Java迴圈效能隨筆，java迴圈隨筆
for iterator做迭代迴圈效能最好 然後是foreach 然後是提前聲明好變數的for迴圈 最後是每次都要計算集合size的for
package test; import java.util.ArrayList;import java.util.HashMap;import java.util.Iterator;import java.util.List;import java.util.Map; /** * 效能測試 * @author LinSir * */public class PerformanceTest { public static void main(String[] args) { // TODO Auto-generated method stub List<Map<String, Object>> list=new ArrayList<Map<String,Object>>(); Map<String, Object> map=new HashMap<String, Object>(); map.put("name", "jams"); map.put("sex", true); map.put("age", 17); for (int i = 0; i <100000; i++) { list.add(map); } System.out.println("第一種起始時間："); long a11=System.currentTimeMillis(); long a1=System.nanoTime(); for (int i = 0; i < list.size(); i++) { list.get(i).get("name"); list.get(i).get("sex"); list.get(i).get("age"); /*list.get(i).put("a", 999); list.get(i).remove("sex");*/ } long a2=System.nanoTime(); long a22=System.currentTimeMillis(); System.out.println("第一種納秒差："+(a2-a1)); System.out.println("第一種毫秒差："+(a22-a11)); System.out.println("-----------------------------"); System.out.println("第二種起始時間："); long b11=System.currentTimeMillis(); long b1=System.nanoTime(); for (Map<String, Object> map2 : list) { map2.get("name"); map2.get("sex"); map2.get("age"); /*map2.put("a", 999); map2.remove("sex");*/ } long b2=System.nanoTime(); long b22=System.currentTimeMillis(); System.out.println("第2種納秒差："+(b2-b1)); System.out.println("第2種毫秒差："+(b22-b11)); System.out.println("-----------------------------"); System.out.println("第3種起始時間："); long c11=System.currentTimeMillis(); long c1=System.nanoTime(); for (Iterator<Map<String, Object>> iterator = list.iterator(); iterator.hasNext();) { Map<String, Object> map2 = (Map<String, Object>) iterator.next(); map2.get("name"); map2.get("sex"); map2.get("age"); /*map2.put("a", 999); map2.remove("sex");*/ } long c2=System.nanoTime(); long c22=System.currentTimeMillis(); System.out.println("第3種納秒差："+(c2-c1)); System.out.println("第3種毫秒差："+(c22-c11)); System.out.println("-----------------------------"); System.out.println("第4種起始時間："); long d11=System.currentTimeMillis(); long d1=System.nanoTime(); int listSize=list.size(); for (int i = 0; i < listSize; i++) { list.get(i).get("name"); list.get(i).get("sex"); list.get(i).get("age"); /*list.get(i).put("a", 999); list.get(i).remove("sex");*/ } long d2=System.nanoTime(); long d22=System.currentTimeMillis(); System.out.println("第4種納秒差："+(d2-d1)); System.out.println("第4種毫秒差："+(d22-d11)); System.out.println("-----------------------------"); } }
//依照以上耗時來判斷for iterator效能大於foreach大於for迴圈
