Java数组倒序的三种写法

将数组元素反转有多种实现方式,这里介绍常见的三种.

• 直接数组元素对换

  @Test
  public void testReverseSelf() throws Exception {
    System.out.println("use ReverseSelf");
    String[] strings = { "ramer", "jelly", "bean", "cake" };
    System.out.println("\t" + Arrays.toString(strings));
    for (int start = 0, end = strings.length - 1; start < end; start++, end--) {
        String temp = strings[end];
        strings[end] = strings[start];
        strings[start] = temp;
    }
    System.out.println("\t" + Arrays.toString(strings));
  }

• 使用ArrayList: ArrayList存入和取出的顺序是一样的,可以利用这里特性暂时存储数组元素.

  @Test
  public void testArrayList() throws Exception {
    System.out.println("use ArrayList method");
    String[] strings = { "ramer", "jelly", "bean", "cake" };
    System.out.println("\t" + Arrays.toString(strings));
    List<String> list = new ArrayList<>(strings.length);
    for (int i = strings.length - 1; i >= 0; i--) {
        list.add(strings[i]);
    }
    strings = list.toArray(strings);
    System.out.println("\t" + Arrays.toString(strings));
  }

• 使用Collections和Arrays工具类

  @Test
  public void testCollectionsReverse() throws Exception {
    System.out.println("use Collections.reverse() method");
    String[] strings = { "ramer", "jelly", "bean", "cake" };
    System.out.println("\t" + Arrays.toString(strings));
    // 这种方式仅针对引用类型,对于基本类型如:
    // char[] cs = {'a','b','c','g','d'};
    // 应该定义或转换成对应的引用类型: 
    // Character[] cs = {'a','b','c','g','d'};
    Collections.reverse(Arrays.asList(strings));
    System.out.println("\t" + Arrays.toString(strings));
  }

• 速度测试: ```java @Test public void testTimeDuration() throws Exception { recordTime(ArrayReverse.class,”testCollectionsReverse”); recordTime(ArrayReverse.class,”testArrayList”); recordTime(ArrayReverse.class,”testReverseSelf”); }

private static String[] strings = new String[1000000]; { for (int i = 0; i < 1000000; i++) { strings[i] = String.valueOf(i); } } /**

• 记录操作执行总时间. *
• @param the generic type
• @param clazz the clazz

• @param methodName the method name */ public void recordTime(Class clazz, String methodName) { long start = System.currentTimeMillis(); System.out.println(“start: “ + start);

  Method[] declaredMethods = clazz.getDeclaredMethods(); for (Method method : declaredMethods) {

   String name = method.getName();
   if (name.equals(methodName)) {
       try {
           method.invoke(clazz.newInstance());
       } catch (Exception e) {
           e.printStackTrace();
       }
   }

  } long end = System.currentTimeMillis(); System.out.println(“end: “ + end); System.out.println(“duration: “ + (end - start) + “ ms”); } ```

• 测试结果:使用Collections和Arrays工具类: 12 ms
  使用ArrayList: 7 ms
  直接数组元素对换: 4 ms
  当数据量越来越大时,使用ArrayList的方式会变得很慢.
  直接使用数组元素对换,总是最快完成.
• 总结: 使用Collections和Arrays工具类反转数组元素更简单,但是在原数组上操作时速度更快,并且占用最少的内存.

原文链接：https://www.cnblogs.com/QQ846300233/p/6683843.html