5f228f9b44
* Added rules Rules For naming variables added and checked the document * fixed: made resources header smaller one size
83 lines
4.5 KiB
Markdown
83 lines
4.5 KiB
Markdown
---
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title: Variables
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---
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# Variables
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Variables store values. They are the most basic entity used to store data such as text, numbers, etc. in a program.
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These are case sensitive ,i.e, small letter nd capital letter must be same as when declared.
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In <a href='https://github.com/FreeCodeCamp/FreeCodeCamp/wiki/Java' target='_blank' rel='nofollow'>Java</a>, variables are <a href='https://en.wikipedia.org/wiki/Strong_and_weak_typing#Definitions_of_.22strong.22_or_.22weak.22' target='_blank' rel='nofollow'>_strongly typed_</a>, which means you have to define the type for each variable whenever you declare it. Otherwise, the compiler will throw an error at <a href='https://en.wikipedia.org/wiki/Compile_time' target='_blank' rel='nofollow'>compile time</a>. Therefore, each variable has an associated '<a href='https://guide.freecodecamp.org/java/data-types' target='_blank' rel='nofollow'>data-type</a>' of one of the following:
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* Primitive Type: `int`, `short`, `char`, `long`, `boolean`, `byte`, `float`, `double`
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* Wrapper Type: `Integer`, `Short`, `Char`, `Long`, `Boolean`, `Byte`, `Float`, `Double`
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* Reference Type: `String`, `StringBuilder`, `Calendar`, `ArrayList`, etc.
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You may have noticed that the **Wrapper Type** consists of types spelled exactly like the **Primitive Type**, except for the capitalised alphabet in the begining (like the **Reference Type**). This is because the Wrapper Types are actually a part of the more general Reference Types, but <i>closely linked</i> with their primitive counterparts via <a href='https://docs.oracle.com/javase/tutorial/java/data/autoboxing.html' target='_blank' rel='nofollow'>autoboxing and unboxing</a>. For now, you just need to know that such a 'Wrapper Type' exists.
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Typically, you can <i>declare</i> (i.e., create) variables as per the following syntax: <<i>data-type</i>> <<i>variableName</i>>;
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```java
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// Primitive Data Type
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int i;
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// Reference Data Type
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Float myFloat;
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```
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You can <i>assign</i> a value to the variable either simultaneously when you are declaring it (which is called <i>initialisation</i>), or anywhere in the code after you have declared it. The symbol **=** is used for the same.
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```java
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// Initialise the variable of Primitive Data Type 'int' to store the value 10
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int i = 10;
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double amount = 10.0;
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boolean isOpen = false;
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char c = 'a'; // Note the single quotes
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String s = "Hello World"; // String is created with double quotes
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//Variables can also be declared in one statement, and assigned values later.
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int j;
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j = 10;
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// initiates an Float object with value 1.0
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// variable myFloat now points to the object
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Float myFloat = new Float(1.0);
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//Bytes are one of the data types in Java and can be
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//represented with this code
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int byteValue = 0B101;
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byte anotherByte = (byte)0b00100001;
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```
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## Variable Data Types
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As evident from the above example, variables of Primitive type behave slightly differently from variables of Reference (& Wrapper) type - while Primitive variables <i>store</i> the actual value, Reference variables <i>refer to</i> an 'object' containing the actual value.
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Java Programming language defines mainly three kind of variables.
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1. Local Variable:
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It is declared inside the method and can only be used within the method itself.
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2. Static variable:
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Any variable which has the keyword static is called a static variable .
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It cannot be created as a local variable. Once created , it can be used among all the instances of the class.
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3. Instance Variable:
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It is declared inside a class but not in a specific method body.
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You can find out more in the sections linked below.
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## Rules for naming Variable
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-> All variable names must begin with a letter of the alphabet, an underscore, or ( _ ), or a dollar sign ($). The convention is to always use a letter of the alphabet. The dollar sign and the underscore are discouraged.
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-> After the first initial letter, variable names may also contain letters and the digits 0 to 9. No spaces or special characters are allowed.
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-> The name can be of any length, but don't get carried away. Remember that you will have to type this name.
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-> Uppercase characters are distinct from lowercase characters. Using ALL uppercase letters are primarily used to identify constant variables. Remember that variable names are case-sensitive.
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-> You cannot use a java keyword (reserved word) for a variable name.
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## Other Resources
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* <a href='https://guide.freecodecamp.org/java/data-types' target='_blank' rel='nofollow'>Data Types</a>
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* <a href='https://guide.freecodecamp.org/java/classes-and-objects' target='_blank' rel='nofollow'>Classes and Objects</a>
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