[Easy-to-understand explanation! ] How to use ArrayList [Java]

1. Prior knowledge

As prior knowledge, the contents of the above link are required.

2. What is ArrayList?

--ʻArrayListis acollection class that implements the List interface. ―― ʻArrayList can be treated like an array as the name ʻArray` suggests.

Difference between ArrayList and array

--The array has a fixed number of elements that can be stored, but the ʻArrayList has a fixed number of elements. --ʻArrayListcannot containprimitive types (int, boolean, etc.)`.

How to use the List method

Method	Description	Description example
`add`	Add a value to the list.	list.add(Element number)
`addAll`	Add a list to the list.	list1.add(list)
`set`	Change the value in the list.	list.set(Element number,Value to substitute)
`get`	Get the value of the list.	list.get(Element number)
`size`	Get the number of elements in the list.	list.size()
`indexOf`	Get the element number of the value from the list.	list.indexOf(Value to search)
`subList`	Specify a range from the list and make a shallow copy.(* Refers to the same data as the original list.)	subList(Starting element number,End element number)
`contains`	Determine if the list contains values.	list.contains(Value to search)
`remove`	Delete the value of the specified element number from the list.	list.remove(Element number)
`distinct`	Remove duplicate values in the list.	list.stream().distinct().collect(Collectors.toList())
`clone`	Copy the list.	list.clone()
`clear`	Empty the list.	list.clear()

Mainly used class

`import`


import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.stream.Collectors;

3. Basic way to write ArrayList

`Test class`


package package name;
public class main class name{
    public static void main(String[] args) {
        //Creating an ArrayList
        ArrayList<Reference type name>Variable name= new ArrayList<Reference type name>();
    }
}

--Basic ʻArrayList` is described as above.

4. Preparation

Start Eclipse and select [File (F)]-> [New (N)]-> [Java Project].
Enter Test1 for the project name and click the Done button.
Select [File (F)] → [New (N)] → [Class].
Enter Test1 for the package and name and click the Done button.
Confirm that Test1.java has been created. Enter Test1 in the package and Hello in the name in the same procedure as in 6.3, and click the Finish button.
Success if Test1.java and Hello.java are created.

5. Description example

`Test1.java`


package Test1;
import java.util.ArrayList;
import java.util.stream.Collectors;

public class Test1 {
    public static void main(String[] args) {
    	//Creating an ArrayList
    	ArrayList<Hello> hello = new ArrayList<Hello>();
    	ArrayList<Hello> subhello = new ArrayList<Hello>();

    	//Add value to ArrayList
    	hello.add(new Hello("A"));
    	hello.add(new Hello("B"));
    	hello.add(new Hello("C"));

    	//Add list to ArrayList
    	subhello.addAll(hello);

    	//Change the value of ArrayList
    	hello.set(0, new Hello("D"));

    	//Number of elements in ArrayList
    	System.out.println("Number of hello elements:"+hello.size());

    	//Matching element numbers between ArrayLists
    	System.out.println("subhello(2)Element number of hello that matches:"+hello.indexOf(subhello.get(2)));

    	//Determine if the ArrayList contains values
    	System.out.println("Does hello contain a value:"+hello.contains(subhello.get(2)));

    	//Delete the value of the specified element number from ArrayList
    	hello.remove(2);

    	//Remove duplicate values in ArrayList.
    	subhello.stream().distinct().collect(Collectors.toList());

    	//Copy the ArrayList.
    	ArrayList<Hello> clonehello = (ArrayList<Hello>) hello.clone();

    	//Display of ArrayList
    	for(Hello h : hello) {
    		h.showGreeting();
    	}
    	System.out.println();
    	for(Hello sub : subhello) {
    		sub.showGreeting();
    	}
    	System.out.println();
    	for(Hello clone : clonehello) {
    		clone.showGreeting();
    	}
    }
}

`Hello.java`


package Test1;
public class Hello {
	//Instance variables
	String name;

	//constructor
    public Hello(String name) {
        this.name = name;
    }
    //Display greetings
    void showGreeting() {
        System.out.println(name+"Hey,.");
    }
}

--Copy the above sentence, specify S-JIS as the character code, save the file name as Test1.java, Hello.java, and execute it. ↓ ↓