

</code>

</program>

<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>

</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>

Unlike Python, where you can create a new object without explicitly using a keyword, Java requires the <c>new</c> keyword to instantiate a new object.

</p>

<program language="java" interactive="activecode" line-numbers="yes">

<code>

// Histo.java // The filename for this example

import java.io.File; // Import the File class, needed for file operations

import java.util.Scanner; // Import the Scanner class, now correctly imported

public class Histo { // Class declaration

public static void main(String[] args) { // Main method declaration

Scanner data = Scanner(new File("test.dat")); // This Line will cause an error

} // End of main method

} // End of class

</code>

</program>

<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>

</section>

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

<title>Forgetting a Semicolon</title>

<p>

A common mistake in Java is to forget that every statement must end with a semicolon (<c>;</c>).

</p>

<program language="java" interactive="activecode" line-numbers="yes">

<code>

// Histo.java

import java.util.Scanner; // Imports Scanner

public class Histo { // Class declaration

public static void main(String[] args) { // Main method declaration

Scanner data = null // The error will point here

System.out.println("This line will not compile.");

}// End of main method

}//End of class

</code>

</program>

<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>

</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>

Java is a statically typed language, meaning you must specify the type of objects that can be stored in a container like an <c>ArrayList</c>. If you forget to declare the type, the compiler will give you an error.

</p>

<program language="java" interactive="activecode" line-numbers="yes">

<code>

// UncheckedWarningDemo.java

import java.util.ArrayList; // Import the ArrayList class

public class UncheckedWarningDemo { // Class declaration

public static void main(String[] args) { // Main method declaration

// This is where the potential for unchecked operations begins.

ArrayList rawList = new ArrayList();

rawList.add("Hello");

//This is the line that will throw an error

System.out.println("Element added: " + rawList.get(0));

} // End of main method

} // End of class

</code>

</program>

<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>

</section>

</chapter>