1 stream常见用法

List<DTO> 转 List<String>

List<String> ids = dtos.stream()
                        .map(DTO::getId)
                        .distinct()
                        .collect(Collectors.toList());

List<DTO> 转 Map<String, List<DTO>>

Map<String, List<DTO>> map = dtos.stream()
                                  .collect(Collectors.groupingBy(DTO::getId));

List<DTO> 转 Map<key, value>

Map<Integer,String> map = dtos.stream()
                               .collect(Collectors.toMap(DTO::getId, DTO::getName));

注意

如果值为空，会报空指针异常，可采用以下方法实现转Map。

Map<String, String> map = dtos.stream().collect(
        HashMap::new, 
        (map, dto) -> map.put(dto.getId(), dto.getName()), 
        HashMap::putAll);

List<DTO> 转 Map<key, DTO>

// 指定key-value，value是对象本身。
// Function.identity()是简洁写法，也是返回对象本身。
// key冲突的解决办法，这里选择第二个key覆盖第一个key
Map<Integer, DTO> map = dtos.stream()
                            .collect(
                                Collectors.toMap(
                                    DTO::getId, Function.identity(), (key1,key2)->key2
                                )
                             );

List<DTO> 转 String拼接

String str = dtos.stream()
                  .map(DTO::getId)
                  .collect(Collectors.joining(","));

List<DTO> 求和

// 使用mapToInteger、mapToDouble等

Integer returnCount = dtos.stream()
                           .mapToInteger(DTO::getNum)
                           .sum(); 

Double returnCount = dtos.stream()
                          .mapToDouble(DTO::getNum)
                          .sum();

List<DTO> 排序

// 使用mapToInteger、mapToDouble等

List<DTO> newDTOS = dtos.stream()
                        .sorted(Comparator.comparing(DTO::getSortNum, Comparator.reverseOrder()))
                        .collect(Collectors.toList()); 

2 类相同属性比较

public class Test {
  public static void main(String[]args)throws NoSuchFieldException {
    Map<String, String> map1 = new HashMap<>();
    Field[] fields = OmsOrderApiVO.class.getDeclaredFields();
    for (Field field : fields) {
      String fieldName = field.getName();
      ApiModelProperty annotation = field.getAnnotation(ApiModelProperty.class);
      String val = "-";
      if (annotation != null) {
        val = annotation.value();
      }
      map1.put(fieldName, val);
    }

    Map<String, String> map2 = new HashMap<>();
    Field[] fields2 = OmsWaybillApiDTO.class.getDeclaredFields();
    for (Field field2 : fields2) {
      String fieldName = field2.getName();
      ApiModelProperty annotation = field2.getAnnotation(ApiModelProperty.class);
      String val = "-";
      if (annotation != null) {
        val = annotation.value();
      }
      map2.put(fieldName, val);
    }
  }
}

3 驼峰字符串转下划线字符串

public class Test {
  public static void main(String[] args) throws NoSuchFieldException {
    Map<String, String> map1 = new HashMap<>();
    Field[] fields = SpmCashBillDetail.class.getDeclaredFields();
    for (Field field : fields) {
      String fieldName = field.getName();
      System.out.println(toUnderlineCase(fieldName).toUpperCase());
    }
  }

  public static String toUnderlineCase(String camelCaseStr) {
    if (camelCaseStr == null) {
      return null;
    }
    // 将驼峰字符串转换成数组
    char[] charArray = camelCaseStr.toCharArray();
    StringBuffer buffer = new StringBuffer();
    //处理字符串
    for (int i = 0, l = charArray.length; i < l; i++) {
      if (charArray[i] >= 65 && charArray[i] <= 90) {
        buffer.append("_").append(charArray[i] += 32);
      } else {
        buffer.append(charArray[i]);
      }
    }
    return buffer.toString();
  }
}

4 Java输入输出流的互相转换

public class Test {
  File file = new File("D:\\test\\1.txt");
  BufferedInputStream bis = null;
  ByteArrayOutputStream arrayOutputStream = null;
  OutputStream bos = null;
  try {
    // 模拟读入一个文件，作为输入流
    bis = new BufferedInputStream(new FileInputStream(file), 1024);
    // 创建ByteArray输出流，所有的输出可以输出到ByteArray中，可以替代一个文件
    arrayOutputStream = new ByteArrayOutputStream();
    //用buffered包装一下，为了提高效率使用缓冲区流
    bos = new BufferedOutputStream(arrayOutputStream);

    int len = -1;
    //读取文件输入流，写入到输出流ByteArray中，输入流转成了输出流
    byte[] buf = new byte[1024];
    while ((len = bis.read(buf)) != -1) {
      bos.write(buf, 0, len);
    }
    bos.flush();//清空缓冲区（非必要）
    //创建ByteArrayResource用ByteArray输出流的字节数组
    InputStreamSource inputStreamSource = new ByteArrayResource(arrayOutputStream.toByteArray());
    //至此把OutputStream已经转换成了InputStreamSource，输出流又转成了输入流
    // 又将输入流转成了输出流
    BufferedInputStream bufferedInputStream = new BufferedInputStream(inputStreamSource.getInputStream());
    int bytesRead;
    byte[] buffer = new byte[4096];
    while ((bytesRead = bufferedInputStream.read(buffer)) != -1) {
      // 将文件发送到客户端
      bos.write(buffer, 0, bytesRead);
      bos.flush();
    }
  } catch(Exception e) {
    System.out.println(e);
  } finally {
    if (bos != null) {
      bos.close();//关闭BufferedOutputStream输出流
    }
    if (arrayOutputStream != null) {
      arrayOutputStream.close();//关闭ByteArray输出流
    }
    if (bis != null) {
      bis.close();
    }
  }
}