Java Map foreach效率

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本文对比了Java中使用Map的entrySet直接进行foreach遍历和先获取entrySet再进行foreach遍历两种方式的字节码，发现直接使用entrySet进行foreach效率更高。

Java Map foreach效率

通过比较源码和字节码，查看在Map 的entrySet在foreach中Java到底做了些什么。

源码：

public class ForEachTest { public void inForEach(int count) { Map<String, String> map = new HashMap<String, String>(); for (Entry<String, String> entry : map.entrySet()) { } } public void outForEach(short count) { Map<String, String> map = new HashMap<String, String>(); Set<Entry<String, String>> entrySet = map.entrySet(); for (Entry<String, String> entry : entrySet) { } } }

字节码

public class com.fpi.sword.controlsdesigner.dataaccess.datamodel.design.ForEachTest extends java.lang.Object{ public com.fpi.sword.controlsdesigner.dataaccess.datamodel.design.ForEachTest(); Code: 0: aload_0 1: invokespecial #8; //Method java/lang/Object."<init>":()V 4: return public void withStrings(int); Code: 0: new #16; //class java/util/HashMap 3: dup 4: invokespecial #18; //Method java/util/HashMap."<init>":()V 7: astore_2 8: aload_2 9: invokeinterface #19, 1; //InterfaceMethod java/util/Map.entrySet:()Ljava/util/Set; 14: invokeinterface #25, 1; //InterfaceMethod java/util/Set.iterator:()Ljava/util/Iterator; 19: astore 4 21: goto 35 24: aload 4 26: invokeinterface #31, 1; //InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object; 31: checkcast #37; //class java/util/Map$Entry 34: astore_3 35: aload 4 37: invokeinterface #39, 1; //InterfaceMethod java/util/Iterator.hasNext:()Z 42: ifne 24 45: return public void withStringBuffer(short); Code: 0: new #16; //class java/util/HashMap 3: dup 4: invokespecial #18; //Method java/util/HashMap."<init>":()V 7: astore_2 8: aload_2 9: invokeinterface #19, 1; //InterfaceMethod java/util/Map.entrySet:()Ljava/util/Set; 14: astore_3 15: aload_3 16: invokeinterface #25, 1; //InterfaceMethod java/util/Set.iterator:()Ljava/util/Iterator; 21: astore 5 23: goto 38 26: aload 5 28: invokeinterface #31, 1; //InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object; 33: checkcast #37; //class java/util/Map$Entry 36: astore 4 38: aload 5 40: invokeinterface #39, 1; //InterfaceMethod java/util/Iterator.hasNext:()Z 45: ifne 26 48: return }

通过比较发现：直接将HashMap.entrySet应用到for each语句中效率更高。