android - 如何从一个 Activity 调用非静态方法到另一个 Activity

Question

我有两个 Activity ，我想从 B 中调用一个清除方法。我这样试过，但出现空指针异常。

class A extends Activity {
    ...
    public void clear(){}

}

class B extends Activity{
    public void clearData() {
        A a=new A();
        a.clear();
    }        
}

Answer 1

class A extends Activity{
static A instance;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    instance = this;
}

public static A getInstance() {
    return instance;
}
....
public void clear(){}

}

在 B 类中:

class B extends Activity {
  public void  clearData(){
      A a = A.getInstance();
      a.clear();
    }
}