Mastering Kotlin ~ Convert Java File to Kotlin File Road to Graduation ~ Part 3

Introduction

When using Kotlin on Android etc., there are many people who automatically convert from Java with "Convert Java File to Kotlin File". It actually works, but it's a memo to take it one step further and make better use of Kotlin. 3rd time This time, you will learn how to process List and for statements. Kotlin version: 1.4

Sample code before Kotlin conversion

An example where there is a process to generate a List from a certain object. There are some conditions for creating a List.

ItemData.java

//Object with 7 Strings
public class ItemData {
    private String item1;
    private String item2;
    private String item3;
    private String item4;
    private String item5;
    private String item6;
    private String item7;

    public ItemData2(String item1, String item2, String item3, String item4,
                     String item5, String item6, String item7) {
        this.item1 = item1;
        this.item2 = item2;
        this.item3 = item3;
        this.item4 = item4;
        this.item5 = item5;
        this.item6 = item6;
        this.item7 = item7;
    }

    public String getItem1() {
        return item1;
    }

    public String getItem2() {
        return item2;
    }

    public String getItem3() {
        return item3;
    }

    public String getItem4() {
        return item4;
    }

    public String getItem5() {
        return item5;
    }

    public String getItem6() {
        return item6;
    }

    public String getItem7() {
        return item7;
    }
}

ListUtil.java

//Util to create List from object
public final class ListUtil {

    private ListUtil() {
    };

    private static final int MAX_SIZE = 4;

    //Make a list with item1, item2, item3, item4, item5 of the object
    //However, if it is null, it will not be added to the List.
    public static List<String> createItemList(final ItemData itemData) {
        if (itemData == null) {
            return new ArrayList<>();
        }
        final List<String> list = new ArrayList<>();
        for (int i = 0; i <= MAX_SIZE; i++) {
            switch (i) {
                case 0:
                    if (itemData.item1 != null) {
                        list.add(itemData.item1);
                    }
                    break;
                case 1:
                    if (itemData.item2 != null) {
                        list.add(itemData.item2);
                    }
                    break;
                case 2:
                    if (itemData.item3 != null) {
                        list.add(itemData.item3);
                    }
                    break;
                case 3:
                    if (itemData.item4 != null) {
                        list.add(itemData.item4);
                    }
                    break;
                case 4:
                    if (itemData.item5 != null) {
                        list.add(itemData.item5);
                    }
                    break;
                default:

            }
        }
        return list;
    }
}

Sample code after Kotlin automatic conversion

If this is automatically converted, it will look like this ... For the time being, it still works.

ItemData.kt

//Object with 7 Strings
class ItemData(
    val item1: String,
    val item2: String,
    val item3: String,
    val item4: String,
    val item5: String,
    val item6: String,
    val item7: String
)

ListUtil.kt

//Util to create List from object
object ListUtil {
    private const val MAX_SIZE = 4
    //Make a list with item1, item2, item3, item4, item5 of the object
    //However, if it is null, it will not be added to the List.
    fun createItemList(itemData: ItemData?): List<String?> {
        if (itemData == null) {
            return ArrayList()
        }
        val list: MutableList<String?> = ArrayList()
        for (i in 0 until MAX_SIZE) {
            when (i) {
                0 -> if (itemData.item1 != null) {
                    list.add(itemData.item1)
                }
                1 -> if (itemData.item2 != null) {
                    list.add(itemData.item2)
                }
                2 -> if (itemData.item3 != null) {
                    list.add(itemData.item3)
                }
                3 -> if (itemData.item4 != null) {
                    list.add(itemData.item4)
                }
                4 -> if (itemData.item5 != null) {
                    list.add(itemData.item5)
                }
                else -> {
                }
            }
        }
        return list
    }
}

Think about how to implement

Since it is implemented in Kotlin, I want to make it simpler. I will fix it immediately

ItemData.kt

//Specify data as a data class with data
data class ItemData(
    val item1: String,
    val item2: String,
    val item3: String,
    val item4: String,
    val item5: String,
    val item6: String,
    val item7: String
)

ListUtil.kt

//Util to create List from object
object ListUtil {
    private const val MAX_SIZE = 4
    //Make a list with item1, item2, item3, item4, item5 of the object
    //However, if it is null, it will not be added to the List.

    //The return value is List<String?>→List<String>Is good
    fun createItemList(itemData: ItemData?): List<String> =
        itemData?.let {
            //If null, don't enter here
            mutableListOf<String?>().also { list ->
                // for (i in 0..MAX_SIZE)Can also be described as
                (0 until MAX_SIZE).forEach { i ->
                    when (i) {
                        0 -> it.item1
                        1 -> it.item2
                        2 -> it.item3
                        3 -> it.item4
                        4 -> it.item5
                        else -> null
                    }.also {
                        list += it
                    }
                }
            }.filterNotNull()
            //This will eliminate the null in the List and make the String NonNull.
            //You don't have to check for null when adding
        } ?: emptyList()
    //Returns an empty List if null
    //Empty List is ArrayList()Not emptyList()return it
}

Very simple!