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Conditional statement if-else

The instruction orif statement, controls the flow of program execution. Depending on a condition, a sequence of instructions is executed or not.

If statement syntax

if (<condition>) <instruction>

The <condition> is a Boolean expression; the result may be true or false. The instruction will be executed, only if the result is true. The <instruction> is usually replaced by a block of statements, which is nothing more than a sequence of statements separated by semicolons.

if (<condition>) {<sequence of statements separated by semicolons>}

The indentation of the statements inside the if, is not mandatory. It is recommended for a better understanding of the code, as it makes it visible that statements are affected by the condition.

Example:

package com.edu4java.javatutorials;import javax.swing.JOptionPane;public class IfSentence {public static void main(String[] args) {String input = JOptionPane.showInputDialog("Enter your age");int age = Integer.parseInt(input);if (age >= 18) {JOptionPane.showMessageDialog(null, "You are an adult");}}}

This program asks your age and if you are older or equal 18, it shows the message "You are an adult".

The statement int age = Integer.parseInt (input); is necessary becauseshowInputDialog returns a String type and we need to convert it to a number, to compare to 18.

If you are under 18, we do not display any messages. This can be improved using anelse block.

If - else syntax

You can add anelse clause to theif statement, which allows you to run a statement or block of statements if the condition is not met.

if (<condition>) <instruction>else<instruction>

Using a block of instructions it would look like this:

if (<condition>) {<sequence of statements separated by semicolons>
} else {<sequence of statements separated by semicolons>}

If the condition is true, the first block of statements is executed, if the condition is false, the second block of statements is executed.

Example:

package com.edu4java.javatutorials;import javax.swing.JOptionPane;public class IfSentence {public static void main(String[] args) {String input = JOptionPane.showInputDialog("Enter your age");int age = Integer.parseInt(input);if (age >= 18) {JOptionPane.showMessageDialog(null, "You are an adult");}else {JOptionPane.showMessageDialog(null, "You are a minor");}}}

Nesting statements

Within the block of statements we can include any instruction we want. It is very common in programming to findif statements withinif statements. We exemplify this by solving a classic problem.

Problem: Given three sides of a triangle, say if it is equilateral, isosceles or scalene:

Equilateral triangle: three equal sides
Isosceles triangle: two equal and one different side
Scalene triangle: three different sides

package com.edu4java.javatutorials;import javax.swing.JOptionPane;public class Triangle {public static void main(String[] args) {int side1 = Integer.parseInt(JOptionPane.showInputDialog("Enter the first side"));int side2 = Integer.parseInt(JOptionPane.showInputDialog("Enter the second side"));int side3 = Integer.parseInt(JOptionPane.showInputDialog("Enter the third side"));if (side1 == side2 && side2 == side3) {JOptionPane.showMessageDialog(null, "It is an equilateral triangle");} else {if (side1 == side2 ||  side2 == side3) {JOptionPane.showMessageDialog(null, "It is an isosceles triangle");} else {JOptionPane.showMessageDialog(null, "It is a scalene triangle");}}}}

The "int side1 = Integer.parseInt (JOptionPane.showInputDialog ("Enter the first side"));" uses as an argument the functionshowInputDialog for theparseInt function. We are replacing the two lines we used to enter a number in the previous example, for a single line.

Explanation of conditions:

If (side1 == side2 && side2 == side3) is true, all of the sides are equal. It is equilateral.
If (side1 == side2 || side2 == side3) is true, there are at least two equal sides.As we know that is not equilateral since we are inside the else block, we can conclude that it is isosceles.
If (side1 == side2 || side2 == side3) is false, we conclude that none of the sides are equal. It is scalene.

Conditional function- Ternary operator

There is a conditional version that is a function instead of an instruction. It receives three parameters; a condition and two expressions. If the condition evaluates to true, the operator returns the first value after the question mark. If it is false it returns the second value.

(condition) ? <expression for true> : <expression for false>

Expressions can be of any type, but both have to be the same type.

Example:

int i = (5>4) ? 10 : 0; // i = 10char c = (false) ? 'a' : 'b'; // c = 'b'

We can rewrite the first example, when we asked about the age, using this function.

package com.edu4java.javatutorials;import javax.swing.JOptionPane;public class IfSentenceConditional {public static void main(String[] args) {int age = Integer.parseInt(JOptionPane.showInputDialog("Enter your age"));String answer = (age >= 18) ? "You are an adult" : "You are a minor";JOptionPane.showMessageDialog(null, answer);}}

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Conditional statement if-else

If statement syntax

If - else syntax

Nesting statements

Conditional function- Ternary operator