setOnClickListener使我的android应用程序崩溃

当我试图在Android应用程序中实现登录屏幕时，我面临一个问题。用户必须用字符串user填写login和password字段。 每当我运行此代码时，我的android虚拟设备就会崩溃。它甚至没有打开，不幸的是，LoginScreen已停止，错误消息出现在设备屏幕上。这是我的代码：

我的主要活动

package com.example.loginscreen;

import android.app.Activity;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends Activity {

Button ok = null;
EditText login = null;
EditText pass = null;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState == null) {
        getFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }

    ok = (Button) findViewById(R.id.ok);  

    login = (EditText) findViewById(R.id.login_text); 
    pass = (EditText) findViewById(R.id.pass_text); 

    ok.setOnClickListener(new View.OnClickListener() { 
            public void onClick(View view) { 
                    String loginStr = login.getText().toString(); 
                    String passStr = pass.getText().toString(); 
                    if (loginStr != null && loginStr.equals("user")) { 
                            if (passStr != null && passStr.equals("user")) {
                                Toast.makeText(getApplicationContext(), "Redirecting...", 
                                Toast.LENGTH_SHORT).show();
                            } else {
                                Toast.makeText(getApplicationContext(), "Wrong Credentials",
                                Toast.LENGTH_SHORT).show();
                            }
                    }
            } 
    }); 
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        return rootView;
    }
}

}

梅因酒店

我在Virtual Box上使用Eclipse+ADT 22.3版本。 该应用程序具有以下配置

Minimum Required SDK: API 18
Target SDK: API 19
Compile With: API 19

请问我做错了什么？

试试这个

因为按钮，EditText都属于fragment_main.xml，所以您必须在PlaceholderFragment.java中这样做

---

您正在初始化活动中fragment_main.xml中的视图。相反，您应该使用rootView.findviewbyd在片段onCreateView中初始化它

findViewById在当前视图层次结构中查找视图。您已将activity_main.xml设置为activity。因此，如果在活动中初始化视图失败，将导致NUllPointerException

添加方法

myMethod() {

//Code hear

}

添加到活动和xml上

android:onClick="myMethod"

---

由于com.example.loginscreen.MainActivity.onCreateMainActivity.java:38上的java.lang.NullPointerException导致的按钮异常。那就是第38行。谢谢你的解释！

public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }
    Button ok = null;
    EditText login = null;
    EditText pass = null;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        ok = (Button) rootView.findViewById(R.id.ok);  

login = (EditText) rootView.findViewById(R.id.login_text); 
pass = (EditText) rootView.findViewById(R.id.pass_text); 

ok.setOnClickListener(new View.OnClickListener() { 
        public void onClick(View view) { 
                String loginStr = login.getText().toString(); 
                String passStr = pass.getText().toString(); 
                if (loginStr != null && loginStr.equals("user")) { 
                        if (passStr != null && passStr.equals("user")) {
                            Toast.makeText(getActivity(), "Redirecting...", 
                            Toast.LENGTH_SHORT).show();
                        } else {
                            Toast.makeText(getActivity(), "Wrong Credentials",
                            Toast.LENGTH_SHORT).show();
                        }
                }
        } 
}); 
        return rootView;
    }
}

public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }
    Button ok = null;
    EditText login = null;
    EditText pass = null;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        ok = (Button) rootView.findViewById(R.id.ok);  

        login = (EditText) rootView.findViewById(R.id.login_text); 
        pass = (EditText) rootView.findViewById(R.id.pass_text); 

        ok.setOnClickListener(new View.OnClickListener() { 
        public void onClick(View view) { 
                String loginStr = login.getText().toString(); 
                String passStr = pass.getText().toString(); 
                if (loginStr != null && loginStr.equals("user")) { 
                        if (passStr != null && passStr.equals("user")) {
                            Toast.makeText(getActivity(), "Redirecting...", 
                            Toast.LENGTH_SHORT).show();
                        } else {
                            Toast.makeText(getActivity(), "Wrong Credentials",
                            Toast.LENGTH_SHORT).show();
                        }
                }
        } 
     }); 
        return rootView;
    }
}

myMethod() {

//Code hear

}

android:onClick="myMethod"