目錄

• Java For Each 迴圈效率分析
• 迴圈方式
• 迴圈效率分析
• REFRENCES
• 更多

Java For Each 迴圈效率分析

迴圈方式

• for i

• for each

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

• students.forEach(p -> {…});

迴圈效率分析

測試程式碼

@Test
public void lamadaForEachTest() {
    List<Student> students = new ArrayList<>();
    List<Long> stat = new ArrayList<>();
    int max = Integer.MIN_VALUE;
    //新增資料
    for (int i = 0; i < 100000; i++) {
        students.add(new Student("s" + i, i));
    }
 

    System.out.println("start for i");
    long t = System.currentTimeMillis();
    //傳統索引迴圈
    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 迴圈
    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();
    //迭代器迴圈
    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迴圈
    students.forEach(p -> {
        Integer.max(max1, p.getAge());
    });
    stat.add(System.currentTimeMillis() - t);
    System.out.println();
    System.out.println(stat);
    //1w-100w 數量級，前三種迴圈速度更快
    //1000W 數量級，四種方法迴圈速度差不多
}

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

   private String name;
   private Integer age;
}

REFRENCES

1. Java8 Lambda表示式和流操作如何讓你的程式碼變慢5倍

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