Java for each实现机制代码原理解析

源测试代码如下

public class ForEachTest { public void test4Iterate(Iterable<String> strings) { for (String str : strings) { System.out.println(str); } } public void test4Array(String[] strings) { for (String str : strings) { System.out.println(str); } }}

执行编译命令

javac ForEachTest.java

使用idea查看反编的Java文件

import java.util.Iterator;public class ForEachTest { public ForEachTest() { } public void test4Iterate(Iterable<String> var1) { Iterator var2 = var1.iterator(); while(var2.hasNext()) { String var3 = (String)var2.next(); System.out.println(var3); } } public void test4Array(String[] var1) { String[] var2 = var1; int var3 = var1.length; for(int var4 = 0; var4 < var3; ++var4) { String var5 = var2[var4]; System.out.println(var5); } }}

执行指令查看对应的汇编指令

javap -c ForEachTest.class

Compiled from 'ForEachTest.java'public class ForEachTest { public ForEachTest(); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object.'<init>':()V 4: return public void test4Iterate(java.lang.Iterable<java.lang.String>); Code: 0: aload_1 1: invokeinterface #2, 1 // InterfaceMethod java/lang/Iterable.iterator:()Ljava/util/Iterator; 6: astore_2 7: aload_2 8: invokeinterface #3, 1 // InterfaceMethod java/util/Iterator.hasNext:()Z 13: ifeq 36 16: aload_2 17: invokeinterface #4, 1 // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object; 22: checkcast #5 // class java/lang/String 25: astore_3 26: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream; 29: aload_3 30: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 33: goto 7 36: return public void test4Array(java.lang.String[]); Code: 0: aload_1 1: astore_2 2: aload_2 3: arraylength 4: istore_3 5: iconst_0 6: istore 4 8: iload 4 10: iload_3 11: if_icmpge 34 14: aload_2 15: iload 4 17: aaload 18: astore 5 20: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream; 23: aload 5 25: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 28: iinc 4, 1 31: goto 8 34: return}

由反编类和汇编指令可以看到对于Iterable类，是通过其迭代方法执行for each，而对于数组是通过传统的for循环方式执行迭代

因为集合类的for each是基于迭代器循环的，所以根据迭代器实现的不同,for each的性能也会不同

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