新手问下 kotlin 的集合.

[nl101531]

本人是服务端编程人员.

这个周末学了下 kotlin,从使用层面上确实比较爽,但是写代码的时候有一种不知道如何下手的感觉.花样太多了,貌似没有一些最佳实践.求哪位大牛出一本 effect kotlin 吧.

另外关于集合的 map, forEach 等高阶函数看了下源码有坑啊.

		var liss = listOf<Int>(1, 2, 3)
        // kotlin 中可以用的写法,每一步都是一个循环处理
        liss.map {x -> x * x  }.forEach(::println)
        
		// Java8 的 Stream, 构建好处理链路,最后一个循环中全部处理
        liss.stream().map { x -> x * x }.forEach(::println)
        

像这样的区别,没有一些最佳实践来指导吗?对于大容量处理必须 Stream 更加高效啊.

[nl101531]

附上 IDEA 中 AMS 插件显示的 Bytecode
```java
// access flags 0x19
public final static main([Ljava/lang/String;)V
@Lkotlin/jvm/JvmStatic;()
@Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
L0
ALOAD 0
LDC "args"
INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull (Ljava/lang/Object;Ljava/lang/String;)V
L1
LINENUMBER 22 L1
ICONST_3
ANEWARRAY java/lang/Integer
DUP
ICONST_0
ICONST_1
INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
AASTORE
DUP
ICONST_1
ICONST_2
INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
AASTORE
DUP
ICONST_2
ICONST_3
INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
AASTORE
INVOKESTATIC kotlin/collections/CollectionsKt.listOf ([Ljava/lang/Object;)Ljava/util/List;
ASTORE 1
L2
LINENUMBER 23 L2
ALOAD 1
CHECKCAST java/lang/Iterable
ASTORE 2
L3
LINENUMBER 28 L3
ALOAD 2
ASTORE 3
NEW java/util/ArrayList
DUP
ALOAD 2
BIPUSH 10
INVOKESTATIC kotlin/collections/CollectionsKt.collectionSizeOrDefault (Ljava/lang/Iterable;I)I
INVOKESPECIAL java/util/ArrayList.<init> (I)V
CHECKCAST java/util/Collection
ASTORE 4
L4
LINENUMBER 29 L4
ALOAD 3
INVOKEINTERFACE java/lang/Iterable.iterator ()Ljava/util/Iterator;
ASTORE 5
L5
FRAME FULL [[Ljava/lang/String; java/util/List java/lang/Iterable java/lang/Iterable java/util/Collection java/util/Iterator] []
ALOAD 5
INVOKEINTERFACE java/util/Iterator.hasNext ()Z
IFEQ L6
ALOAD 5
INVOKEINTERFACE java/util/Iterator.next ()Ljava/lang/Object;
ASTORE 6
L7
LINENUMBER 30 L7
ALOAD 4
ALOAD 6
CHECKCAST java/lang/Number
INVOKEVIRTUAL java/lang/Number.intValue ()I
ISTORE 7
ASTORE 11
L8
LINENUMBER 23 L8
ILOAD 7
ILOAD 7
IMUL
L9
INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
ASTORE 12
ALOAD 11
ALOAD 12
INVOKEINTERFACE java/util/Collection.add (Ljava/lang/Object;)Z
POP
L10
LINENUMBER 29 L10
GOTO L5
L6
LINENUMBER 31 L6
FRAME SAME
ALOAD 4
L11
CHECKCAST java/util/List
L12
CHECKCAST java/lang/Iterable
ASTORE 2
L13
LINENUMBER 23 L13
NOP
L14
LINENUMBER 32 L14
ALOAD 2
INVOKEINTERFACE java/lang/Iterable.iterator ()Ljava/util/Iterator;
ASTORE 3
L15
FRAME FULL [[Ljava/lang/String; java/util/List java/lang/Iterable java/util/Iterator java/lang/Object] []
ALOAD 3
INVOKEINTERFACE java/util/Iterator.hasNext ()Z
IFEQ L16
ALOAD 3
INVOKEINTERFACE java/util/Iterator.next ()Ljava/lang/Object;
ASTORE 4
L17
ALOAD 4
CHECKCAST java/lang/Number
INVOKEVIRTUAL java/lang/Number.intValue ()I
ISTORE 5
L18
LINENUMBER 23 L18
GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
ILOAD 5
INVOKEVIRTUAL java/io/PrintStream.println (I)V
L19
NOP
L20
GOTO L15
L16
LINENUMBER 33 L16
FRAME SAME
NOP
L21
LINENUMBER 25 L21
ALOAD 1
INVOKEINTERFACE java/util/List.stream ()Ljava/util/stream/Stream;
GETSTATIC com/itoolshub/task/fetch/FetchSite$main$3.INSTANCE : Lcom/itoolshub/task/fetch/FetchSite$main$3;
CHECKCAST java/util/function/Function
INVOKEINTERFACE java/util/stream/Stream.map (Ljava/util/function/Function;)Ljava/util/stream/Stream;
GETSTATIC com/itoolshub/task/fetch/FetchSite$main$4.INSTANCE : Lcom/itoolshub/task/fetch/FetchSite$main$4;
CHECKCAST kotlin/jvm/functions/Function1
DUP
IFNULL L22
ASTORE 2
NEW com/itoolshub/task/fetch/FetchSiteKt$sam$Consumer$89e5f0a8
DUP
ALOAD 2
INVOKESPECIAL com/itoolshub/task/fetch/FetchSiteKt$sam$Consumer$89e5f0a8.<init> (Lkotlin/jvm/functions/Function1;)V
L22
FRAME FULL [[Ljava/lang/String; java/util/List java/lang/Object java/util/Iterator java/lang/Object] [java/util/stream/Stream java/lang/Object]
CHECKCAST java/util/function/Consumer
INVOKEINTERFACE java/util/stream/Stream.forEach (Ljava/util/function/Consumer;)V
L23
LINENUMBER 26 L23
RETURN
L24
LOCALVARIABLE x I L8 L9 7
LOCALVARIABLE $i$a$1$map I L8 L9 8
LOCALVARIABLE item$iv$iv Ljava/lang/Object; L7 L10 6
LOCALVARIABLE $receiver$iv$iv Ljava/lang/Iterable; L4 L11 3
LOCALVARIABLE destination$iv$iv Ljava/util/Collection; L4 L11 4
LOCALVARIABLE $i$f$mapTo I L4 L11 9
LOCALVARIABLE $receiver$iv Ljava/lang/Iterable; L3 L12 2
LOCALVARIABLE $i$f$map I L3 L12 10
LOCALVARIABLE p1 I L18 L19 5
LOCALVARIABLE $i$a$2$unknown I L18 L19 6
LOCALVARIABLE element$iv Ljava/lang/Object; L17 L20 4
LOCALVARIABLE $receiver$iv Ljava/lang/Iterable; L14 L21 2
LOCALVARIABLE $i$f$forEach I L14 L21 7
LOCALVARIABLE liss Ljava/util/List; L2 L24 1
LOCALVARIABLE args [Ljava/lang/String; L0 L24 0
MAXSTACK = 4
MAXLOCALS = 13

```

[sorra]

放心，没坑
seq 方法生成 lazy view，类似 Java 8 Stream

[mx1700]

不想循环多次，你需要
list.asSequence()

[nl101531]

@mx1700 感谢,这些知识哪里有呢?感觉官方文档太简略了.

[nl101531]

@sorra 感谢,请问有一些实践的资料吗?

[mx1700]

翻翻官方 API 文档？可能比较枯燥

[zhuangzhuang1988]

学习啊 linq 就知道了。