Y.A.M の雑記帳: 8月 2017

2017年8月24日木曜日

Kotlin メモ : forEachIndexed

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/for-each-indexed.html

Java


int pos = 1;
for (Item item : items) {
    log(pos, item);
    pos++;
}

Kotlin 変換直後


var pos = 1
for (item in items) {
    log(pos, item)
    pos++
}

forEachIndexed 使用


items.forEachIndexed { index, item -> log(index + 1, item)}

2017年8月22日火曜日

Kotlin メモ : mapNotNull

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/map-not-null.html


val list = (0 until adapter.count)
        .map { adapter.item(it) }
        .filterNotNull()
        .toList()

mapNotNull 使用


val list = (0 until adapter.count)
        .mapNotNull { adapter.item(it) }
        .toList()

Kotlin メモ : until

https://kotlinlang.org/docs/reference/ranges.html
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.ranges/until.html


val list = (0..adapter.count - 1)
        .map { adapter.item(it) }
        .filterNotNull()
        .toList()

until 使用


val list = (0 until adapter.count)
        .map { adapter.item(it) }
        .filterNotNull()
        .toList()

Kotlin メモ : filterNotNull

filterNotNull

Java


final List<Item> list = new ArrayList<>();
for (int i = 0, count = adapter.getCount(); i < count; i++) {
    final Item item = adapter.getItem(i);
    if (item != null) {
        list.add(item);
    }
}

Kotlin 自動変換直後


val list = ArrayList<Item>()
var i = 0
val count = adapter.count
while (i < count) {
    val item = adapter.item(i)
    if (item != null) {
        list.add(item)
    }
    i++
}

range, let 使用


val list = ArrayList<Item>()
for(i in 0..adapter.count - 1) {
    adapter.item(i)?.let { 
        list.add(it)
    }
}

map, filterNotNull 使用


val list = (0..adapter.count - 1)
        .map { adapter.item(it) }
        .filterNotNull()
        .toList()

2017年8月16日水曜日

Kotlin メモ : indexOfFirst

indexOfFirst

predicate にマッチする最初の位置の index を返す。マッチするものが無い場合は -1 を返す。

Java


@Override
public int getSectionForPosition(int position) {
    final Object[] sections = getSections();
    if (sections == null) {
        return 0;
    }
    int section = 0;
    for (int i = 0; i < sections.length; i++) {
        final MonthSection ms = (MonthSection) sections[i];
        if (ms.position > position) {
            return section;
        }
        section = i;
    }
    return section;
}

Kotlin


override fun getSectionForPosition(position: Int): Int {
    val sections = getSections() ?: return 0
    return sections.indexOfFirst { it.position > position }
            .let {
                when {
                    it > 0 -> it - 1
                    it == 0 -> 0
                    else -> sections.lastIndex
                }
            }
}

2017年8月10日木曜日

Kotlin メモ : ArrayAdapter

Java


class MyAdapter extends ArrayAdapter<MyData> {

    private final LayoutInflater inflater;

    MyAdapter(Context context, List<MyData> objects) {
        super(context, 0, objects);
        inflater = LayoutInflater.from(context);
    }

    ...

    @NonNull
    @Override
    public View getView(int position, View convertView, @NonNull ViewGroup parent) {
        final MyViewHolder holder;
        if (convertView == null) {
            holder = MyViewHolder.create(inflater, parent);
            convertView = holder.view;
            convertView.setTag(holder);
        } else {
            holder = (MyViewHolder) convertView.getTag();
        }

        final MyData data = getItem(position);
        assert data != null;

        holder.bind(data);

        return convertView;
    }
}

Kotlin


class MyAdapter(context: Context, objects: List<MyData>) : ArrayAdapter<MyData>(context, 0, objects) {

    private val inflater = LayoutInflater.from(context)

    ...

    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {
        val view: View = convertView ?: MyViewHolder.create(inflater, parent)
                .also { it.view.tag = it }
                .view
        getItem(position)?.let {
            (view.tag as MyViewHolder).bind(it)
        }
        return view
    }
}

2017年8月3日木曜日

Kotlin メモ : joinToString

kotlin-stdlib / kotlin.collections / joinToString

Java


/**
 * 00 11 22 33 44 55 66 77
 */
@NonNull
public String expression(byte[] bytes) {
    final StringBuilder sb = new StringBuilder();
    boolean firstTime = true;
    for (byte each : bytes) {
        if (firstTime) {
            firstTime = false;
        } else {
            sb.append(" ");
        }
        sb.append(hex(each));
    }
    return sb.toString();
}

Java その2


/**
 * 00 11 22 33 44 55 66 77
 */
@NonNull
public String expression(byte[] bytes) {
    final List<String> tokens = new ArrayList<>();
    for (byte each : bytes) {
        tokens.add(hex(each));
    }
    return TextUtils.join(" ", tokens);
}

Kotlin


/**
 * 00 11 22 33 44 55 66 77
 */
fun expression(bytes : ByteArray): String {
    return bytes.joinToString(separator = " ", transform = { hex(it) })
}

2017年8月1日火曜日

Kotlin メモ : takeIf

Java


public Hoge createFromParcel(Parcel source) {
    final int length = source.readInt();
 final byte[] data;
    if (length > 0) {
        data = new byte[length];
        source.readByteArray(data);
    } else {
        data = null;
    }

    return new Hoge(data);
}

Kotlin 変換直後


override fun createFromParcel(source: Parcel): Hoge {
    val length = source.readInt()
    val data: ByteArray?
    if (length > 0) {
        data = ByteArray(length)
        source.readByteArray(data)
    } else {
        data = null
    }

    return Hoge(data)
}

source.run {} を使う


override fun createFromParcel(source: Parcel): Hoge = source.run {
    val length = readInt()
    val data: ByteArray?
    if (length > 0) {
        data = ByteArray(length)
        readByteArray(data)
    } else {
        data = null
    }

    Hoge(data)
}

also を使う


override fun createFromParcel(source: Parcel): Hoge = source.run {
    val length = readInt()
    val data: ByteArray?
    if (length > 0) {
        data = ByteArray(length).also { readByteArray(it) }
    } else {
        data = null
    }

    Hoge(data)
}

if 式にしてみる


override fun createFromParcel(source: Parcel): Hoge = source.run {
    val length = readInt()
    val data: ByteArray? = if (length > 0) {
        ByteArray(length).also { readByteArray(it) }
    } else {
        null
    }

    Hoge(data)
}

length に let を使う


override fun createFromParcel(source: Parcel): Hoge = source.run {
    val length = readInt()
    val data: ByteArray? = if (length > 0) {
        length.let { ByteArray(it).also { readByteArray(it) } }
    } else {
        null
    }

    Hoge(data)
}

readInt() に takeIf を使う


override fun createFromParcel(source: Parcel): Hoge = source.run {
    val data: ByteArray? = readInt()
            .takeIf { it > 0 }
            ?.let { ByteArray(it).also { readByteArray(it) } }

    Hoge(data)
}

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