java foreach實現原理
阿新 • • 發佈:2021-08-30
java foreach實現原理
在平時Java程式中,應用比較多的就是對Collection集合類的foreach遍歷,foreach之所以能工作,是因為這些集合類都實現了Iterable介面,該介面中定義了Iterator迭代器的
產生方法,並且foreach就是通過Iterable介面在序列中進行移動。
Iterable介面API:
package java.lang; import java.util.Iterator; public interface Iterable { public abstract Iterator iterator(); }
該介面中定義了產生Iterator迭代器的方法;
package java.util; public interface Iterator { public abstract boolean hasNext(); public abstract Object next(); public abstract void remove(); }
那麼為什麼實現了Iterable介面就支援foreach操作了呢?
我們寫段foreach小程式看看位元組碼是怎麼樣的吧.
package test; import java.util.List; /** * Created by vino on 2016/5/6. */ public class TestForeach { List<Integer> integers; public void testForeach(){ for(Integer i : integers){ } } }
其中TestForeach方法的詳細位元組碼如下:
public void testForeach(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=3, args_size=1 0: aload_0 1: getfield #2 // Field integers:Ljava/util/List; 4: invokeinterface #3, 1 // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator; 9: astore_1 10: aload_1 11: invokeinterface #4, 1 // InterfaceMethod java/util/Iterator.hasNext:()Z 16: ifeq 32 19: aload_1 20: invokeinterface #5, 1 // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object; 25: checkcast #6 // class java/lang/Integer 28: astore_2 29: goto 10 32: return LineNumberTable: line 11: 0 line 13: 29 line 14: 32 LocalVariableTable: Start Length Slot Name Signature 29 0 2 i Ljava/lang/Integer; 0 33 0 this Ltest/TestForeach; }