在Android Studio的ListView中显示数据

Question

I am new to Android, and i am trying to display the data from sqlite to new Activity in List view.

**MainActivity.xml**
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/relative"
    android:background="@color/App_background">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:gravity="center_vertical"
        android:text="Fill the Details Here"
        android:textSize="30sp"
        android:layout_marginLeft="80dp"
        android:layout_marginTop="25dp"
        android:textColor="@color/color1"
        />

    <TextView
        android:id="@+id/name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Name"
        android:textSize="35sp"
        android:layout_marginTop="105sp"
        android:layout_marginLeft="30sp"/>

    <EditText
        android:id="@+id/edit1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/name"
        />
    <TextView
        android:id="@+id/email"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Email"
        android:textSize="35sp"
        android:layout_marginLeft="30sp"
        android:layout_marginTop="10sp"
        android:layout_below="@+id/edit1"/>
    <EditText
        android:id="@+id/edit2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/email"
        />
    <TextView
        android:id="@+id/mobile"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Mobile"
        android:textSize="35sp"
        android:layout_marginTop="10sp"
        android:layout_marginLeft="30sp"
        android:layout_below="@+id/edit2"/>
    <EditText
        android:id="@+id/edit3"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/mobile"
        />
    <TextView
        android:id="@+id/password"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Password"
        android:textSize="35sp"
        android:layout_marginTop="10sp"
        android:layout_marginLeft="30sp"
        android:layout_below="@+id/edit3"/>
    <EditText
        android:id="@+id/edit4"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/password"
        />
    <Button
        android:id="@+id/btn1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/edit4"
        android:text="Sign Up"
        android:background="@color/teal_700"
        android:textStyle="bold"
        android:layout_marginTop="25dp" />
    <Button
        android:id="@+id/btn2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Show Data"
        android:background="@color/teal_700"
        android:textStyle="bold"
        android:layout_below="@id/btn1"
        android:layout_marginTop="25dp" />
</RelativeLayout>

**Show_data.xml**
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".show_data">

    <ListView
        android:id="@+id/lv_customerList"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</androidx.constraintlayout.widget.ConstraintLayout>

**MainActivity.java**
package com.example.databaseconnection;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    Button signup,showdata;
    EditText name,email,Mobile,Password;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        signup = findViewById(R.id.btn1);
        name = findViewById(R.id.edit1);
        email = findViewById(R.id.edit2);
        Mobile = findViewById(R.id.edit3);
        Password = findViewById(R.id.edit4);
        showdata = findViewById(R.id.btn2);


        signup.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                CustomerModel customerModel = null;
                try{
                    customerModel = new CustomerModel(-1,
                            name.getText().toString(),
                            email.getText().toString(),
                            Integer.parseInt(Mobile.getText().toString()),
                            Password.getText().toString());
                }
                catch (Exception e){
                    Toast.makeText(MainActivity.this, "Error in Registration",Toast.LENGTH_SHORT).show();
                }
                DataBaseHelper dataBaseHelper = new DataBaseHelper(MainActivity.this); ///check why MainActivity. this
                boolean b = dataBaseHelper.addOne(customerModel);
                Toast.makeText(MainActivity.this,"Data Added Sccessfully", Toast.LENGTH_SHORT).show();

            }
        });
       showdata.setOnClickListener(new View.OnClickListener() {
           @Override
           public void onClick(View v) {
               Intent intent = new Intent(MainActivity.this,show_data.class);
               startActivity(intent);

           }
       });

    }
}

**DatabaseHelper.java**
package com.example.databaseconnection;

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

import androidx.annotation.Nullable;

import java.util.ArrayList;
import java.util.List;

public class DataBaseHelper extends SQLiteOpenHelper {
    public static final String CUSTOMER_TABLE = "CUSTOMER_TABLE";
    public static final String NAME = "NAME";
    public static final String EMAIL = "EMAIL";
    public static final String MOBILE = "MOBILE";
    public static final String PASSWORD = "PASSWORD";
    public static final String ID = "ID";

    public DataBaseHelper(@Nullable Context context)
    {
        super(context, "customer.db", null, 1);
    }

    // This is called the first time a database is accessed. there should be code in here to create a new database
    @Override
    public void onCreate(SQLiteDatabase db) {
        String createTableStatement = "CREATE TABLE " + CUSTOMER_TABLE + "(" + ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + NAME + " TEXT, " + EMAIL + " TEXT, " + MOBILE + " TEXT, " + PASSWORD + " TEXT)";
        db.execSQL(createTableStatement);

    }
    // this is called if hte database version number changes.
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }
    public boolean addOne(CustomerModel customerModel)
    {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues cv = new ContentValues();
        cv.put(NAME,customerModel.getName());
        cv.put(EMAIL, customerModel.getEmail());
        cv.put(MOBILE,customerModel.getMobile());
        cv.put(PASSWORD,customerModel.getPassword());
        db.insert(CUSTOMER_TABLE,null,cv);
        return true;
    }
    public List<CustomerModel> getEveryOne()
    {
        List<CustomerModel> returnList = new ArrayList<>();
        String queryString = " Select * FROM " + CUSTOMER_TABLE;
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(queryString,null); // rawQuery returns a Cursor.
        if(cursor.moveToFirst()){
            do{
                int id = cursor.getInt(0);
                String Name = cursor.getString(1);
                String Email = cursor.getString(2);
                int Mobile = cursor.getInt(3);
                String Password = cursor.getString(4);
                CustomerModel newcustomer = new CustomerModel(id,Name,Email,Mobile,Password);
                returnList.add(newcustomer);

            } while (cursor.moveToNext());

        }
        else {
            //If nothing in the database, do not add anything to the list.

        }
        cursor.close();
        db.close();
        return returnList;
    }
}

**CustomerModel.java**
package com.example.databaseconnection;

public class CustomerModel {
    int id;
    private String name;
    private String email;
    private int mobile;
    private String password;`enter code here`
    @Override
    public String toString() {
        return "CustomerModel{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", email='" + email + '\'' +
                ", mobile=" + mobile +
                ", password='" + password + '\'' +
                '}';
    }
    public CustomerModel(int id, String name, String email, int mobile, String password) {
        this.id = id;
        this.name = name;
        this.email = email;
        this.mobile = mobile;
        this.password = password;
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    public int getMobile() {
        return mobile;
    }
    public void setMobile(int mobile) {
        this.mobile = mobile;
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }
}

**show_data. java**
public class show_data extends AppCompatActivity {
    ListView lv_customerList;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_show_data);
        DataBaseHelper dataBaseHelper = new DataBaseHelper(show_data.this); //check why MainActivity.this
        List<CustomerModel> everyone = dataBaseHelper.getEveryOne();
        // Toast.makeText(MainActivity.this, everyone.toString(),Toast.LENGTH_SHORT).show();
        ArrayAdapter customerArrayAdapter = new ArrayAdapter<CustomerModel>(this, android.R.layout.simple_list_item_1, everyone);
       lv_customerList.setAdapter(customerArrayAdapter);
    }}`enter code here`

lt Adds the data successfully but the the functionality to view the data does not work. My app crashes an throws the below error.

Caused by: java.lang.NullPointerException : Attempt to invoke virtual method void android.widget.ListView.setAdapter(android.widget.ListAdapter) on a null object reference

Answer 1

You haven't called findViewById() on lv_customerList

after setContentView(R.layout.activity_show_data) in show_data.java add below code

lv_customerList = findViewById(R.id.lv_customerList)