调用在其他类中实现的接口方法时出现空指针异常 [英] null pointer exception on calling interface method implemented in other class

问题描述

我正在尝试调用接口 OnSelectedListener 的方法 getFailureDialog().该方法在MainActivity.java 中实现.但是当我调用该方法时，我得到了空指针异常.

I am trying to call the method getFailureDialog() of the interface OnSelectedListener. The method is implemented in MainActivity.java. But when I call the method, I am getting the null pointer exception.

我知道这是因为 OnSelectedListener 仍未初始化，而您正在对未初始化的对象调用 getFailureDialog().显然，接口方法永远不会被初始化.但是，我如何从我的类 Common.java 中调用方法 getFailureDialog()?

I know that its because OnSelectedListener is still not initialized and you are calling getFailureDialog() on uninitialized object. Obviously, interface methods are never initialized. But then how do I call the method getFailureDialog() from my class Common.java?

我只在下面放置相关的源代码-

I am placing only the relevant source code below-

源代码:

SharedFragment.java

public class SharedFragment extends DialogFragment 
{       
    Bundle bundle = getArguments();
    final String email = bundle.getString("email");

    Thread t=new Thread(new Runnable() 
    {
        public void run() {
        common.myRecord(email);
    }
    });   t.start(); 
}

Common.java

public class Common
{
OnSelectedListener mCallback;


    public interface OnSelectedListener 
    {
        public void getFailureDialog();
    }

    public void myRecord(String email)
    {
        mCallback.getFailureDialog();  //null pointer exception here
    }
}

MainActivity.java

public class MainActivity implements Common.OnSelectedListener
{

@Override
    public void getFailureDialog()
    {

        RecordFailure fd = new RecordFailure(); 
        fd.show(getSupportFragmentManager(), "dialog");
    }
}

错误日志

03-22 15:50:39.032: W/dalvikvm(20796): threadid=16: thread exiting with uncaught exception (group=0x4204c450)
03-22 15:50:39.052: E/AndroidRuntime(20796): FATAL EXCEPTION: Thread-30126
03-22 15:50:39.052: E/AndroidRuntime(20796): java.lang.NullPointerException
03-22 15:50:39.052: E/AndroidRuntime(20796):    at com.cornmail.util.Common.myRecord(Common.java:2062)

推荐答案

OnSelectedListener mCallback;

从未被初始化，或者被初始化为空值.

is never getting initialized, or is being initialized with a null value.

public class Common
{
    OnSelectedListener mCallback = new OnSelectedListener(){
        public void getFailureDialog(){
            JOptionPane.showMessageDialog(null, "An Error Has Occurred.");
        }
    };


    public interface OnSelectedListener 
    {
        public void getFailureDialog();
    }

    public void myRecord(String email)
    {
        mCallback.getFailureDialog();  //now this works.
    }
}

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