尝试在空对象引用上调用虚方法'void android.widget.EditText.setText（java.lang.CharSequence）'

Question

我是Android开发的新手，在这里我尝试使用sqlite创建一个简单的数据库应用程序 我想要做的是当用户选择搜索它将要求id并且匹配id的相应数据应该替换该特定数据的editText字段

ActivityMain.xml

    <?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.ashish.project.MainActivity">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Personal Details"
        android:id="@+id/textView"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:gravity="center" />
<ScrollView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fillViewport="true">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">
        <RelativeLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent">
    <GridLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/gridLayout"
        android:layout_centerVertical="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true">

        <LinearLayout
            android:orientation="horizontal"
            android:layout_width="match_parent"
            android:layout_height="50dp"
            android:layout_row="0"
            android:layout_column="0">

            <EditText
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:inputType="textPersonName"
                android:ems="10"
                android:id="@+id/editText"
                android:layout_weight="1"
                android:hint="First Name" />

            <EditText
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:inputType="textPersonName"
                android:ems="10"
                android:id="@+id/editText_LastName"
                android:layout_weight="1"
                android:hint="Last Name" />
        </LinearLayout>

        <EditText
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:inputType="number"
            android:ems="10"
            android:id="@+id/editText_number"
            android:layout_row="1"
            android:layout_column="0"
            android:hint="Contact Number" />

        <EditText
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:inputType="textEmailAddress"
            android:ems="10"
            android:id="@+id/editText_email"
            android:layout_row="2"
            android:layout_column="0"
            android:hint="E-mail" />

        <RelativeLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_row="3"
            android:layout_column="0">

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Submit Details"
                android:id="@+id/Button_Submit"
                android:layout_weight="1"
                android:layout_row="4"
                android:layout_column="0" />

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Show Details"
                android:id="@+id/button_Show_Details"
                android:layout_gravity="right"
                android:layout_weight="1"
                android:layout_row="5"
                android:layout_column="0"
                android:layout_alignParentTop="true"
                android:layout_alignParentRight="true"
                android:layout_alignParentEnd="true" />
            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:id="@+id/button_search"
                android:layout_below="@id/Button_Submit"
                android:text="search"/>

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Clear Table"
                android:id="@+id/button_clear"
                android:gravity="left|center_horizontal"
                android:layout_below="@+id/button_Show_Details"
                android:layout_alignParentRight="true"
                android:layout_alignParentEnd="true" />
        </RelativeLayout>

    </GridLayout>

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="number"
        android:ems="10"
        android:id="@+id/editText_srch"
        android:hint="Enter id to search"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="search"
        android:id="@+id/button_show_id_result"
        android:layout_alignParentBottom="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true" />
        </RelativeLayout>
    </LinearLayout>
</ScrollView>
</RelativeLayout>

MainActivity.java

   package com.example.ashish.project;

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.inputmethod.InputMethodManager;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    Button button_submit,button_show_details,button_clear,button_search,button_show_id_result;
    EditText firstName,lastName,number,email,editText_id_search;
    String firstnameString,lastnameString,emailString,num;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        button_submit=(Button)findViewById(R.id.Button_Submit);
        button_show_id_result=(Button) findViewById(R.id.button_show_id_result);
        editText_id_search=(EditText) findViewById(R.id.editText_srch);
        button_show_details=(Button)findViewById(R.id.button_Show_Details);
        firstName=(EditText)findViewById(R.id.editText);
        button_search=(Button) findViewById(R.id.button_search);
        button_clear=(Button)findViewById(R.id.button_clear);
        lastName=(EditText)findViewById(R.id.editText_LastName);
        number=(EditText)findViewById(R.id.editText_number);
        email=(EditText)findViewById(R.id.editText_email);

        editText_id_search.setVisibility(View.GONE);
        button_show_id_result.setVisibility(View.GONE);

        button_clear.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                DatabaseHelper db = new DatabaseHelper(getApplicationContext());
                db.clear();
            }
        });



        button_search.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
/*
*/
                editText_id_search.setVisibility(View.VISIBLE);
                button_show_id_result.setVisibility(View.VISIBLE);

                button_show_id_result.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        String  qry = editText_id_search.getText().toString();
                        DatabaseHelper db = new DatabaseHelper(getApplicationContext());
                        db.search(qry);
                    }
                });
            }
        });
        button_submit.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                firstnameString = firstName.getText().toString();
                lastnameString=lastName.getText().toString();
                num=number.getText().toString();
                emailString=email.getText().toString();

