H.K.I.T
Login
Free Signup
H.K.I.T
Login Free Signup
H.K.I.T
Java Introduction
What is Java?
Java is a popular programming language, created in 1995.
It is owned by Oracle, and more than 3 billion devices run Java.
It is used for:
Mobile applications (specially Android apps)
Desktop applications
Web applications
Web servers and application servers
Games
Database connection
And much, much more!
Why Use Java?
Java works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc.)
It is one of the most popular programming language in the world
It is easy to learn and simple to use
It is open-source and free
It is secure, fast and powerful
It has a huge community support (tens of millions of developers)
Java is an object oriented language which gives a clear structure to programs and allows code to be reused, lowering development costs
As Java is close to C++ and C#, it makes it easy for programmers to switch to Java or vice versa
Get Started
It is not necessary to have any prior programming experience.Java Getting Started
Java Install
Some PCs might have Java already installed.
To check if you have Java installed on a Windows PC, search in the start bar for Java or type the following in Command Prompt (cmd.exe):
C:\Users\Your Name>java -version
If Java is installed, you will see something like this (depending on version):
java version "11.0.1" 2018-10-16 LTS
Java(TM) SE Runtime Environment 18.9 (build 11.0.1+13-LTS)
Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.1+13-LTS, mixed mode)
If you do not have Java installed on your computer, you can download it for free at oracle.com.
Note: In this tutorial, we will write Java code in a text editor. However, it is possible to write Java in an Integrated Development Environment, such as IntelliJ IDEA, Netbeans or Eclipse, which are particularly useful when managing larger collections of Java files.
Setup for Windows
To install Java on Windows:
Go to "System Properties" (Can be found on Control Panel > System and Security > System > Advanced System Settings)
Click on the "Environment variables" button under the "Advanced" tab
Then, select the "Path" variable in System variables and click on the "Edit" button
Click on the "New" button and add the path where Java is installed, followed by \bin. By default, Java is installed in C:\Program Files\Java\jdk-11.0.1 (If nothing else was specified when you installed it). In that case, You will have to add a new path with: C:\Program Files\Java\jdk-11.0.1\bin
Then, click "OK", and save the settings
At last, open Command Prompt (cmd.exe) and type java -version to see if Java is running on your machine
Java Quickstart
In Java, every application begins with a class name, and that class must match the filename.
Let's create our first Java file, called Main.java, which can be done in any text editor (like Notepad).
The file should contain a "Hello World" message, which is written with the following code:
Main.java
public class Main {
public static void main(String[] args) {
System.out.println("Hello World");
}
}
Try it Yourself >
Don't worry if you don't understand the code above - we will discuss it in detail in later chapters. For now, focus on how to run the code above.
Save the code in Notepad as "Main.java". Open Command Prompt (cmd.exe), navigate to the directory where you saved your file, and type "javac Main.java":
C:\Users\Your Name>javac Main.java
This will compile your code. If there are no errors in the code, the command prompt will take you to the next line. Now, type "java Main" to run the file:
C:\Users\Your Name>java Main
The output should read:
Hello World
Congratulations! You have written and executed your first Java program.
Try it Yourself >
Java Syntax
Java Syntax
In the previous chapter, we created a Java file called Main.java, and we used the following code to print "Hello World" to the screen:
Main.java
public class Main {
public static void main(String[] args) {
System.out.println("Hello World");
}
}
Example explained
Every line of code that runs in Java must be inside a class. In our example, we named the class Main. A class should always start with an uppercase first letter.
Note: Java is case-sensitive: "MyClass" and "myclass" has different meaning.
The name of the java file must match the class name. When saving the file, save it using the class name and add ".java" to the end of the filename. To run the example above on your computer, make sure that Java is properly installed: Go to the Get Started Chapter for how to install Java. The output should be:
Hello World
The main Method
The main() method is required and you will see it in every Java program:
public static void main(String[] args)
Any code inside the main() method will be executed. You don't have to understand the keywords before and after main. You will get to know them bit by bit while reading this tutorial.
For now, just remember that every Java program has a class name which must match the filename, and that every program must contain the main() method.
System.out.println()
Inside the main() method, we can use the println() method to print a line of text to the screen:
public static void main(String[] args) {
System.out.println("Hello World");
}
Try it Yourself >
Note: The curly braces {} marks the beginning and the end of a block of code.
