For those who have just started learning programming including the Java language, and those who have already learned it, for review This time I'm writing to learn about ** arrays **.
[Introduction to Java Table of Contents] -Variables and types ・ Type conversion -Variable Scope -String operation ・ Array operation ← Now here ・ Operator ・ Conditional branch (in preparation) ・ Repeat processing (in preparation) ・ About class (in preparation) ・ Abstract class (in preparation) ・ Interface (in preparation) ・ Encapsulation (in preparation) ・ About the module (in preparation) -Exception handling ・ About lambda expression ・ About Stream API
A variable is a container that holds one value data for one variable (Click here for variables).
By using an array, you can manage multiple data of the same data type in one array. ``
When there are 500 values, in the case of variables, you have to prepare 500 values, In the case of an array, it means that one array can be prepared and 500 pieces of data can be stored in it.
You can also easily sort the data in the array and get the largest value.
When creating an array, it is necessary to first decide the data type of what kind of value to handle like a variable, give it a name, and declare it.
Declare the data type [] array name.
Main.java
class Main {
public static void main(String[] args) {
int[] numbers; //Declaration of an array that can handle int type values
// []Can be after the array name.
String names[]; //Declaration of an array that can handle String type values
}
}
It is necessary to secure an area for how much the value is packed in the declared array.
Determine the size of the area to be secured by new [number of elements].
Main.java
class Main {
public static void main(String[] args) {
int[] numbers; //Declaration of an array that can handle int type values
String names[]; //Declaration of an array that can handle String type values
numbers = new int[50]; //Reserve an area to store 50 values in the numbers array
names = new String[3]; //Reserve an area to store 3 values in the name array
}
}
** It is also possible to declare an array and secure an area at the same time. ** **
Main.java
class Main {
public static void main(String[] args) {
int[] numbers = new int[50];//Declaration of array that can handle int type value and secure area for 50
String names[] = new String[3]; //Declaration of array that can handle String type value and secure area for 3
}
}
** Please note that if you do not specify the number of elements, a compile error will occur. ** **
Main.java
class Main {
public static void main(String[] args) {
int[] numbers = new int[];//Compile error because the number of elements is not specified
String names[] = new String[]; //Compile error because the number of elements is not specified
}
}
The number of elements in the array cannot be changed later, Keep in mind that if you first specify the number of elements in the array as 5, then the number of elements after that will be fixed at 5 all the time.
Also, keep in mind that the number of elements must be an integer.
The array is declared and instantiated and ready to store the values.
Use the subscript (index) to assign a value to an array.
Subscripts (indexes) are serial numbers assigned to each element of the array, starting at 0.
Main.java
class Main {
public static void main(String[] args) {
int[] numbers = new int[5]; //Define a numbers array that can store 5 elements of int type
numbers[0] = 100; //First, 100
numbers[1] = 200; //Second, 200
numbers[2] = 300; //Third, 300
numbers[3] = 400; //Fourth, 400
numbers[4] = 500; //Fifth, 500
}
}
Subscripts (indexes) are also used to access each element of the array.
Main.java
class Main {
public static void main(String[] args) {
int[] numbers = new int[5]; //Define a numbers array that can store 5 elements of int type
numbers[0] = 100; //First, 100
numbers[1] = 200; //Second, 200
numbers[2] = 300; //Third, 300
numbers[3] = 400; //Fourth, 400
numbers[4] = 500; //Fifth, 500
System.out.println(numbers[0]); //100 is output
System.out.println(numbers[3]); //400 is output
}
}
Up to the above, array declaration, area allocation, and value (initial value) assignment were performed step by step.
You can declare an array, secure an area, and assign a value (initial value) at the same time. (Array initialization)
Enclose the value in {} and describe each element separated by commas.
Data type [] Array name = {Initial value 1, Initial value 2, Initial value 3, Initial value 4, Initial value 5};
Also, use lengthto find out how manyelements you have created.
You can get the number of elements with array name.length;.
