Kotlin メモ : forEachIndexed

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


Java

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1. int pos = 1;  
2. for (Item item : items) {  
3.     log(pos, item);  
4.     pos++;  
5. }  

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

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1. var pos = 1  
2. for (item in items) {  
3.     log(pos, item)  
4.     pos++  
5. }  

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

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

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

Kotlin メモ : mapNotNull

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

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

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

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

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

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

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

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

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

Kotlin メモ : filterNotNull

filterNotNull


Java

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1. final List<Item> list = new ArrayList<>();  
2. for (int i = 0, count = adapter.getCount(); i < count; i++) {  
3.     final Item item = adapter.getItem(i);  
4.     if (item != null) {  
5.         list.add(item);  
6.     }  
7. }  

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

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1. val list = ArrayList<Item>()  
2. var i = 0  
3. val count = adapter.count  
4. while (i < count) {  
5.     val item = adapter.item(i)  
6.     if (item != null) {  
7.         list.add(item)  
8.     }  
9.     i++  
10. }  

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

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1. val list = ArrayList<Item>()  
2. for(i in 0..adapter.count - 1) {  
3.     adapter.item(i)?.let {   
4.         list.add(it)  
5.     }  
6. }  

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

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

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

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

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1. @Override  
2. public int getSectionForPosition(int position) {  
3.     final Object[] sections = getSections();  
4.     if (sections == null) {  
5.         return 0;  
6.     }  
7.     int section = 0;  
8.     for (int i = 0; i < sections.length; i++) {  
9.         final MonthSection ms = (MonthSection) sections[i];  
10.         if (ms.position > position) {  
11.             return section;  
12.         }  
13.         section = i;  
14.     }  
15.     return section;  
16. }  

@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

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1. override fun getSectionForPosition(position: Int): Int {  
2.     val sections = getSections() ?: return 0  
3.     return sections.indexOfFirst { it.position > position }  
4.             .let {  
5.                 when {  
6.                     it > 0 -> it - 1  
7.                     it == 0 -> 0  
8.                     else -> sections.lastIndex  
9.                 }  
10.             }  
11. }  

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 } } }

Kotlin メモ : ArrayAdapter

Java

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1. class MyAdapter extends ArrayAdapter<MyData> {  
2.   
3.     private final LayoutInflater inflater;  
4.   
5.     MyAdapter(Context context, List<MyData> objects) {  
6.         super(context, 0, objects);  
7.         inflater = LayoutInflater.from(context);  
8.     }  
9.   
10.     ...  
11.   
12.     @NonNull  
13.     @Override  
14.     public View getView(int position, View convertView, @NonNull ViewGroup parent) {  
15.         final MyViewHolder holder;  
16.         if (convertView == null) {  
17.             holder = MyViewHolder.create(inflater, parent);  
18.             convertView = holder.view;  
19.             convertView.setTag(holder);  
20.         } else {  
21.             holder = (MyViewHolder) convertView.getTag();  
22.         }  
23.   
24.         final MyData data = getItem(position);  
25.         assert data != null;  
26.   
27.         holder.bind(data);  
28.   
29.         return convertView;  
30.     }  
31. }  

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

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1. class MyAdapter(context: Context, objects: List<MyData>) : ArrayAdapter<MyData>(context, 0, objects) {  
2.   
3.     private val inflater = LayoutInflater.from(context)  
4.   
5.     ...  
6.   
7.     override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {  
8.         val view: View = convertView ?: MyViewHolder.create(inflater, parent)  
9.                 .also { it.view.tag = it }  
10.                 .view  
11.         getItem(position)?.let {  
12.             (view.tag as MyViewHolder).bind(it)  
13.         }  
14.         return view  
15.     }  
16. }  

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 } }

Kotlin メモ : joinToString

kotlin-stdlib / kotlin.collections / joinToString

Java

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1. /** 
2.  * 00 11 22 33 44 55 66 77 
3.  */  
4. @NonNull  
5. public String expression(byte[] bytes) {  
6.     final StringBuilder sb = new StringBuilder();  
7.     boolean firstTime = true;  
8.     for (byte each : bytes) {  
9.         if (firstTime) {  
10.             firstTime = false;  
11.         } else {  
12.             sb.append(" ");  
13.         }  
14.         sb.append(hex(each));  
15.     }  
16.     return sb.toString();  
17. }  

/** * 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

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1. /** 
2.  * 00 11 22 33 44 55 66 77 
3.  */  
4. @NonNull  
5. public String expression(byte[] bytes) {  
6.     final List<String> tokens = new ArrayList<>();  
7.     for (byte each : bytes) {  
8.         tokens.add(hex(each));  
9.     }  
10.     return TextUtils.join(" ", tokens);  
11. }  

/** * 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

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1. /** 
2.  * 00 11 22 33 44 55 66 77 
3.  */  
4. fun expression(bytes : ByteArray): String {  
5.     return bytes.joinToString(separator = " ", transform = { hex(it) })  
6. }  

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

Kotlin メモ : takeIf

Java

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1. public Hoge createFromParcel(Parcel source) {  
2.     final int length = source.readInt();  
3.  final byte[] data;  
4.     if (length > 0) {  
5.         data = new byte[length];  
6.         source.readByteArray(data);  
7.     } else {  
8.         data = null;  
9.     }  
10.   
11.     return new Hoge(data);  
12. }  

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

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1. override fun createFromParcel(source: Parcel): Hoge {  
2.     val length = source.readInt()  
3.     val data: ByteArray?  
4.     if (length > 0) {  
5.         data = ByteArray(length)  
6.         source.readByteArray(data)  
7.     } else {  
8.         data = null  
9.     }  
10.   
11.     return Hoge(data)  
12. }  

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

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1. override fun createFromParcel(source: Parcel): Hoge = source.run {  
2.     val length = readInt()  
3.     val data: ByteArray?  
4.     if (length > 0) {  
5.         data = ByteArray(length)  
6.         readByteArray(data)  
7.     } else {  
8.         data = null  
9.     }  
10.   
11.     Hoge(data)  
12. }  

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

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1. override fun createFromParcel(source: Parcel): Hoge = source.run {  
2.     val length = readInt()  
3.     val data: ByteArray?  
4.     if (length > 0) {  
5.         data = ByteArray(length).also { readByteArray(it) }  
6.     } else {  
7.         data = null  
8.     }  
9.   
10.     Hoge(data)  
11. }  

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

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1. override fun createFromParcel(source: Parcel): Hoge = source.run {  
2.     val length = readInt()  
3.     val data: ByteArray? = if (length > 0) {  
4.         ByteArray(length).also { readByteArray(it) }  
5.     } else {  
6.         null  
7.     }  
8.   
9.     Hoge(data)  
10. }  

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

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1. override fun createFromParcel(source: Parcel): Hoge = source.run {  
2.     val length = readInt()  
3.     val data: ByteArray? = if (length > 0) {  
4.         length.let { ByteArray(it).also { readByteArray(it) } }  
5.     } else {  
6.         null  
7.     }  
8.   
9.     Hoge(data)  
10. }  

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

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1. override fun createFromParcel(source: Parcel): Hoge = source.run {  
2.     val data: ByteArray? = readInt()  
3.             .takeIf { it > 0 }  
4.             ?.let { ByteArray(it).also { readByteArray(it) } }  
5.   
6.     Hoge(data)  
7. }  

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