java - 代码中的 NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 6 年前。

我正在尝试建立 Oracle 连接。但是这个 NullPointerException 真的让我烦透了。
请提供任何帮助，我们将不胜感激。
帮助我解决任何错误。

这是我的代码:

public class MainActivity extends AppCompatActivity {

public Button btn_connection;
public TextView tv;
public OracleConnection oc;
public String Result;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    tv = (TextView) findViewById(R.id.textView);
    try {

        btn_connection = (Button) findViewById(R.id.connectionAttempt);
        btn_connection.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Result = oc.ConnectionMade();
                tv.setText(Result);
            }
        });

    }
    catch(NullPointerException e) {
        tv.setText("Doll");

    }

}



}

主要 Activity 布局:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.tahir.onlineshopping12.MainActivity">


<TextView
    android:id="@+id/textView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="65dp" />

<Button
    android:id="@+id/connectionAttempt"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_centerVertical="true"
    android:text="Connect" />

OracleConnection 类:

public class OracleConnection {



public String ConnectionMade(){
        try {
            Class.forName("oracle.jdbc.OracleDriver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

        Connection co = null;
        String Result = " ";

        try{
            co = DriverManager.getConnection(ipAddress,user,pass);
            DriverManager.registerDriver(new    oracle.jdbc.driver.OracleDriver());
            Result = "Successfully Connected";


        } catch (SQLException e) {

            Result = "Not Connected!!!!!!!!!" + e.toString();
        }


    return Result;

}

这是错误

java.lang.NullPointerException at com.example.tahir.onlineshopping12.MainActivity$1.onClick(MainActivity.java:28) at android.view.View.performClick(View.java:4461) at android.view.View$PerformClick.run(View.java:18514) at android.os.Handler.handleCallback(Handler.java:733) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:136) at android.app.ActivityThread.main(ActivityThread.java:5129) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:515) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:606) at dalvik.system.NativeStart.main(Native Method)

Answer 1

您声明类变量:

公共(public) OracleConnection oc；

但没有实例化，尝试使用它。

结果 = oc.ConnectionMade();