如何设置组合键并从中获取价值

Question

我想设置的值cid使用学生类SubjectMark->private String cid; 如何在我的控制器中设置和获取价值。 实体和控制器的方法如下：

@Entity
public class Student implements Serializable {
    private static final long serialVersionUID = 1L;
    @EmbeddedId
    private SubjectMark id;
    private String fullName;
    private Integer totalMarks;
    private Double percentage;
    private String grade;
    //Setters and getters
}


//Composit class
@Embeddable
public class SubjectMark implements Serializable {
//Composit key
private String cid;
//Setters and getters

}

在我的控制器中，我尝试像这样设置值：

@RequestMapping(value="getstdata",method=RequestMethod.GET)
@ResponseBody
public String getstdata(@RequestParam(value="cid")String cid){  
//Some code
try{

Student st=new Student();
st.getId().setCid(cid);//Set value like this but it is getting null pointer exception
//some code

//retuen some value
}

请帮我！

Answer 1

您需要在您的类中编写一个get set方法...尝试编写类似

私有字符串cid;

公共字符串Cid {get => cid; 设置=> CID =值; }

Answer 2

问题的第一部分：

我正在尝试使用SubjectMark-> private String cid在学生班级中设置值cid; 如何在我的控制器中设置和获取价值。

@Entity
@Table
public class Student implements Serializable {
private static final long serialVersionUID = 1L;
@EmbeddedId
private SubjectMark subjectMarkId;
private String otherField;
// setters, getters
}

//Composite class
@Embeddable
public class SubjectMark implements Serializable {
private String cId;
// setter, getter
}


//Controller
@GetMapping(value = "getstdata")
public String getStData(@RequestParam(value="cid") String cid) {
Student student = new Student();

student.setSubjectMark(new SubjectMark());
student.getSubjectMark().setCId(cid);//cid value dynamic
// some other code
return "";
}

问题的第二部分：

现在，当您尝试从null对象调用方法（setter或getter）时，发生null异常的原因之一。