Note: Each code statement must end with a semicolon.
Test Yourself With Exercises
Exercise:
Insert the missing part of the code below to output "Hello World".
public class MyClass {
public static void main(String[] args) {
.
.
("Hello World");
}
}
Start the Exercise
Java Comments
Java Comments
Comments can be used to explain Java code, and to make it more readable. It can also be used to prevent execution when testing alternative code.
Single-line Comments
Single-line comments start with two forward slashes (//).
Any text between // and the end of the line is ignored by Java (will not be executed).
This example uses a single-line comment before a line of code:
Example
// This is a comment
System.out.println("Hello World");
This example uses a single-line comment at the end of a line of code:
Example
System.out.println("Hello World"); // This is a comment
Try it Yourself >
Java Multi-line Comments
Multi-line comments start with /* and ends with */.
Any text between /* and */ will be ignored by Java.
This example uses a multi-line comment (a comment block) to explain the code:
Example
/* The code below will print the words Hello World
to the screen, and it is amazing */
System.out.println("Hello World");
Single or multi-line comments?
It is up to you which you want to use. Normally, we use // for short comments, and /* */ for longer.
Try it Yourself >
Test Yourself With Exercises
Exercise:
Insert the missing part to create two types of comments.
This is a single-line comment
This is a multi-line comment
Java Variables
Java Variables
Variables are containers for storing data values.
In Java, there are different types of variables, for example:
String - stores text, such as "Hello". String values are surrounded by double quotes
int - stores integers (whole numbers), without decimals, such as 123 or -123
float - stores floating point numbers, with decimals, such as 19.99 or -19.99
char - stores single characters, such as 'a' or 'B'. Char values are surrounded by single quotes
boolean - stores values with two states: true or false
Declaring (Creating) Variables
To create a variable, you must specify the type and assign it a value:
Syntax
type variableName = value;
Where type is one of Java's types (such as int or String), and variableName is the name of the variable (such as x or name). The equal sign is used to assign values to the variable.
To create a variable that should store text, look at the following example:
Example
Create a variable called name of type String and assign it the value "John":
String name = "John";
System.out.println(name);
To create a variable that should store a number, look at the following example:
Try it Yourself >
Example
Create a variable called myNum of type int and assign it the value 15:
int myNum = 15;
System.out.println(myNum);
Try it Yourself >
You can also declare a variable without assigning the value, and assign the value later:
Example
int myNum;
myNum = 15;
System.out.println(myNum);
Try it Yourself >
Note that if you assign a new value to an existing variable, it will overwrite the previous value:
Example
Change the value of myNum from 15 to 20:
int myNum = 15;
myNum = 20; // myNum is now 20
System.out.println(myNum);
Try it Yourself >
Final Variables
However, you can add the final keyword if you don't want others (or yourself) to overwrite existing values (this will declare the variable as "final" or "constant", which means unchangeable and read-only):
Example
final int myNum = 15;
myNum = 20; // will generate an error: cannot assign a value to a final variable
Try it Yourself >
Other Types
A demonstration of how to declare variables of other types:
Example
int myNum = 5;
float myFloatNum = 5.99f;
char myLetter = 'D';
boolean myBool = true;
String myText = "Hello";
You will learn more about data types in the next chapter.
Display Variables
The println() method is often used to display variables.
To combine both text and a variable, use the + character:
Example
String name = "John";
System.out.println("Hello " + name);
You can also use the + character to add a variable to another variable:
Try it Yourself >
Example
String firstName = "John ";
String lastName = "Doe";
String fullName = firstName + lastName;
System.out.println(fullName);
Try it Yourself >
For numeric values, the + character works as a mathematical operator (notice that we use int (integer) variables here):
Example
int x = 5;
int y = 6;
System.out.println(x + y); // Print the value of x + y
Try it Yourself >
From the example above, you can expect:
x stores the value 5
y stores the value 6
Then we use the println() method to display the value of x + y, which is 11
Declare Many Variables
To declare more than one variable of the same type, use a comma-separated list:
Example
int x = 5, y = 6, z = 50;
System.out.println(x + y + z);
Try it Yourself >
Java Identifiers
All Java variables must be identified with unique names.
These unique names are called identifiers.
Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).
Note: It is recommended to use descriptive names in order to create understandable and maintainable code:
Example
// Good
int minutesPerHour = 60;
// OK, but not so easy to understand what m actually is
int m = 60;
Try it Yourself >
The general rules for naming variables are:
Names can contain letters, digits, underscores, and dollar signs
Names must begin with a letter
Names should start with a lowercase letter and it cannot contain whitespace
Names can also begin with $ and _ (but we will not use it in this tutorial)
Names are case sensitive ("myVar" and "myvar" are different variables)
Reserved words (like Java keywords, such as int or boolean) cannot be used as names
Test Yourself With Exercises
Exercise:
Create a variable named carName and assign the value Volvo to it.
=
;
Start the Exercise
Java Data Types
Java Data Types
As explained in the previous chapter, a variable in Java must be a specified data type:
Example
int myNum = 5; // Integer (whole number)
float myFloatNum = 5.99f; // Floating point number
char myLetter = 'D'; // Character
boolean myBool = true; // Boolean
String myText = "Hello"; // String
Data types are divided into two groups:
Primitive data types - includes byte, short, int, long, float, double, boolean and char
Non-primitive data types - such as String, Arrays and Classes (you will learn more about these in a later chapter)
Primitive Data Types
A primitive data type specifies the size and type of variable values, and it has no additional methods.
There are eight primitive data types in Java:
Data Type Size Description
byte 1 byte Stores whole numbers from -128 to 127
short 2 bytes Stores whole numbers from -32,768 to 32,767
int 4 bytes Stores whole numbers from -2,147,483,648 to 2,147,483,647
long 8 bytes Stores whole numbers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
float 4 bytes Stores fractional numbers. Sufficient for storing 6 to 7 decimal digits
double 8 bytes Stores fractional numbers. Sufficient for storing 15 decimal digits
boolean 1 bit Stores true or false values
char 2 bytes Stores a single character/letter or ASCII values
Numbers
Primitive number types are divided into two groups:
Integer types stores whole numbers, positive or negative (such as 123 or -456), without decimals. Valid types are byte, short, int and long. Which type you should use, depends on the numeric value.
Floating point types represents numbers with a fractional part, containing one or more decimals. There are two types: float and double.
Even though there are many numeric types in Java, the most used for numbers are int (for whole numbers) and double (for floating point numbers). However, we will describe them all as you continue to read.
Integer Types
Byte
The byte data type can store whole numbers from -128 to 127. This can be used instead of int or other integer types to save memory when you are certain that the value will be within -128 and 127:
Example
byte myNum = 100;
System.out.println(myNum);
Try it Yourself >
Short
The short data type can store whole numbers from -32768 to 32767:
Example
short myNum = 5000;
System.out.println(myNum);
Try it Yourself >
Int
The int data type can store whole numbers from -2147483648 to 2147483647. In general, and in our tutorial, the int data type is the preferred data type when we create variables with a numeric value.
Example
int myNum = 100000;
System.out.println(myNum);
Try it Yourself >
Long
The long data type can store whole numbers from -9223372036854775808 to 9223372036854775807. This is used when int is not large enough to store the value. Note that you should end the value with an "L":
Example
long myNum = 15000000000L;
System.out.println(myNum);
Try it Yourself >
Floating Point Types
You should use a floating point type whenever you need a number with a decimal, such as 9.99 or 3.14515.
Float
The float data type can store fractional numbers from 3.4e−038 to 3.4e+038. Note that you should end the value with an "f":
Example
float myNum = 5.75f;
System.out.println(myNum);
Try it Yourself >
Double
The double data type can store fractional numbers from 1.7e−308 to 1.7e+308. Note that you should end the value with a "d":
Example
double myNum = 19.99d;
System.out.println(myNum);
Try it Yourself >
Use float or double?
The precision of a floating point value indicates how many digits the value can have after the decimal point. The precision of float is only six or seven decimal digits, while double variables have a precision of about 15 digits. Therefore it is safer to use double for most calculations.
