Question

我很难弄清楚为什么我在接收它的方法中分配它时，我的全班级变量为空。我曾尝试阅读有关变量，对象等的引用之间的区别的其他问题，但我无法真正解决这个问题。

public class Class1 {
  private TestActivity testActivity;

  // This line of code is ran when an event is triggered:
  // But  testActivity seems  to be null and I get
  // Caused by: java.lang.NullPointerException: 
  //Attempt to invoke virtual method on this line:
  testActivity.getBackData(var);

  public void setParam(TestActivity testA){
    // Running testA.getBackData(var) here works fine.  
    // But assigning this.testActivty makes testActivity 
    //null outside of this method.
    this.testActivity = testA;
  } 
}

活动：

public class TestActivity extends Activity {
  protected void onCreate...{
    Class1  c = new Class1();

    c.setParam(TestActivity.this);
  }

  public String getDataBack(String str){}

}

谢谢。

Answer 1

您尝试使用以下方法调用TestActivity方法：

public class Class1 {
  private TestActivity testActivity;

  // calling the method
  testActivity.getBackData(var);

}

这是一个逻辑错误。

您只能像这样在方法内部调用类的特定方法：

public class Class1 {
  private TestActivity testActivity;

  public void setParam(TestActivity testA){
    this.testActivity = testA;

    // calling the method
    testActivity.getBackData(var);
  } 
}

您的以下评论：

  // This line of code is ran when an event is triggered:
  // But  testActivity seems  to be null and I get
  // Caused by: java.lang.NullPointerException: 
  //Attempt to invoke virtual method on this line:
   testActivity.getBackData(var);

不是Java的工作方式。

将其分配给全班级变量并使用它时，作为参数传递的Class / Activity实例为null

1 个答案: