Java List遍历效率对比

遍历效率测试：

public class TestCollection {

    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        for (int i = 0; i < 3000 * 10000; i++) {
            list.add("list - " + i);
        }

        long time = System.currentTimeMillis();

        list.forEach(new Consumer<String>() {
            @Override
            public void accept(String str) {
                stringF(str);
            }
        });

        System.out.println("forEach *** " + (System.currentTimeMillis() - time));
        time = System.currentTimeMillis();

        for (String str : list) {
            stringF(str);
        }

        System.out.println("for : *** " + (System.currentTimeMillis() - time));
        time = System.currentTimeMillis();

        for (int i = 0; i < list.size(); i++) {
            stringF(list.get(i));
        }

        System.out.println("for size *** " + (System.currentTimeMillis() - time));
        time = System.currentTimeMillis();

        for (Iterator<String> it = list.iterator(); it.hasNext(); ) {
            stringF(it.next());
        }

        System.out.println("Iterator *** " + (System.currentTimeMillis() - time));
    }

    private static void stringF(String str) {
        if (str != null && str.trim().length() > 3) {
            String temp = str.substring(0, 2);
            if (temp.contains("s")) {
                temp = temp + "666";
            } else if (temp.contains("A")) {
                temp = temp + "555";
            } else if (temp.contains("b")) {
                temp = temp + "444";
            } else if (temp.contains("c")) {
                temp = temp + "333";
            } else if (temp.contains("d")) {
                temp = temp + "222";
            } else if (temp.contains("1")) {
                temp = temp + "555";
            } else if (temp.contains("2")) {
                temp = temp + "444";
            } else if (temp.contains("3")) {
                temp = temp + "333";
            } else if (temp.contains("4")) {
                temp = temp + "222";
            } else if (temp.contains("5")) {
                temp = temp + "555";
            } else if (temp.contains("7")) {
                temp = temp + "444";
            } else if (temp.contains("8")) {
                temp = temp + "333";
            } else if (temp.contains("9")) {
                temp = temp + "222";
            } else if (temp.contains("0")) {
                temp = temp + "555";
            } else if (temp.contains("q")) {
                temp = temp + "444";
            } else if (temp.contains("w")) {
                temp = temp + "333";
            } else if (temp.contains("e")) {
                temp = temp + "222";
            } else if (temp.contains("r")) {
                temp = temp + "555";
            } else if (temp.contains("t")) {
                temp = temp + "444";
            } else if (temp.contains("y")) {
                temp = temp + "333";
            } else if (temp.contains("u")) {
                temp = temp + "222";
            }
        }
    }

}

1. size=3000*10000, stringF() 方法不为空（执行如上 if-else）

forEach *** 3773
for : *** 2466
for size *** 2468
Iterator *** 2482

2. size=1000*10000, stringF() 方法不为空（执行如上 if-else）

forEach *** 1042
for : *** 956
for size *** 914
Iterator *** 923

3. size=3000*10000, stringF() 为空方法

forEach *** 142
for : *** 140
for size *** 143
Iterator *** 137

4. size=300*10000, stringF() 方法不为空（执行如上 if-else）

forEach *** 251
for : *** 286
for size *** 233
Iterator *** 244

5. size=300*10000, stringF() 为空方法

forEach *** 17
for : *** 18
for size *** 16
Iterator *** 17

6. size=3*10000, stringF() 方法不为空（执行如上 if-else）

forEach *** 20
for : *** 4
for size *** 4
Iterator *** 4

6. size=0.3*10000, stringF() 方法不为空（执行如上 if-else）

forEach *** 5
for : *** 2
for size *** 2
Iterator *** 1

7. size=0.03*10000, stringF() 方法不为空（执行如上 if-else）

forEach *** 4
for : *** 0
for size *** 1
Iterator *** 0

貌似for (int i = 0; i < list.size(); i++) 与for (Iterator<String> it = list.iterator(); it.hasNext(); ) 在各种情况下表现都比较稳定