Scientific Numbers
A floating point number can also be a scientific number with an "e" to indicate the power of 10:
Example
float f1 = 35e3f;
double d1 = 12E4d;
System.out.println(f1);
System.out.println(d1);
Try it Yourself >
Booleans
A boolean data type is declared with the boolean keyword and can only take the values true or false:
Example
boolean isJavaFun = true;
boolean isFishTasty = false;
System.out.println(isJavaFun); // Outputs true
System.out.println(isFishTasty); // Outputs false
Try it Yourself >
Boolean values are mostly used for conditional testing, which you will learn more about in a later chapter.
Characters
The char data type is used to store a single character. The character must be surrounded by single quotes, like 'A' or 'c':
Example
char myGrade = 'B';
System.out.println(myGrade);
Try it Yourself >
Alternatively, you can use ASCII values to display certain characters:
Example
char myVar1 = 65, myVar2 = 66, myVar3 = 67;
System.out.println(myVar1);
System.out.println(myVar2);
System.out.println(myVar3);
Try it Yourself >
Tip: A list of all ASCII values can be found in our ASCII Table Reference.
Strings
The String data type is used to store a sequence of characters (text). String values must be surrounded by double quotes:
Example
String greeting = "Hello World";
System.out.println(greeting);
Try it Yourself >
The String type is so much used and integrated in Java, that some call it "the special ninth type".
A String in Java is actually a non-primitive data type, because it refers to an object. The String object has methods that are used to perform certain operations on strings. Don't worry if you don't understand the term "object" just yet. We will learn more about strings and objects in a later chapter.
Non-Primitive Data Types
Non-primitive data types are called reference types because they refer to objects.
The main difference between primitive and non-primitive data types are:
Primitive types are predefined (already defined) in Java. Non-primitive types are created by the programmer and is not defined by Java (except for String).
Non-primitive types can be used to call methods to perform certain operations, while primitive types cannot.
A primitive type has always a value, while non-primitive types can be null.
A primitive type starts with a lowercase letter, while non-primitive types starts with an uppercase letter.
The size of a primitive type depends on the data type, while non-primitive types have all the same size.
Examples of non-primitive types are Strings, Arrays, Classes, Interface, etc. You will learn more about these in a later chapter.
Java Type Casting
Java Type Casting
Type casting is when you assign a value of one primitive data type to another type.
In Java, there are two types of casting:
Widening Casting (automatically) - converting a smaller type to a larger type size
byte -> short -> char -> int -> long -> float -> double
Narrowing Casting (manually) - converting a larger type to a smaller size type
double -> float -> long -> int -> char -> short -> byte
Widening Casting
Widening casting is done automatically when passing a smaller size type to a larger size type:
Example
public class Main {
public static void main(String[] args) {
int myInt = 9;
double myDouble = myInt; // Automatic casting: int to double
System.out.println(myInt); // Outputs 9
System.out.println(myDouble); // Outputs 9.0
}
}
Try it Yourself >
Narrowing Casting
Narrowing casting must be done manually by placing the type in parentheses in front of the value:
Example
public class Main {
public static void main(String[] args) {
double myDouble = 9.78d;
int myInt = (int) myDouble; // Manual casting: double to int
System.out.println(myDouble); // Outputs 9.78
System.out.println(myInt); // Outputs 9
}
}
Try it Yourself >
Java Operators
Operators are used to perform operations on variables and values.
In the example below, we use the + operator to add together two values:
Example
int x = 100 + 50;
Although the + operator is often used to add together two values, like in the example above, it can also be used to add together a variable and a value, or a variable and another variable:
Example
int sum1 = 100 + 50; // 150 (100 + 50)
int sum2 = sum1 + 250; // 400 (150 + 250)
int sum3 = sum2 + sum2; // 800 (400 + 400)
Java divides the operators into the following groups:
Arithmetic operators
Assignment operators
Comparison operators
Logical operators
Bitwise operators
Arithmetic Operators
Arithmetic operators are used to perform common mathematical operations.
Operator Name Description Example Try it
+ Addition Adds together two values x + y Try it >
- Subtraction Subtracts one value from another x - y Try it >
* Multiplication Multiplies two values x * y Try it >
/ Division Divides one value by another x / y Try it >
% Modulus Returns the division remainder x % y Try it >
++ Increment Increases the value of a variable by 1 ++x Try it >
-- Decrement Decreases the value of a variable by 1 --x Try it >
Java Assignment Operators
Assignment operators are used to assign values to variables.
