<?xml version="1.0"?>

<chapter xml:id="conditionals">

<title>Conditionals</title>

<section xml:id="simple-if">

<title>Using the Simple <c>if</c> Statement</title>

<p><idx>conditional statements</idx>

Conditional statements in Python and Java are very similar.

In Python we have three patterns:

</p>

<p>

In Python the simple <c>if</c> statement is written as:

</p>

<program interactive="activecode" language="python">

<code>

score = 95

if score &gt;= 90:

print("Excellent work!")

</code>

</program>

<p>

In Java, this same pattern requires two changes: the condition must be in parentheses <c>()</c>, and the code block must be enclosed in curly braces <c>{}</c>.

</p>

<program interactive="activecode" language="java">

<code>

public class SimpleIfExample {

public static void main(String[] args) {

int score = 70;

if (score &lt;= 70) {

System.out.println("Needs work!");

}

}

}

</code>

</program>

<p>

Once again you can see that in Java the curly braces define a block rather than indentation.

In Java the parenthesis around the condition are required because it is technically a function that evaluates to <c>True</c> or <c>False</c>.

</p>

</section>

<section xml:id="if-else">

<title>Using the <c>if</c> - <c>else</c> Statement</title>

<p>The Java equivalent follows the same syntactical rules as before.</p>

<program interactive="activecode" language="python">

<code>

age = 16

if age &gt;= 18:

print("You can vote.")

else:

print("You are not yet eligible to vote.")

</code>

</program>

<program language="java">

<code>

public class IfElseExample {

public static void main(String[] args) {

int age = 16;

if (age &gt;= 18) {

System.out.println("You can vote.");

} else {

System.out.println("You are not yet eligible to vote.");

}

}

}

</code>

</program>

</section>

<section xml:id="elif">

<title>Can we use <c>elif</c>?</title>

<p><idx><c>elif</c> statement</idx>

Java does not have an <c>elif</c> pattern like Python.

In Java you can get the functionality of an <c>elif</c> statement by nesting <c>if</c> and <c>else</c>.

Here is a simple example in both Python and Java.

</p>

<program interactive="activecode" language="python">

<code>

grade = int(input('enter a grade'))

if grade &lt; 60:

print('F')

elif grade &lt; 70:

print('D')

elif grade &lt; 80:

print('C')

elif grade &lt; 90:

print('B')

else:

print('A')

</code> <tests> </tests>

</program>

<p>

In Java we have a couple of ways to write this.

</p>

<program interactive="activecode" language="java">

<code>

public class ElseIf {

public static void main(String args[]) {

int grade = 85;

if (grade &lt; 60) {

System.out.println('F');

} else {

if (grade &lt; 70) {

System.out.println('D');

} else {

if (grade &lt; 80) {

System.out.println('C');

} else {

if (grade &lt; 90) {

System.out.println('B');

} else {

System.out.println('A');

}

}

}

}

}

}

</code> <tests> </tests>

</program>

<p>

We can get even closer to the <c>elif</c> statement by taking advantage of the Java rule that a single statement does not need to be enclosed in curly braces. Since the if is the only statement used in each else we can get away with the following.

</p>

<program interactive="activecode" language="java">

<code>

public class ElseIf {

public static void main(String args[]) {

int grade = 85;

if (grade &lt; 60) {

System.out.println('F');

} else if (grade &lt; 70) {

System.out.println('D');

} else if (grade &lt; 80) {

System.out.println('C');

} else if (grade &lt; 90) {

System.out.println('B');

} else System.out.println('A');

}

}

</code> <tests> </tests>

</program>

</section>

<section xml:id="switch">

<title>Using the <c>switch</c> Statement</title>

<p>

Java also supports a <c>switch</c> statement that acts something like the <c>elif</c> statement of Python under certain conditions. To write the grade program using a <c>switch</c> statement we would use the following:

</p>

<note>

<p>

Depending on your knowledge and experience with Python you may already be familiar and questioning why we are not using the <c>match</c> statement in our Python examples. The answer is that this book currently runs its active code examples on Python 3.7, which does not support the <c>match</c> statement. The <c>match</c> statement was introduced in Python 3.10. Below is an example of the <c>match</c> statement similar to our grade method.

</p>

<program language="python">

<title>Match Case Example</title>

<code>

grade = 100 // 10

def grading(grade):

match grade:

case 10 | 9:

return 'A'

case 8:

return 'B'

case 7:

return 'C'

case 6:

return 'D'

case _:

return 'F'

print(grading(grade))

</code>

</program>

</note>

<p><idx><c>switch</c></idx>

The <c>switch</c> statement in Java provides a clean and efficient alternative to chaining multiple <c>if-else</c> conditions, especially when comparing a single variable against several constant values. It supports a variety of data types, including primitive types (<c>byte</c>, <c>short</c>, <c>char</c>, <c>int</c>), their wrapper classes, <c>enumerations</c>, and <c>String</c> (introduced in Java 7). Each <c>case</c> within a <c>switch</c> must be defined using a constant expression, and duplicate <c>case</c> values are not permitted. By default, control flow "<c>falls through</c>" from one <c>case</c> to the next unless a <c>break</c>, <c>return</c>, or <c>throw</c> statement is used to terminate execution.

