11 - Enumerations in Java/001 Enumerations in Java_en.srt

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Hello team, In this lesson we 
will learn what enumerations are and 

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how we can use them in 
Java. We are also going to 

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discuss how to declare enum and 
how we can compare them. I 

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will show you how enums can 
have properties and custom methods. But 

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first of all – what is 
Enum type? An enum type is 

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a special data type that enables 
for a variable to be a 

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set of predefined constants. The variable 
of enum type always must be 

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equal to one of the values 
that have been predefined for it. 

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When do we need enum types? 
We need enum types to express 

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some set of values that are 
not infinite. You need to use 

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enum for data sets where you 
know all possible values at compile 

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time. For example, seasons of the 
year – WINTER, SPRING, SUMMER, FALL. 

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There are very low chances that 
you will need to add additional 

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seasons in your program because no 
other exists. Or let’s review real 

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life scenario: together with students we 
implemented simple truth or dare game 

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for android. There are thousands of 
cards in the game. Each card 

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has its type and it can 
be either TRUTH or DARE. So, 

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in our program CardType will be 
enumeration. Does it make sense? To 

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make it clearer - let’s take 
a look at examples. To create 

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Enum make mouse right click on 
the package and select Enum. Here 

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you can specify Enum name and 
package. I’ve already created Enums to 

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save our time during the lesson. 
Here I have ‘Priority’ enum type. 

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In my program I’m going to 
have only three types of priority, 

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they are: high, medium, and low. 
I want to draw your attention 

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to the code style. Taking 
into account that all enum values 

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are constant we use capital letters 
according to java naming convention. Let’s 

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take a look how I use 
this enum. I can declare variable 

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of type Priority. As I mentioned 
before - the variable of enum 

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type always must be equal to 
one of the values that have 

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been predefined for it. You remember 
that one of the data types 

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that can be used in switch 
statement – is enum type. And 

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here is how you can use 
it. Based on the value of 

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‘priority’ variable during the runtime, different 
case block will be executed. In 

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this particular case when the value 
of our priority variable is HIGH 

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– this block will be executed. 
So now you know how to 

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declare custom enum type and how 
to create variable of enum type 

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to use it in the program. 
Let’s move on. One more option 

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to initialize enum variable. Let’s pretend 
you are dealing with user input 

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and you need to create enum 
based on the user input. You 

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can use ‘valueOf’ method to initialize 
enum variable. Just use enum type 

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name, call valueOf method and pass 
string to this method. In case 

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there are no enum value for 
the string you entered – IllegalArgumentException 

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will be thrown. You see when 
I pass ‘high’ in lowercase exception 

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is thrown. Take this into consideration 
while doing your homework. To compare 

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enumerations, you can use ‘equals to’ 
operator. And again, you remember that 

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Enumeration is not primitive type of 
data. Thus, you might think that 

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you should use ‘equals’ method to 
compare enums. Not necessarily. For example, 

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in this case we will get 
false as expected. And in this 

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case, we will get true as 
expected. Why? Because each enum variable 

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has its own ordinal. Take a 
look here one more time: you 

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have set of values which are 
known during the compilation. And they 

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are listed in the strict sequence. 
The HIGH has zero ordinal, the 

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MEDIUM - one and the LOW 
- two. Taking into account each 

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enum value has its own ordinal 
we can easily compare them via 

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‘equals to’ operator. You can use 
‘ordinal()’ method to get ordinal of 

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each enum value. In some cases 
you might want to iterate over 

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all values from specific enum type. 
Just be aware that each enum 

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type has ‘values()’ method that returns 
array of enum values. You can 

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iterate over this array as you 
wish. You already know all 

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iteration statements in Java. And now 
probably the most interesting part. Each 

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enum value may have its own 
properties and methods. Let’s take a 

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look at this ‘Month’ enum type. 
We have next values: January, February, 

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March and so on. The list 
of constants is always terminated by 

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a semicolon. Definition of properties and 
methods always should go after the 

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list of constants. And here I 
declared a property – daysAmount. Each 

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of these months should have this 
property of int type. After that 

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I declare special method to create 
each enum. This special method is 

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called ‘constructor’. It is used to 
construct objects. Very soon we will 

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get to the object-oriented programming and 
I will explain you in detail 

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what constructor is. Also ignore this 
‘private’ modifiers as of now, we 

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will discuss them during the OOP 
topic when we will talk about 

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encapsulation. Here you can see that 
I can pass int value to 

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constructor, and after I passed this 
value, I initialize ‘daysAmount’ property for 

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each enum. Here in parentheses I 
call constructor and pass int. Similar 

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to the method call, I call 
constructor and pass value to it. 

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So, for January, I pass thirty-one 
and daysAmount is initialized with thirty-one. 

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And you see ‘this’ keyword. This 
is new to you. That means 

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for THIS enum, in this specific 
case for JANUARY, the daysAmount will 

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be equal to thirty-one. And you 
see that local variable name is 

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also ‘daysAmount’. But ‘this.daysAmount’ and method 
argument ‘daysAmount’ are different variables. Because 

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‘this.daysAmount’ is this variable and ‘daysAmount’ 
is just this local variable.  

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For February I 
pass twenty-eight, and daysAmount is initialized 

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with twenty-eight value. For THIS enum, 
days amount will be twenty-eight. Then, 

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I can declare that each enum 
has its own behavior. Namely: each 

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month can tell me how many 
days it has. ‘this’ keyword means 

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THIS particular month. It can be 
January, February, March or any other. 

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Let’s take a look how we 
can use properties and methods of 

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Enum constants. We can call method 
getDaysAmount for January enum variable. And 

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for THIS month, for January, this 
is thirty-one. Hope it is clear 

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now. That’s it. Let’s take a 
look at what we have learned 

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from this lesson. From this lesson 
we learned what enum types are. 

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We can initialize enum variables, know how 
to compare them. Also we learned how 

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to iterate over all constants of 
specific enum type and learned that 

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enum type can have custom properties and 
methods. Now you know everything you need 

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to know to complete your homework. 
Let’s take a look at the 

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task. You need to implement simple console 
application with two enums. Try to implement 

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it by yourself and after that 
take a look at my solution. 

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And as always – you are welcome 
to ask questions in case you have 

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any. Thank you for your attention. 
See you in the next lesson.