In the example below, we use the assignment operator (=) to assign the value 10 to a variable called x:
Example
int x = 10;
The addition assignment operator (+=) adds a value to a variable:
Example
int x = 10;
x += 5;
A list of all assignment operators:
Operator Example Same As Try it
= x = 5 x = 5 Try it >
+= x += 3 x = x + 3 Try it >
-= x -= 3 x = x - 3 Try it >
*= x *= 3 x = x * 3 Try it >
/= x /= 3 x = x / 3 Try it >
%= x %= 3 x = x % 3 Try it >
&= x &= 3 x = x & 3 Try it >
|= x |= 3 x = x | 3 Try it >
^= x ^= 3 x = x ^ 3 Try it >
>>= x >>= 3 x = x >> 3 Try it >
<<= x <<= 3 x = x << 3 Try it >
Java Comparison Operators
Comparison operators are used to compare two values:
Operator Name Example Try it
== Equal to x == y && Logical and Returns true if both statements are true x < 5 && x < 10 Try it >
!= Not equal x != y Try it >
> Greater than x > y Try it >
< Less than x < y Try it >
>= Greater than or equal to x >= y Try it >
<= Less than or equal to x <= y Try it >
Java Logical Operators
Logical operators are used to determine the logic between variables or values:
Operator Name Description Example Try it
&& Logical and Returns true if both statements are true x < 5 && x < 10 Try it >
|| Logical or Returns true if one of the statements is true x < 5 || x < 4
&& Logical and Returns true if both statements are true x < 5 && x < 10 Try it >
! Logical not Reverse the result, returns false if the result is true !(x < 5 && x < 10)
&& Logical and Returns true if both statements are true x < 5 && x < 10 Try it >
Java Strings
Strings are used for storing text.
A String variable contains a collection of characters surrounded by double quotes:
Example
Create a variable of type String and assign it a value:
String greeting = "Hello";
String Length
A String in Java is actually an object, which contain methods that can perform certain operations on strings. For example, the length of a string can be found with the length() method:
Example
String txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
System.out.println("The length of the txt string is: " + txt.length());
More String Methods
There are many string methods available, for example toUpperCase() and toLowerCase():
Example
String txt = "Hello World";
System.out.println(txt.toUpperCase()); // Outputs "HELLO WORLD"
System.out.println(txt.toLowerCase()); // Outputs "hello world"
Finding a Character in a String
The indexOf() method returns the index (the position) of the first occurrence of a specified text in a string (including whitespace):
Example
String txt = "Please locate where 'locate' occurs!";
System.out.println(txt.indexOf("locate")); // Outputs 7
Java counts positions from zero.
0 is the first position in a string, 1 is the second, 2 is the third ...
String Concatenation
The + operator can be used between strings to combine them. This is called concatenation:
Example
String firstName = "John";
String lastName = "Doe";
System.out.println(firstName + " " + lastName);
Note that we have added an empty text (" ") to create a space between firstName and lastName on print.
You can also use the concat() method to concatenate two strings:
Example
String firstName = "John ";
String lastName = "Doe";
System.out.println(firstName.concat(lastName));
Special Characters
Because strings must be written within quotes, Java will misunderstand this string, and generate an error:
String txt = "We are the so-called "Vikings" from the north.";
The solution to avoid this problem, is to use the backslash escape character.
The backslash (\) escape character turns special characters into string characters:
Escape character Result Description
\' ' Single quote
\" " Double quote
\\ \ Backslash
The sequence \" inserts a double quote in a string:
Example
String txt = "We are the so-called \"Vikings\" from the north.";
The sequence \' inserts a single quote in a string:
Example
String txt = "It\'s alright.";
The sequence \\ inserts a single backslash in a string:
Example
String txt = "The character \\ is called backslash.";
Six other escape sequences are valid in Java:
Code Result Try it
\n New Line Try it >
\r Carriage Return Try it >
\t Tab Try it >
\b Backspace Try it >
\f Form Feed Try it >
Adding Numbers and Strings
WARNING!
Java uses the + operator for both addition and concatenation.
Numbers are added. Strings are concatenated.
