Three ways to reverse the order of Java array elements (summary)

There are many ways to reverse array elements. Here are three common ways

Direct array element exchange

@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));
}

Use ArrayList: ArrayList is stored and retrieved in the same order. You can use this feature to temporarily store array elements

@Test
public void testArrayList() throws Exception {
  System.out.println("use ArrayList method");

  String[] strings = { "ramer","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));
}

Using collections and arrays utility classes

@Test
public void testCollectionsReverse() throws Exception {
  System.out.println("use Collections.reverse() method");

  String[] strings = { "ramer","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','d'};
  Collections.reverse(Arrays.asList(strings));
  System.out.println("\t" + Arrays.toString(strings));
}

Speed test:

@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 <T> the generic type
 * @param clazz the clazz
 * @param methodName the method name
 */
public <T> void recordTime(Class<T> 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");
}

Test results:

Summary: it is easier to invert array elements using collections and arrays tool classes, but it is faster and takes up the least memory when operating on the original array

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