Main.java
class Main {
public static void main(String[] args) {
int[] numbers = {100, 200, 300, 400, 500}; //Array initialization
int size = numbers.length; //Get the number of elements in the numbers array In this case, 5 is stored
System.out.println(numbers[0]); //Output as 100
System.out.println(numbers[1]); //Output as 200
System.out.println(numbers[2]); //Output as 300
System.out.println(numbers[3]); //Output as 400
System.out.println(numbers[4]); //Output as 500
System.out.println(size); //Is output as 5
//Array initialization using this description method is also effective.
int[] id = new int[]{1, 2, 3};
System.out.println(id[0]); //Is output as 1
System.out.println(id[1]); //Is output as 2
System.out.println(id[2]); //Is output as 3
System.out.println(id.length); //Is output as 3
}
}
I used to use subscripts (indexes) to access the elements of the array, When trying to access outside the elements of an array, I don't get a compile error, but I get a run-time error (exception). For more information, see Exception Handling Articles (https://qiita.com/morioheisei/items/d25d2b67e7530c1aab9d).
Let's take a quick look here as well.
Main.java
class Main {
public static void main(String[] args) {
int[] id = {1, 2, 3, 4, 5}; //The number of elements in the array is 5
//Turn the loop 6 times with the for statement=Exceeding elements of id array
for(int i = 0; i < 6; i++) {
//Output one by one
System.out.println(id[i]);
}
System.out.println("I finished outputting all the contents of the id array.");
}
}
The output result is
Terminal
1
2
3
4
5
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 5 out of bounds for length 5
at Main.main(Main.java:195)
It was output as above. An ArrayIndexOutOfBoundsException exception has occurred. And I finished outputting all the contents of the id array. Is not displayed. When you try to access the array like this, if you are accessing outside the element, Be careful as an exception will occur and processing will stop in the middle.
There are also two-dimensional arrays that manage two subscripts (indexes), and more multidimensional arrays. This time, I will explain the two-dimensional array.
In a two-dimensional array, use two [] to
Declare the data type [] [] array name.
Main.java
class Main {
public static void main(String[] args) {
int[][] numbers; //Declaration of a two-dimensional array that can handle int type values
// [][]Can be after the array name.
String strs[][]; //Declaration of a two-dimensional array that can handle String type values
}
}
As in the case of a one-dimensional array, it is necessary to secure an area for how much the declared array should be filled with values.
Main.java
class Main {
public static void main(String[] args) {
int[][] numbers; //Declaration of a two-dimensional array that can handle int type values
String strs[][]; //Declaration of a two-dimensional array that can handle String type values
//An array with three elements, numbers[0]From numbers[2]Secure an area with 4 elements for each element of
numbers = new int[3][4];
//An array with two elements, strs[0]From strs[1]Secure an area with two elements for each element of
strs = new String[2][2];
}
}
Like a one-dimensional array ** It is also possible to declare an array and secure an area at the same time. ** **
Main.java
class Main {
public static void main(String[] args) {
//Reserve a 2D array declaration and area that can handle int type values
int[][] numbers = new int[3][4];
//Allocate a 2D array declaration and area that can handle String type values
String strs[][] = new String[2][2];
}
}
It is also possible to secure only the area of the `first dimension array. `` In that case, you can decide the number of elements in the second dimension later.
Main.java
class Main {
public static void main(String[] args) {
int[][] array; //Declaration of a two-dimensional array that can handle int type values
array = new int[3][]; //Reserve an area for an array with three elements
array[0] = new int[5]; //The first array of array can store 5 elements
array[1] = new int[3]; //The second array of array can store 3 elements
array[2] = new int[4]; //The third array of array can store 4 elements
//The same applies when declaring an array and securing an area at the same time.
String[][] strs = new String[2][]; //Declaration of String type array, allocation of area
strs[0] = new String[6]; //The first array of strs can store 6 elements
strs[1] = new String[3]; //The second array of strs can store three elements
}
}
Even in the case of a two-dimensional array, the number of elements cannot be changed later, and the number of elements must be an integer.
Values are stored using subscripts (indexes) in the same way as one-dimensional arrays.
Main.java
class Main {
public static void main(String[] args) {
int[][] numbers = new int[2][2]; //Declaration of a two-dimensional array that can handle int type values
numbers[0][0] = 100;
numbers[0][1] = 200;
numbers[1][0] = 300;
numbers[1][1] = 400;
System.out.println(numbers[0][0]); //Output as 100
System.out.println(numbers[0][1]); //Output as 200
System.out.println(numbers[1][0]); //Output as 300
System.out.println(numbers[1][1]); //Output as 400
}
}
Even in a two-dimensional array
You can declare an array, secure an area, and assign a value (initial value) at the same time. (Array initialization)
Similarly, enclose the value in {} and describe each element separated by commas.
