[Java] Basic summary of Java not covered by Progate ~ Part 2 · List ~

list

Progate talks about arrays, but not lists. Arrays and lists are similar, but what's the difference?

Array

python

int result[] = new int[5];

result[0] = 85;
result[1] = 78;
result[2] = 92;
result[3] = 62;
result[4] = 69;

If you compare it to a container, the array result [] must first determine the size of the container. Furthermore, once the size is decided, it cannot be changed after that.

list

python

List<String> list = new ArrayList<String>();

On the other hand, the size of the list is variable, and elements can be added / deleted.

Interface

In java, an interface (like a specification) for handling lists is defined. This List interface is an interface with the roots of Collection.

Package import required

You need to ʻimrport`` java.util.List` to use List.

python

import java.util.List;

Also, since List is an interface (specification), it has no implementation itself, so It is necessary to ʻimport`` class` which implements List interface separately.

ArrayList and LinkedList

There are ʻArrayList and LinkedListinclass` that implements List interface.

Try creating an instance of the ArrayList class

python

import java.util.list;
import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    List<String> list = new ArrayList<>();
    System.out.println(list);
  }
}


///Execution result
/// >[]

List<String> list = new ArrayList<>(); It declares that the variable is of type List and that the data handled by this List is of type String.

Instantiate LinkedList class

python

import java.util.list;
import java.util.LinkedList;

public class Main {
  public static void main(String[] args) {
    List<String> list = new LinkedList<>();
    System.out.println(list);
  }
}


///Execution result
/// >[]

LinkedList defines some unique methods that List does not have, but they are omitted here. (Almost not in ArrayList)

For lists dealing with primitive types

List cannot directly handle primitive types such as ʻintandlong. String` is a reference type. See the table in the previous article for more information on primitive and reference types. https://qiita.com/hiroshimahiroshi/items/01de02cfe1caacd07540#3-%E5%9F%BA%E6%9C%AC%E3%83%87%E3%83%BC%E3%82%BF%E5%9E%8B%E3%81%A8%E5%8F%82%E7%85%A7%E5%9E%8B

So when dealing with primitive types such as ʻintandlong, the wrapper class ʻInteger of ʻint, Use the long wrapper class Long`.

python

    List<Integer> list = new LinkedList<>();

Primitive type? Wrapper class? I'm not sure, but for the time being, I just want to know the list first! In that case, think that something is strange if the type of List <type> starts with a lowercase letter for the time being. Let's learn about primitive types, reference types, and wrapper classes separately.

At the end

I edited the article significantly. This article is for people like me who have finished Progate and feel like understanding Java. I've only touched on it, but I think it's important basic knowledge, so let's deepen your understanding with CODEPREP.