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

字节码

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

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