第一个选项应该更快。您可以通过在使用之前调整集合大小来使其更快。通常,如果您预计会有少量重复:
Set<String> undefined = new HashSet<String>(pairs.size(), 1);
请注意,我使用 1 作为负载因子以防止任何调整大小。
出于好奇,我进行了测试(下面的代码) - 结果是(编译后):
测试 1(注意:预热需要几分钟)
原始列表的大小 = 3,000,没有重复:
设置:8数组列表
:668
链接列表:1166
测试 2
原始列表的大小 = 30,000 - 所有字符串相同:
设置:25数组列表
:11
链接列表:13
这种说法是有道理的:
- 当有很多重复时,
List#contains
将运行得相当快,因为会更快地找到重复并且分配大集合的成本+散列算法正在惩罚
- 当没有重复或重复很少时,该组以很大的优势获胜。
public class TestPerf {
private static int NUM_RUN;
private static Random r = new Random(System.currentTimeMillis());
private static boolean random = false; //toggle to false for no duplicates in original list
public static void main(String[] args) {
List<String> list = new ArrayList<>();
for (int i = 0; i < 30_000; i++) {
list.add(getRandomString());
}
//warm up
for (int i = 0; i < 10_000; i++) {
method1(list);
method2(list);
method3(list);
}
NUM_RUN = 100;
long sum = 0;
long start = System.nanoTime();
for (int i = 0; i < NUM_RUN; i++) {
sum += method1(list);
}
long end = System.nanoTime();
System.out.println("set: " + (end - start) / 1000000);
sum = 0;
start = System.nanoTime();
for (int i = 0; i < NUM_RUN; i++) {
sum += method2(list);
}
end = System.nanoTime();
System.out.println("arraylist: " + (end - start) / 1000000);
sum = 0;
start = System.nanoTime();
for (int i = 0; i < NUM_RUN; i++) {
sum += method3(list);
}
end = System.nanoTime();
System.out.println("linkelist: " + (end - start) / 1000000);
System.out.println(sum);
}
private static int method1(final List<String> list) {
Set<String> set = new HashSet<>(list.size(), 1);
for (String s : list) {
set.add(s);
}
return set.size();
}
private static int method2(final List<String> list) {
List<String> undefined = new ArrayList<>();
for (String s : list) {
if (!undefined.contains(s)) {
undefined.add(s);
}
}
return undefined.size();
}
private static int method3(final List<String> list) {
List<String> undefined = new LinkedList<>();
Iterator<String> it = list.iterator();
while (it.hasNext()) {
String value = it.next();
if (!undefined.contains(value)) {
undefined.add(value);
}
}
return undefined.size();
}
private static String getRandomString() {
if (!random) {
return "skdjhflkjrglajhsdkhkjqwhkdjahkshd";
}
int size = r.nextInt(100);
StringBuilder sb = new StringBuilder();
for (int i = 0; i < size; i++) {
char c = (char) ('a' + r.nextInt(27));
sb.append(c);
}
System.out.println(sb);
return sb.toString();
}
}