Java Program แปลง LinkedList ↔ Array

LinkedList ↔ Array Conversion

LinkedList<E> และ Array สามารถแปลงกันได้ 2 ทิศทาง

กรณีที่ 1: LinkedList → Array

1. toArray() - ไม่มี Argument (คืน Object[])

LinkedList list = new LinkedList<>(Arrays.asList("A", "B", "C"));
Object[] objArray = list.toArray(); // Object[] - ต้อง Cast

2. toArray(T[] a) - มี Argument (Type-Safe) ⭐ แนะนำ

LinkedList list = new LinkedList<>(Arrays.asList("A", "B", "C"));
String[] strArray = new String[list.size()];
list.toArray(strArray); // ไปไว้ใน strArray

// หรือ Short form (Java 11+)
String[] strArray2 = list.toArray(String[]::new);

ตัวอย่างที่ 1: LinkedList → Array

import java.util.LinkedList;

public class Demo {
    public static void main(String[] args) {
        System.out.println("The required packages have been imported");
        
        LinkedList inputList = new LinkedList<>();
        inputList.add("Java");
        inputList.add("Python");
        inputList.add("Scala");
        inputList.add("Mysql");
        
        System.out.println("The list is defined as: " + inputList);
        
        // สร้าง Array ขนาดเท่า List
        String[] resultArray = new String[inputList.size()];
        
        // แปลง LinkedList → Array
        inputList.toArray(resultArray);
        
        System.out.print("\nThe result array is: ");
        for (String element : resultArray) {
            System.out.print(element + " ");
        }
    }
}

ผลลัพธ์

The required packages have been imported
The list is defined as: [Java, Python, Scala, Mysql]
The result array is: Java Python Scala Mysql

กรณีที่ 2: Array → LinkedList

1. Arrays.asList() + LinkedList Constructor

String[] array = {"Java", "Python", "Scala", "Mysql"};
LinkedList list = new LinkedList<>(Arrays.asList(array));

2. Collections.addAll()

String[] array = {"Java", "Python", "Scala", "Mysql"};
LinkedList list = new LinkedList<>();
Collections.addAll(list, array);

3. Stream API (Java 8+)

String[] array = {"Java", "Python", "Scala", "Mysql"};
LinkedList list = Arrays.stream(array)
    .collect(Collectors.toCollection(LinkedList::new));

ตัวอย่างที่ 2: Array → LinkedList

import java.util.Arrays;
import java.util.LinkedList;

public class Demo {
    public static void main(String[] args) {
        System.out.println("The required packages have been imported");
        
        String[] resultArray = {"Java", "Python", "Scala", "Mysql"};
        System.out.println("The elements of the result_array are defined as: " + 
            Arrays.toString(resultArray));
        
        // แปลง Array → LinkedList
        LinkedList resultList = new LinkedList<>(Arrays.asList(resultArray));
        
        System.out.println("\nThe elements of the result list are: " + resultList);
    }
}

ผลลัพธ์

The required packages have been imported
The elements of the result_array are defined as: [Java, Python, Scala, Mysql]
The elements of the result list are: [Java, Python, Scala, Mysql]

LinkedList vs ArrayList สำหรับ toArray()

ทั้งคู่ implement List interface มี toArray() เหมือนกัน

Multidimensional / Nested Collections

// LinkedList of Arrays
LinkedList listOfArrays = new LinkedList<>();
listOfArrays.add(new int[]{1, 2});
listOfArrays.add(new int[]{3, 4, 5});

// Convert to 2D Array
int[][] twoD = listOfArrays.toArray(new int[0][]);

Primitive Types (int, double, etc.)

Generics ไม่รองรับ Primitive ต้องใช้ Wrapper หรือ Library:

// ❌ ไม่ได้: LinkedList

// ✅ ใช้ Wrapper
LinkedList list = new LinkedList<>();
list.add(1); list.add(2); list.add(3);

// toArray() → Integer[]
Integer[] boxed = list.toArray(new Integer[0]);

// แปลงเป็น primitive int[] (Manual/Stream)
int[] primitive = list.stream().mapToInt(Integer::intValue).toArray();

// หรือ Library (Apache Commons Lang, Guava)
int[] primitive2 = ArrayUtils.toPrimitive(list.toArray(new Integer[0]));

Best Practices

• ใช้ toArray(new Type[0]) หรือ toArray(Type[]::new) (Java 11+) สำหรับ Type Safety
• หลีกเลี่ยง toArray() ไม่มี Argument (คืน Object[])
• Array → List: ใช้ new LinkedList<>(Arrays.asList(array))
• Primitive Arrays: ใช้ Stream .mapToInt(Integer::intValue).toArray()

สรุป

• LinkedList → Array: list.toArray(new String[list.size()]) หรือ list.toArray(String[]::new)
• Array → LinkedList: new LinkedList<>(Arrays.asList(array))
• toArray(T[]) Type-Safe และประสิทธิภาพดีกว่า toArray()
• Primitive Types ต้องใช้ Wrapper Class + Stream mapToInt