首先先声明两个类

public class Person {    ..... }  public class Student {     public Person A;     public Student(Person a) {         A = a;     }     ..... }

MainActivity在实现抽象activity时出现空的返回值

public abstract class Main2Activity extends AppCompatActivity{     Student B = getStudent();     oncreate(...){     ......         getStudent().getA();         //A不为null         B.getA();         //A为null     }     public abstract Student getStudent(); }

MainActivity

public class MainActivity extends Main2Activity{     Person person = new Person(10);     @Override     public Student getStudent() {         Student student = new Student(person);         return student;     } }

为什么会出现成员变量A为null的情况,但是在声明周期中赋值时并不会出现这个问题.两种情况下成员变量B都不会是null,请问是我哪里理解错误了.