                DatabaseHelper db = new DatabaseHelper(getApplicationContext());
                db.insertData(firstnameString,lastnameString,num,emailString);
                Toast.makeText(MainActivity.this,"Record Updated",Toast.LENGTH_LONG).show();
                InputMethodManager input = (InputMethodManager) getSystemService(INPUT_METHOD_SERVICE);
                input.hideSoftInputFromWindow(email.getWindowToken(),0);
                firstName.setText("");
                lastName.setText("");
                number.setText("");
                email.setText("");

                db.close();

            }
        });

        button_show_details.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent i = new Intent(MainActivity.this,List_Result.class);

                startActivity(i);
            }
        });


    }



    public void show(String string, String cString, String s, String string1) {

        firstName.setText(string);
        lastName.setText(cString);
        number.setText(s);
        email.setText(string1);
    }
}

DatabaseHelper.java

package com.example.ashish.project;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

import java.util.ArrayList;

/**
 * Created by ashis on 7/21/2016.
 */
public class DatabaseHelper extends SQLiteOpenHelper {

    private static int DATABASE_VERSION = 6;
    private static String DATABSE_NAME="DATA";
    private static String TABLE_NAME="PERSONAL_INFORMATION";
    private static String COLLUMN_ID= "_ID";
    private static String COLLUMN_FIRST_NAME="FIRST_NAME";
    private static String COLLUMN_LAST_NAME="LAST_NAME";
    private static String COLLUMN_NUMBER = "NUMBER";
    private static String COLLUMN_EMAIL="EMAIL";

    public DatabaseHelper(Context context)
    {
        super(context,DATABSE_NAME,null,DATABASE_VERSION);

    }


    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {

        String CREATE_TABLE = "CREATE TABLE "+TABLE_NAME + "( "
                                + COLLUMN_ID +" INTEGER PRIMARY KEY, "
                                + COLLUMN_FIRST_NAME + " TEXT, "
                                + COLLUMN_LAST_NAME + " TEXT, "
                                + COLLUMN_NUMBER + " TEXT , "
                                + COLLUMN_EMAIL + " TEXT"+" )";
        sqLiteDatabase.execSQL(CREATE_TABLE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {

        sqLiteDatabase.execSQL("DROP TABLE IF EXISTS"+TABLE_NAME);
        this.onCreate(sqLiteDatabase);

    }


    public void insertData(String firstnameString, String lastnameString, String num, String emailString) {

        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(COLLUMN_FIRST_NAME,firstnameString);
        values.put(COLLUMN_LAST_NAME,lastnameString);
        values.put(COLLUMN_NUMBER,num);
        values.put(COLLUMN_EMAIL,emailString);
        db.insertOrThrow(TABLE_NAME,null,values);
        db.close();
    }

    public ArrayList<String> readData() {

        ArrayList<String> list = new ArrayList<>();
        String readQry = "SELECT * FROM "+TABLE_NAME;
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor c = db.rawQuery(readQry,null);
        if (c.moveToFirst())
        {
            do {
                list.add(c.getString(0)+ ": "+c.getString(1) +" "+ c.getString(2)+","+c.getString(3)+","+c.getString(4));


            }while (c.moveToNext());

        }
        c.close();
        db.close();



        return list;

    }

    public void clear() {
        SQLiteDatabase db= this.getWritableDatabase();
        String clrqry = "DROP TABLE " + TABLE_NAME;
        db.execSQL(clrqry);

        this.onCreate(db);