</p>

<p><idx><c>switch</c> expressions</idx>

Java 14 introduced <c>switch</c> <term>expressions</term>, enhancing functionality by allowing the <c>switch</c> to return values and eliminating <c>fall-through</c> via the <c>-&gt;</c> arrow syntax. These expressions can even use <c>yield</c> within code blocks for more complex evaluations. It’s important to note that traditional <c>switch</c> statements do not support <c>null</c> values and will throw a <c>NullPointerException</c> if evaluated with <c>null</c>. As the language evolves, newer versions of Java continue to extend <c>switch</c> capabilities with features like <c>pattern matching</c> and enhanced <c>type handling</c>, making it a more powerful and expressive tool for decision-making in Java programs.

</p>

<program interactive="activecode" language="java">

<code>

public class SwitchUp {

public static void main(String args[]) {

int grade = 85;

int tempgrade = grade / 10;

switch(tempgrade) {

case 10:

case 9:

System.out.println('A');

break;

case 8:

System.out.println('B');

break;

case 7:

System.out.println('C');

break;

case 6:

System.out.println('A');

break;

default:

System.out.println('F');

}

}

}

</code> <tests> </tests>

</program>

<p>

The <c>switch</c> statement is not used very often, and we recommend you do not use it. First, it is not as powerful as the <c>else if</c> model because the switch variable can only be compared for equality with an integer or enumerated constant. Second, it is very easy to forget to put in the <c>break</c> statement, so it is more error-prone. If the break statement is left out then then the next alternative will be automatically executed. For example, if the grade was 95 and the <c>break</c> was omitted from the <c>case 9:</c> alternative then the program would print(out both A and B.)

</p>

</section>

<section xml:id="boolean-operators">

<title>Boolean Operators</title>

<p><idx>Boolean operators</idx>

The conditionals used in the if statement can be Boolean variables, simple comparisons, and compound Boolean expressions.

</p>

<p><idx>ternary operator</idx>

Java also supports the <c>boolean</c> expression using the ternary operator

<c>condition ? trueValue : falseValue</c>. This operator tests a condition as part

of an assignment statement. The following table summarizes how this works:

</p>

<table>

<title>Ternary Operator in Java</title>

<tabular>

<row>

<cell><term>Component</term></cell>

<cell><term>Description</term></cell>

</row>

<row>

<cell><c>condition</c></cell>

<cell>The Boolean expression that is evaluated (e.g., <c>a % 2 == 0</c>).</cell>

</row>

<row>

<cell><c>?</c></cell>

<cell> This is the ternary operator that separates the condition from the <c>trueValue</c>.</cell>

</row>

<row>

<cell><c>trueValue</c></cell>

<cell>The value assigned if the condition is true (e.g., <c>a * a</c>).</cell>

</row>

<row>

<cell><c>:</c></cell>

<cell> This is the ternary operator that separates the <c>trueValue</c> from the <c>falseValue</c>.</cell>

</row>

<row>

<cell><c>falseValue</c></cell>

<cell>The value assigned if the condition is false (e.g., <c>3 * x - 1</c>).</cell>

</row>

<row>

<cell>Example Usage</cell>

<cell><c>a = a % 2 == 0 ? a * a : 3 * x - 1</c></cell>

</row>

<row>

<cell>Equivalent if-else Code</cell>

<cell>Can also be written with a regular <c>if-else</c> statement, but the ternary form is more concise.</cell>

</row>

</tabular>

</table>

<p>

Using this operator can make code shorter and more readable in cases where a simple conditional assignment is needed.

</p>

<program interactive ="activecode" language ="java">

<code>

class Main {

public static void main(String[] args) {

int a = 4;

int x = 2;

// Using the ternary operator

a = (a % 2 == 0) ? a * a : 3 * x - 1;

System.out.println("Result: " + a);

}

}

</code>

</program>

<p>

In this example we are using this ternary operator to assign a value to <c>a</c> based on whether <c>a</c> is even or odd. If <c>a</c> is even, it will be squared; if odd, it will be instead be calculated as <c>3 * x - 1</c>. This is a concise way to write conditional assignments in Java. However, it should be used reasonably, as it can make code less readable if overused or used in complex expressions.

</p>

</section>

</chapter>