What I learned in Java (Part 2) What are variables?

Looking back on Java

I would like to take a quick look back at what I have been studying Java since the last time.

• Please see the previous article if you like. What I learned in Java (1) Java development flow and overview

What is a variable?

Java is an image of building a system while making boxes one by one.

Variables are the smallest unit in the box. For variables, prepare a box to store data, Putting in and out of data in it can do.

As an example

public class Main {
	public static void main(String[] args) {
		int age;
		age = 20;
		System.out.println(age);
	}
}

• Age Execution result What the above source code does

1. Create variable "age"
2. Put the integer "20" in "age" (substitution)
3. Display the contents of "age" on the console The image looks like this! I would like to explain in order from 1.

Variable declaration

First, create a box (variable declaration) to store data. A box does not mean anything. In Java you have to explicitly specify what the box is for Don't.

The "age" mentioned earlier is a box for storing an integer called "int type". Only integers (character strings and decimal points) can be entered. In Java, it is necessary to decide in advance the type of data to be inserted (* hereinafter referred to as "data type"). This is called a "variable declaration".

int age;

The above is a variable declaration that "creates a box called int type age".

Data assignment

int age;
age = 20;

Substitute the integer "20" in the created variable "age". As mentioned earlier, characters and decimal points cannot be entered in this int type variable "age". When I try to put it in, it cannot be compiled (converted to a computer-readable sentence) and I am asked to correct it.

• Assignment can also be written together with the variable declaration.

int age = 20;

Data reference

After data substitution

System.out.println(age);

System.out.println (); is a process to display the inside of () at the end on the console (where you talk to the computer). Since "age" is specified here, the "20" that was assigned earlier is displayed. So what happens if we substitute "30" for the variable "age" that contains "20"?

public class Main {
	public static void main(String[] args) {
		int age;
		age = 20;
		age = 30;
		System.out.println(age);
	}
}

Execution result As the name "number" changes, it will be overwritten with the most recently assigned one.

Data type

The data type is not limited to int type. There is also a data type for entering the "character string" and "decimal point number" mentioned earlier. I would like to summarize below.

• Double precision floating point number

At the beginning of study When using an integer int When using a decimal point double When using the alternative boolean When using a string String It's okay if you remember!

Primitive and reference types

There are two types of data types: primitive types and reference types. Primitive types put values directly in the "box". Integer (long, int, short, byte) Decimal point number (double, float) Boolean value (boolean) Character (char) Reference types, on the other hand, assign addresses in memory rather than specific data. String type and I would like to summarize it later, but "array", "List", etc. are also included in this reference type. Instead of entering the "character string" directly, ... Specifically, one cushion is already included, but as shown in the figure, the first address of the "character" sequence of the name is substituted.

• I'm sorry if it's hard to understand.

Why use variables?

The use of variables has the following two advantages. ・ High versatility. ・ Easy to change and modify.

If you compare it to the sales of apples

public class Main {
	public static void main(String[] args) {
		//The price of an apple is 100 yen
		int apple = 100;
		
		//Sales in each region
		int tokyo = 100 * apple;
		int kanagawa = 80 * apple;
		int tiba = 70 * apple;
		
		//Total sales
		int totalSales = tokyo + kanagawa + tiba; 
		
		//Display of total sales
		System.out.println(totalSales);
	}
}

Execution result The total sales are displayed. Here, if the price of apples is wrong, or if you want to get a quote when you raise the price, etc. There are various situations, but if you want to change the price of apples, you only need to make one correction.

public class Main {
    public static void main(String[] args) {
        //Adjust the price of apples from 100 yen to 120 yen
        int apple = 120;

        //Sales in each region
        int tokyo = 100 * apple;
        int kanagawa = 80 * apple;
        int tiba = 70 * apple;

        //Total sales
        int totalSales = tokyo + kanagawa + tiba; 

        //Display of total sales
        System.out.println(totalSales);
    }
}

Execution result

It may be difficult to understand, but I think you will understand it as you study.

References

https://book.impress.co.jp/books/1113101090

Next time, I would like to write about "instruction execution statements"!