【Java】常用方法合集

以 DemoVo 为实体

import lombok.Data;
import com.alibaba.excel.annotation.ExcelProperty;
import com.alibaba.excel.annotation.ExcelIgnoreUnannotated;@Data
@ExcelIgnoreUnannotated
public class ExportPromoteUnitResult {private String id;@ExcelProperty(value = "学号")private String stuNo;@ExcelProperty(value = "姓名")private String stuName;
}

1. 将一个实体的属性名取出来，整合为一个 List<String[]>格式

    private static List<String[]> getData() {// new一个实体DemoVoDemoVo demo = new DemoVo();List<String[]> propertyNames = getPropertyNames(demo);return propertyNames;}public static List<String[]> getPropertyNames(Object obj) {List<String[]> properties = new ArrayList<>();Class<?> clazz = obj.getClass(); // 获取对象的Class对象Field[] fields = clazz.getDeclaredFields(); // 获取所有字段for (Field field : fields) {if (field.isAccessible()) { // 如果字段可访问，则添加其名称到列表中（对于私有字段可能需要设置可访问）properties.add(new String[]{field.getName()}); // 将属性名添加到List<String[]>中，每个String[]包含一个属性名} else {field.setAccessible(true); // 对于私有字段，需要设置可访问才能获取其名称（注意：这可能会破坏封装性）properties.add(new String[]{field.getName()}); // 添加属性名到List<String[]>中}}return properties; // 返回包含所有属性名的List<String[]>}

2. 将一个实体的属性名整合为 List< String > 格式

	// new一个实体DemoVo,取出属性名DemoVo demo = new DemoVo();List<String[]> propertyNames = getPropertyNames(demo.getClass());System.out.print("propertyNames的输出结果为：" + propertyNames); // propertyNames的输出结果为：[id,stuNo,stuName]// 获取实体类的所有字段名public List<String> getPropertyNames(Class<?> clazz) {Field[] fields = clazz.getDeclaredFields(); // 获取所有字段List<String> propertyNames = new ArrayList<>(); // 新建一个数组propertyNamesfor (Field field : fields) {propertyNames.add(field.getName()); // 将所有字段放于数组propertyNames中}return propertyNames;}

3. 根据属性名/字段名获取对应的值

    List<Map<String, Object>> maps = combinFixedLists(stuClassId);System.out.print("maps的输出结果为：" + maps);// maps的输出结果为：[{id=1,stuNo="2024001",stuName="李四",stuSex="1"},{id=2,stuNo="2024002",stuName="张三",stuSex="1"}]private List<Map<String, Object>> combinFixedLists(Long stuClassId) throws IllegalAccessException, NoSuchFieldException {// 先根据stuClassId查询数据,此处不过多写sql语句了List<StudentClassResult> listObjects = this.queryStuResult(stuClassId);System.out.print("假如得出的结果为：" + listObjects);// 假如得出的结果为：[{id:1,stuNo:"2024001",stuName:"李四",stuSex:"1"},{id:2,stuNo:"2024002",stuName:"张三",stuSex:"1"}]List<Map<String, Object>> combinedList = new ArrayList<>();for (StudentClassResult object : listObjects) {Map<String, Object> fieldMap = new HashMap<>();Field[] fields = object.getClass().getDeclaredFields();for (Field field : fields) {String name = field.getName();field.setAccessible(true);fieldMap.put(field.getName(), field.get(object));}combinedList.add(fieldMap);}return combinedList;}

4. 去掉List< String >中指定元素

	removePropertyFromString(propertyNames, "id");// 去掉指定属性名private static void removePropertyFromString(List<String> fixList, String fixElement) {Iterator<String> iterator = fixList.iterator();while (iterator.hasNext()) {String next = iterator.next();if (next == fixElement) {iterator.remove();}}}

5. 取出JSON中的某属性名对应的属性值

	System.out.print(stuData)// stuData={"stuData":{"stuId":57,"photoList":{"fileName":["照片.png"],"filePath":["/qcbucket/2024/10/22/01.照片.png"]},"stuClassId":"3","stuName":"张三"}}Object stuId = null;Object photoList = null;Object stuClassId = null;Object stuName = null;JSONObject jsonObject = JSONObject.parseObject(stuData.toString());stuId = jsonObject.getJSONObject("stuData").get("stuId ");// 57photoList = jsonObject.getJSONObject("stuData").get("photoList");// "fileName":["照片.png"],"filePath":["/qcbucket/2024/10/22/01.照片.png"]stuClassId = jsonObject.getJSONObject("stuData").get("stuClassId");// "3"stuName = jsonObject.getJSONObject("stuData").get("stuName");// "张三"

6. 保留两位数

（1）setScale

String num = "123.4567";
BigDecimal val = new BigDecimal(num);
BigDecimal roundedVal = val.setScale(2, RoundingMode.HALF_UP);
System.out.println(roundedVal); // 输出: 123.46

（2）sql语句

7. 原数组 String[] originalArray，在指定元素后添加n个空字符串 “”

    // 原数组originalArray = new String[]{"apple","banana"}// 查找指定字符串String[] target = {"apple"}// 在指定字符串后追加num个 空字符串int num = 2;String[] array = modifyArray(originalArray, target, num);System.out("array的结果为：" + array);// array的结果为：["apple","","","banana"]private static String[] modifyArray(String[] originalArray, String[] target, int n) {List<String> resultList = new ArrayList<>();for (String element : originalArray) {resultList.add(element);for (String add : target) {if (element.equals(add)) {for (int i = 0; i < n; i++) {resultList.add("");}}}}return resultList.toArray(new String[0]);}

8. 获取 resources 下 指定文件的路径
   

URL resource = getClass().getClassLoader().getResource("ip2region.xdb");
String XDB_PATH = resource.getPath();