如何從一個活動調用非靜態方法到另一個活動

Question

我有兩個活動&我想調用一個方法清除B. 我試過這樣，我得到空指針異常。

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(){} 

} 

和B類：

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