If you add two numbers, the result will be a number:
Example
int x = 10;
int y = 20;
int z = x + y; // z will be 30 (an integer/number)
Try it >
If you add two strings, the result will be a string concatenation:
Example
String x = "10";
String y = "20";
String z = x + y; // z will be 1020 (a String)
Try it >
If you add a number and a string, the result will be a string concatenation:
Example
String x = "10";
int y = 20;
String z = x + y; // z will be 1020 (a String)
Try it >
Complete String Reference
For a complete reference of String methods, go to our Java String Methods Reference.
The reference contains descriptions and examples of all string methods.
Java Math
The Java Math class has many methods that allows you to perform mathematical tasks on numbers.
Math.max(x,y)
The Math.max(x,y) method can be used to find the highest value of x and y:
Example
Math.max(5, 10);
Math.min(x,y)
The Math.min(x,y) method can be used to find the lowest value of x and y:
Example
Math.min(5, 10);
Try it >
Math.sqrt(x)
The Math.sqrt(x) method returns the square root of x:
Example
Math.sqrt(64);
Try it >
Math.abs(x)
The Math.abs(x) method returns the absolute (positive) value of x:
Example
Math.abs(-4.7);
Try it >
Random Numbers
Math.random() returns a random number between 0.0 (inclusive), and 1.0 (exclusive):
Example
Math.random();
Try it >
To get more control over the random number, e.g. you only want a random number between 0 and 100, you can use the following formula:
Example
int randomNum = (int)(Math.random() * 101); // 0 to 100
Try it >
Complete Math Reference
For a complete reference of Math methods, go to our Java Math Methods Reference.
Java Booleans
Very often, in programming, you will need a data type that can only have one of two values, like:
YES / NO
ON / OFF
TRUE / FALSE
For this, Java has a boolean data type, which can take the values true or false.
Boolean Values
A boolean type is declared with the boolean keyword and can only take the values true or false:
Example
boolean isJavaFun = true;
boolean isFishTasty = false;
System.out.println(isJavaFun); // Outputs true
System.out.println(isFishTasty); // Outputs false
However, it is more common to return boolean values from boolean expressions, for conditional testing (see below).
Boolean Expression
A Boolean expression is a Java expression that returns a Boolean value: true or false.
You can use a comparison operator, such as the greater than (>) operator to find out if an expression (or a variable) is true:
Example
int x = 10;
int y = 9;
System.out.println(x > y); // returns true, because 10 is higher than 9
Try it >
Or even easier:
Example
System.out.println(10 > 9); // returns true, because 10 is higher than 9
Try it >
In the examples below, we use the equal to (==) operator to evaluate an expression:
Example
int x = 10;
System.out.println(x == 10); // returns true, because the value of x is equal to 10
Try it >
Example
System.out.println(10 == 15); // returns false, because 10 is not equal to 15
Try it >
The Boolean value of an expression is the basis for all Java comparisons and conditions.
You will learn more about conditions in the next chapter.
Java If ... Else
Java Conditions and If Statements
Java supports the usual logical conditions from mathematics:
Less than: a < b
Less than or equal to: a <= b
Greater than: a > b
Greater than or equal to: a >= b
Equal to a == b
Not Equal to: a != b
You can use these conditions to perform different actions for different decisions.
Java has the following conditional statements:
Use if to specify a block of code to be executed, if a specified condition is true
Use else to specify a block of code to be executed, if the same condition is false
Use else if to specify a new condition to test, if the first condition is false
Use switch to specify many alternative blocks of code to be executed
The if Statement
Use the if statement to specify a block of Java code to be executed if a condition is true.
Syntax
if (condition) {
// block of code to be executed if the condition is true
}
Note that if is in lowercase letters. Uppercase letters (If or IF) will generate an error.
In the example below, we test two values to find out if 20 is greater than 18. If the condition is true, print some text:
Example
if (20 > 18) {
System.out.println("20 is greater than 18");
}
We can also test variables:
Example
int x = 20;
int y = 18;
if (x > y) {
System.out.println("x is greater than y");
}
Example explained
In the example above we use two variables, x and y, to test whether x is greater than y (using the > operator). As x is 20, and y is 18, and we know that 20 is greater than 18, we print to the screen that "x is greater than y".
The else Statement
Use the else statement to specify a block of code to be executed if the condition is false.
Syntax
if (condition) {
// block of code to be executed if the condition is true
} else {
// block of code to be executed if the condition is false
}
Example
int time = 20;
if (time < 18) {
System.out.println("Good day.");
} else {
System.out.println("Good evening.");
}
// Outputs "Good evening."
Example explained
In the example above, time (20) is greater than 18, so the condition is false. Because of this, we move on to the else condition and print to the screen "Good evening". If the time was less than 18, the program would print "Good day".
The else if Statement
Use the else if statement to specify a new condition if the first condition is false.
Syntax
if (condition1) {
// block of code to be executed if condition1 is true
} else if (condition2) {
// block of code to be executed if the condition1 is false and condition2 is true
} else {
// block of code to be executed if the condition1 is false and condition2 is false
}
Example
int time = 22;
if (time < 10) {
System.out.println("Good morning.");
} else if (time < 20) {
System.out.println("Good day.");
} else {
System.out.println("Good evening.");
}
// Outputs "Good evening."
Try it >
Example explained
In the example above, time (22) is greater than 10, so the first condition is false. The next condition, in the else if statement, is also false, so we move on to the else condition since condition1 and condition2 is both false - and print to the screen "Good evening".
However, if the time was 14, our program would print "Good day."
Short Hand If...Else (Ternary Operator)
There is also a short-hand if else, which is known as the ternary operator because it consists of three operands. It can be used to replace multiple lines of code with a single line. It is often used to replace simple if else statements:
Syntax
variable = (condition) ? expressionTrue : expressionFalse;
Instead of writing:
Example
int time = 20;
if (time < 18) {
System.out.println("Good day.");
} else {
System.out.println("Good evening.");
}
Try it >
You can simply write:
Example
int time = 20;
String result = (time < 18) ? "Good day." : "Good evening.";
System.out.println(result);
Try it >
Java Switch
Java Switch Statements
Use the switch statement to select one of many code blocks to be executed.
Syntax
switch(expression) {
case x:
// code block
break;
case y:
// code block
break;
default:
// code block
}
Try it >
This is how it works:
The switch expression is evaluated once.
The value of the expression is compared with the values of each case.
If there is a match, the associated block of code is executed.
The break and default keywords are optional, and will be described later in this chapter
The example below uses the weekday number to calculate the weekday name:
Example
int day = 4;
switch (day) {
case 1:
System.out.println("Monday");
break;
case 2:
System.out.println("Tuesday");
break;
case 3:
System.out.println("Wednesday");
break;
case 4:
System.out.println("Thursday");
break;
case 5:
System.out.println("Friday");
break;
case 6:
System.out.println("Saturday");
break;
case 7:
System.out.println("Sunday");
break;
}
// Outputs "Thursday" (day 4)
Try it >
The break Keyword
When Java reaches a break keyword, it breaks out of the switch block.
This will stop the execution of more code and case testing inside the block.
When a match is found, and the job is done, it's time for a break. There is no need for more testing.
A break can save a lot of execution time because it "ignores" the execution of all the rest of the code in the switch block.
The default Keyword
The default keyword specifies some code to run if there is no case match:
Example
int day = 4;
switch (day) {
case 6:
System.out.println("Today is Saturday");
break;
case 7:
System.out.println("Today is Sunday");
break;
default:
System.out.println("Looking forward to the Weekend");
}
// Outputs "Looking forward to the Weekend"
Try it >
Note that if the default statement is used as the last statement in a switch block, it does not need a break.
Java While Loop
Loops
Loops can execute a block of code as long as a specified condition is reached.
Loops are handy because they save time, reduce errors, and they make code more readable.
Java While Loop
The while loop loops through a block of code as long as a specified condition is true:
Syntax
while (condition) {
// code block to be executed
}
In the example below, the code in the loop will run, over and over again, as long as a variable (i) is less than 5:
Example
int i = 0;
while (i < 5) {
System.out.println(i);
i++;
}
Try it >
Note: Do not forget to increase the variable used in the condition, otherwise the loop will never end!
The Do/While Loop
The do/while loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true.
Syntax
do {
// code block to be executed
}
while (condition);
The example below uses a do/while loop. The loop will always be executed at least once, even if the condition is false, because the code block is executed before the condition is tested:
Example
int i = 0;
do {
System.out.println(i);
i++;
}
while (i < 5);
Try it >
Do not forget to increase the variable used in the condition, otherwise the loop will never end!