    }

    public void search(String srch) {

        SQLiteDatabase db = this.getReadableDatabase();
        MainActivity obj = new MainActivity();
        String sqlqry = "SELECT * FROM "+TABLE_NAME +"  WHERE  "+ COLLUMN_ID + " = ?";
        Cursor c=    db.rawQuery(sqlqry,new  String[]{srch});
        if (c.moveToFirst())
        {
            do {

                        obj.show(c.getString(1),c.getString(2),c.getString(3),c.getString(4));
            }
            while (c.moveToNext());
        }

    }
}

Activity_list.xml

<?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.ashish.project.List_Result">

    <ListView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/listView2"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:background="@android:color/darker_gray" />
</RelativeLayout>

Activity_list.java

package com.example.ashish.project;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import java.util.ArrayList;

public class List_Result extends AppCompatActivity {

    ListView listview1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_list__result);
        listview1=(ListView) findViewById(R.id.listView2);


        loadlistviewData();
    }

    private void loadlistviewData() {


        DatabaseHelper db = new DatabaseHelper(getApplicationContext());
        ArrayList<String> list = db.readData();
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(getApplicationContext(),android.R.layout.simple_list_item_1,list);
        adapter.setDropDownViewResource(android.R.layout.simple_list_item_1);
        listview1.setAdapter(adapter);

    }
}

这是我输入id

后点击搜索时的logcat

07-25 14:27:34.033 668-668/com.example.ashish.project W/System: ClassLoader referenced unknown path: /data/app/com.example.ashish.project-2/lib/arm
07-25 14:27:34.256 668-668/com.example.ashish.project W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
07-25 14:27:34.541 668-668/com.example.ashish.project D/android.widget.GridLayout: vertical constraints: y4-y0>=864, y4-y3<=192, y3-y2<=91, y2-y1<=91, y1-y0<=100 are inconsistent; permanently removing: y4-y3<=192. 
07-25 14:27:39.235 668-668/com.example.ashish.project D/AndroidRuntime: Shutting down VM
07-25 14:27:39.236 668-668/com.example.ashish.project E/AndroidRuntime: FATAL EXCEPTION: main
                                                                        Process: com.example.ashish.project, PID: 668
                                                                        java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.EditText.setText(java.lang.CharSequence)' on a null object reference
                                                                            at com.example.ashish.project.MainActivity.show(MainActivity.java:103)
                                                                            at com.example.ashish.project.DatabaseHelper.search(DatabaseHelper.java:108)
                                                                            at com.example.ashish.project.MainActivity$2$1.onClick(MainActivity.java:60)
                                                                            at android.view.View.performClick(View.java:5201)
                                                                            at android.view.View$PerformClick.run(View.java:21163)
                                                                            at android.os.Handler.handleCallback(Handler.java:746)
                                                                            at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                            at android.os.Looper.loop(Looper.java:148)
                                                                            at android.app.ActivityThread.main(ActivityThread.java:5443)
                                                                            at java.lang.reflect.Method.invoke(Native Method)
                                                                            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
                                                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)

任何帮助将不胜感激

Answer 1

您可以通过创建新的MainActivity实例来调用 show（）。因此，MainActivity的所有变量都将为null。最好你应该使这些静态，以便所有MainActivity实例的初始化相同。尝试

static EditText firstName,lastName,number,email,editText_id_search;

Answer 2

不要在View小部件中使用静态。

将DatabaseHelper.java中的搜索功能替换为下面的一个：

public void search(String srch, MainActivity activity) {

    SQLiteDatabase db = this.getReadableDatabase();
    String sqlqry = "SELECT * FROM "+TABLE_NAME +"  WHERE  "+ COLLUMN_ID + " = ?";
    Cursor c=    db.rawQuery(sqlqry,new  String[]{srch});
    if (c.moveToFirst())
    {
        do {

                    activity.show(c.getString(1),c.getString(2),c.getString(3),c.getString(4));
        }
        while (c.moveToNext());
    }

}

在MainActivity.java中，替换如下：

db.search(qry);

使用：

db.search(qry, MainActivity.this);

Answer 3

只需尝试包装即可解决问题。