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Learning Java Programming: Variables and Data Types

Learning Java Programming: Understanding Variables and Data Types
A variable is a place to store temporary values.
That is all.
Serious?
Yeah seriously, if you want a longer one ...
... let's see the meaning from Wikipedia:
Variable: (Lat) 1. changeable, not fixed; 2. declaration of something that has a variety of values ​​3. different in programming languages ​​also called symbols that represent certain values, variables known in sub-programs are called local variables. while what is generally known in one program is called a global variable.
Well, do you understand?
If not, understand enough: Variables as a place to store temporary values.
Then, what is the data type?
Data type is a type of data stored in a variable.

Types of Data Types

The following are various types of data in Java:
  • char: Character data type, example Z
  • int: numbers or integers, for example 29
  • float: decimal number, for example 2.1
  • double: decimal number too, but more capacity, for example 2.1
  • String: a collection of characters that make up text, for example Hello World!
  • boolean: data types that are only valuable trueandfalse

Making Variables

The thing you need to know in making variables in java is how to write them.
Format it like this:
<tipe data> namaVariabel;
Example :
Create an empty variable of type integer:
int namaVariabel;
Creates an integer type variable and is immediately filled with a value:
int namaVariabel = 19;
Create a set of variables whose data types are the same:
int a, b, c;
Then, where was it written?
  1. Inside functions main(): variables written in other functions main()and functions are called local variables.
  2. In the class: This variable is called a class or global variable.
Confused…?

Let's Try Exercises by Creating a DataDiri Program

Create a new program called DataDiri. Please add a class (java class) just package (package) pertemuan1 .
Creating Java Classes in Netbeans
In the window that appears, fill in the name of the class with DataDiri . After that, please follow the following code.
DataDiri Program
After that, try running by pressing the [Shift]button [F6]Analyze and understand the intent of the codes above.
Self data program with Java
The output seems wrong.
How old is the address, why is that?
That is because we call the variable alamatat the age line .
So what will appear is the contents of the variable alamat, not the contents of the variable usia.
Repair the code to be like this:
Improvement of the Self Data Program
And, ... please try running the program again.
DataDiri Program Output Results
Yep! the result is correct.

Variable Writing Rules

Apparently not allowed to create variables.
There are rules that must be followed, including:
  • The variable name may not use the keywords of the Java (reserved word) such as ifforswitch, etc.
  • Variable names may use letters, numbers (0-9), underscore , and dollar symbols ($), but the use of underscores and symbols is better avoided.
  • The variable name must start with a lowercase letter, because Java uses the CamelCase style .
  • If the variable name is more than 1 syllable, then the second word is written by starting with a capital letter and so on, for example namaVariabel.

2nd Exercise: Broad Circle Program

Let us establish understanding by making a Broad Circularprogram .
This program functions to calculate the area of ​​a circle.
We can calculate the area of ​​a circle using the formula .PI x r2
Before starting to program, we should first understand the algorithm and flowchart:

Algorithm

Deklarasi:
    Double luas, PI
    int r
Deskripsi:
    - Input
        PI = 3.14
        r = 18
    - Proses
        luas = PI * r * r
    - Output
        cetak luas

Flow chart:

Flow Chart Area of ​​Circles
Well, it's clear right?
Now we are just coding .
Java Code Area of ​​Circles
Please execute and refine the results.

Data Type Conversion

Conversion means changing to another type.
Why do we need to convert data types?
To answer this, I want to show the following illustration:

Water that is in liquid form cannot be stored in cardboard. Therefore, water must be converted into a solid form (ice) so that it can be stored in cardboard.
Likewise with variables.
Data string types can not be stored in variables with integertypes .

How to Convert Data Types

Example: Convert to integer type
Method 1:
Integer.perseInt(variabel);
Integer.perseInt(1.2);
Method 2:
Integer.valueOf(2.1);
Method 3:
objek.toInt();
Method 4:
(int) variabel;
(int) 2.1;
Apparently, there are many ways.

Let's Try in the Program ...

Make a new class (java class) in the meeting package2 .
Give a name: Extensive Triangle .
Then follow the following program code:
package pertemuan2;

import java.util.Scanner;

public class LuasSegitia {

    public static void main(String[] args) {
        // deklarasi
        Double luas;
        int alas, tinggi;

        // mebuat scanner baru
        Scanner baca = new Scanner(System.in);

        // Input
        System.out.println("== Program hitung luas Segitiga ==");
        System.out.print("Input alas: ");
        alas = baca.nextInt();
        System.out.print("Input tinggi: ");
        tinggi = baca.nextInt();

        // proses
        luas = Double.valueOf((alas * tinggi) / 2);

        // output
        System.out.println("Luas = " + luas);
    }

}
Variables of luastype Double data , meaning the value that can be stored is Double .
While the variables alasand tinggitypes are Integer .
In order for the results of integer data operations to be stored in a variable of type double, it needs to be converted.
If executed, it will produce:
The output of the program area is triangle with java
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