Java For Loop
Java For Loop
When you know exactly how many times you want to loop through a block of code, use the for loop instead of a while loop:
Syntax
for (statement 1; statement 2; statement 3) {
// code block to be executed
}
Statement 1 is executed (one time) before the execution of the code block.
Statement 2 defines the condition for executing the code block.
Statement 3 is executed (every time) after the code block has been executed.
The example below will print the numbers 0 to 4:
Example
for (int i = 0; i < 5; i++) {
System.out.println(i);
}
Try it >
Example explained
Statement 1 sets a variable before the loop starts (int i = 0).
Statement 2 defines the condition for the loop to run (i must be less than 5). If the condition is true, the loop will start over again, if it is false, the loop will end.
Statement 3 increases a value (i++) each time the code block in the loop has been executed.
Another Example
This example will only print even values between 0 and 10:
Example
for (int i = 0; i <= 10; i = i + 2) {
System.out.println(i);
}
Try it >
For-Each Loop
There is also a "for-each" loop, which is used exclusively to loop through elements in an array:
Syntax
for (type variableName : arrayName) {
// code block to be executed
}
The following example outputs all elements in the cars array, using a "for-each" loop:
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
System.out.println(i);
}
Try it >
Note: Don't worry if you don't understand the example above. You will learn more about Arrays in the Java Arrays chapter.
Java Break and Continue
Java Break
You have already seen the break statement used in an earlier chapter of this tutorial. It was used to "jump out" of a switch statement.
The break statement can also be used to jump out of a loop.
This example stops the loop when i is equal to 4:
Example
for (int i = 0; i < 10; i++) {
if (i == 4) {
break;
}
System.out.println(i);
}
Try it >
Java Continue
The continue statement breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.
This example skips the value of 4:
Example
for (int i = 0; i < 10; i++) {
if (i == 4) {
continue;
}
System.out.println(i);
}
Try it >
Break and Continue in While Loop
You can also use break and continue in while loops:
Break Example
int i = 0;
while (i < 10) {
System.out.println(i);
i++;
if (i == 4) {
break;
}
}
Try it >
Java Arrays
Java Arrays
Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.
To declare an array, define the variable type with square brackets:
String[] cars;
We have now declared a variable that holds an array of strings. To insert values to it, we can use an array literal - place the values in a comma-separated list, inside curly braces:
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
To create an array of integers, you could write:
int[] myNum = {10, 20, 30, 40};
Access the Elements of an Array
You access an array element by referring to the index number.
This statement accesses the value of the first element in cars:
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
System.out.println(cars[0]);
// Outputs Volvo
Try it >
Note: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.
Change an Array Element
To change the value of a specific element, refer to the index number:
Example
cars[0] = "Opel";
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
cars[0] = "Opel";
System.out.println(cars[0]);
// Now outputs Opel instead of Volvo
Try it >
Array Length
To find out how many elements an array has, use the length property:
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
System.out.println(cars.length);
// Outputs 4
Try it >
Loop Through an Array
You can loop through the array elements with the for loop, and use the length property to specify how many times the loop should run.
The following example outputs all elements in the cars array:
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (int i = 0; i < cars.length; i++) {
System.out.println(cars[i]);
}
Try it >
Loop Through an Array with For-Each
There is also a "for-each" loop, which is used exclusively to loop through elements in arrays:
Syntax
for (type variable : arrayname) {
...
}
The following example outputs all elements in the cars array, using a "for-each" loop:
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
System.out.println(i);
}
Try it >
The example above can be read like this: for each String element (called i - as in index) in cars, print out the value of i.
If you compare the for loop and for-each loop, you will see that the for-each method is easier to write, it does not require a counter (using the length property), and it is more readable.
Multidimensional Arrays
A multidimensional array is an array of arrays.
To create a two-dimensional array, add each array within its own set of curly braces:
Example
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
myNumbers is now an array with two arrays as its elements.
Try it >
To access the elements of the myNumbers array, specify two indexes: one for the array, and one for the element inside that array. This example accesses the third element (2) in the second array (1) of myNumbers:
Example
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
int x = myNumbers[1][2];
System.out.println(x); // Outputs 7
Try it >
We can also use a for loop inside another for loop to get the elements of a two-dimensional array (we still have to point to the two indexes):