Data type [] [] Array name = { {Initial value 1, Initial value 2, Initial value 3, Initial value 4, Initial value 5}, {Initial value 6, initial value 7, initial value 8, initial value 9, initial value 10} };
Also, use lengthto find out how manyelements you have created.
You can get the number of elements with array name.length;.
To get the length of the array in that array,
You can get the number of elements with array name [subscript (index)] .length.
Main.java
class Main {
public static void main(String[] args) {
//As with a one-dimensional array, it is possible to declare an array, secure an area, and assign values at once.
int[][] numbers = {
{1, 2, 3, 4, 5},
{6, 7, 8, 9, 10},
{11, 12, 13},
{16, 17}
};
System.out.println("numbers[0][4]The value of the: " + numbers[0][4]); // numbers[0][4]The value of the:Is output as 5
System.out.println("numbers[3][0]The value of the: " + numbers[3][0]); // numbers[3][0]The value of the:Is output as 16
System.out.println("numbers length: " + numbers.length); // numbers length:Is output as 4
System.out.println("numbers[0]Length of: " + numbers[0].length); // numbers[0]Length of:Is output as 5
System.out.println("numbers[2]Length of: " + numbers[2].length); // numbers[2]Length of:Is output as 3
System.out.println("numbers[3]Length of: " + numbers[3].length); // numbers[3]Length of:Is output as 2
}
}
From here, I will introduce a simple usage of 1D array and 2D array.
Remember to import first as we will be using the java.util.Arrays class. Click here for Oracle's Arrays class
Main.java
import java.util.Arrays;
class Main {
public static void main(String[] args) {
//Initialize the numbers array (the order of the integers in it is random)
int[] numbers = {10, 1, 5, 6, 9};
//Sort in ascending order using the sort method of the Arrays class
Arrays.sort(numbers);
//Output numbers array one by one
for(int number : numbers) {
System.out.print(number + " "); //Output as 1 5 6 9 10
}
}
}
Numbers, which stores random integers, can be sorted in ascending order by using the sort method.
Next, let's sort the strings.
Main.java
import java.util.Arrays;
class Main {
public static void main(String[] args) {
//Initialize the names array (the order of the names in it is random)
String[] names = {"tanaka", "abe", "suzuki", "maeda"};
//Sort alphabetically using the sort method of the Arrays class
Arrays.sort(names);
//Output names array one by one
for(String name : names) {
System.out.print(name + " "); //Output as abe maeda suzuki tanaka
}
}
}
Names, which contained random character strings, can be sorted alphabetically by using the sort method.
Main.java
class Main {
public static void main(String[] args) {
//Initialize the two-dimensional array numbers (the order of the integers in it is random)
int[][] numbers = {
{2, 5, 6, -10, 100, 3},
{-1000, 1, 20},
{999, 12, 300, 50}
};
//Define max variable to put the maximum value
int max = 0;
//Define a min variable to put the minimum value
int min = 0;
//Look at the first dimension of the numbers array one by one
for(int i = 0; i < numbers.length; i++) {
// numbers[0]、numbers[1]、numbers[2]Look at the contents of, one by one
for(int j = 0; j < numbers[i].length; j++) {
// numbers[0]、numbers[1]、numbers[2]If there is a number larger than max in
if(max < numbers[i][j]) {
//Substitute that number for the max variable
max = numbers[i][j];
}
// numbers[0]、numbers[1]、numbers[2]If there is a number smaller than min in
if(min > numbers[i][j]) {
//Substitute that number for the min variable
min = numbers[i][j];
}
}
}
System.out.println("Maximum value in the numbers array: " + max); // Maximum value in the numbers array:Output as 999
System.out.println("Minimum value in the numbers array: " + min); // Minimum value in the numbers array: -Output as 1000
}
}
The elements of the array are compared one by one using the for statement (a separate article will be described for iterative processing).
If there is a large number or a small number, the value is assigned to the maximum value max variable and the minimum value min variable each time.
It's easy, but I learned about arrays.
Arrays have a fixed number of elements, but there is also a variable List. I would like to cover that in another article.
Since multiple data can be used, there are many opportunities to use it. I want to hold it firmly.
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