Java For Each 循环效率分析

循环方式

• for i

• for each

• while (iterator.hasNext()) {} / for( Iterator<Student> iterator = students.iterator();iterator.hasNext;){}

• students.forEach(p -> {...});

循环效率分析

测试代码

@Test
public void lamadaForEachTest() {
    List<Student> students = new ArrayList<>();
    List<Long> stat = new ArrayList<>();
    int max = Integer.MIN_VALUE;
    //添加数据
    for (int i = 0; i < 100000; i++) {
        students.add(new Student("s" + i, i));
    }
    System.out.println("start for i");
    long t = System.currentTimeMillis();
    //传统索引循环
    for (int i = 0; i < students.size(); i++) {
        max = Integer.max(max, students.get(i).getAge());
    }
    stat.add(System.currentTimeMillis() - t);
    System.out.println();
    System.out.println("end for i");
    System.out.println("start for each");
    t = System.currentTimeMillis();
    //for each 循环
    for (Student s : students) {
        max = Integer.max(max, s.getAge());
    }
    stat.add(System.currentTimeMillis() - t);
    System.out.println("end for each");
    System.out.println("start for iterator");
    t = System.currentTimeMillis();
    //迭代器循环
    Iterator<Student> iterator = students.iterator();
    while (iterator.hasNext()) {
        max = Integer.max(max, iterator.next().getAge());
    }
    stat.add(System.currentTimeMillis() - t);
    System.out.println("start for lamda");
    t = System.currentTimeMillis();
    int max1 = Integer.MIN_VALUE;
    //JDK8 forEach循环
    students.forEach(p -> {
        Integer.max(max1, p.getAge());
    });
    stat.add(System.currentTimeMillis() - t);
    System.out.println();
    System.out.println(stat);
    //1w-100w 数量级，前三种循环速度更快
    //1000W 数量级，四种方法循环速度差不多
}

@Data
@ToString
@Accessors(chain = true)
@AllArgsConstructor
class Student {

   private String name;
   private Integer age;
}

数据级(10^1)	循环耗时
1	[0, 0, 0, 79]
2	[0, 0, 0, 84]
3	[1, 0, 0, 63]
4	[2, 1, 1, 57]
5	[3, 29, 7, 53]
6	[15, 17, 17, 58]
7	[96, 93, 97, 137]
8	java.lang.OutOfMemoryError: GC overhead limit exceeded (PC-8G)

REFRENCES

1. Java8 Lambda表达式和流操作如何让你的代码变慢5倍

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