java - 崩溃到主 Activity 屏幕

Question

我从我创建的数据库登录，当我尝试登录时，屏幕应该转到 HomeActivity，但看起来它崩溃回到 mainActivity 尝试更改位置仍然崩溃，myabie有人有什么想法吗？

来自登录类的代码

    package com.example.laivumusis;

    import androidx.appcompat.app.AppCompatActivity;

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

    public class Login extends AppCompatActivity {
    EditText username, password;
    Button login, register;
    DatabaseLogin databaseLogin;


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

        username = findViewById(R.id.username);
        password = findViewById(R.id.password);
        login = findViewById(R.id.login);
        register = findViewById(R.id.register);
        databaseLogin = new DatabaseLogin(this);

        login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String usernameValue = username.getText().toString(); //???????
                String passwordValue = password.getText().toString();

                if(databaseLogin.isLoginValid(usernameValue, passwordValue)){ //should go to Home activity but crashes to main
                    Intent intent = new Intent(Login.this, HomeActivity.class);
                    startActivity(intent);
                    Toast.makeText(Login.this, "Login Successful!", Toast.LENGTH_SHORT).show();                   //Pop up
                }
                    else{
                    Toast.makeText(Login.this, "Invalid username or password", Toast.LENGTH_SHORT).show();

                }
            }
        });

        register.setOnClickListener(new View.OnClickListener() {       //goes to register tab
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(Login.this, Register.class);

                startActivity(intent);

            }
        });



    }
}

调试错误:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable android.widget.EditText.getText()' on a null object reference
        at com.example.laivumusis.Login$1.onClick(Login.java:34)

向显示问题的行添加了问号

添加 xml 代码 myabie 你会看到问题:(

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:orientation="vertical"
    tools:context=".Login">

    <TextView
        android:text="Login form"
        android:textSize="24sp"
        android:gravity="center"
        android:background="#971919"
        android:textColor="#fff"
        android:padding="10dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <EditText
        android:id="@+id/Username"
        android:layout_gravity="center"
        android:hint="Username"
        android:layout_width="match_parent"
        android:maxLines="1"
        android:layout_height="wrap_content" />



    <EditText
        android:maxLines="1"
        android:id="@+id/password"
        android:inputType="textPassword"
        android:layout_gravity="center"
        android:hint="Password"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />


    <LinearLayout
        android:layout_marginTop="24dp"
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >

        <Button
            android:id="@+id/login"
            android:text="Login"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:layout_width="0dp" />

        <Button
            android:id="@+id/register"
            android:text="Register"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:layout_width="0dp" />

    </LinearLayout>



</LinearLayout>

更改 R.id，但不会从调试器中删除另一个问题

E/SQLiteLog: (1) near "''": syntax error in "Select count (*) from user where username=' 'and password''"
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.laivumusis, PID: 21418
    android.database.sqlite.SQLiteException: near "''": syntax error (code 1 SQLITE_ERROR): , while compiling: Select count (*) from user where username=' 'and password''

数据库代码:

    package com.example.laivumusis;

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

import androidx.annotation.Nullable;

public class DatabaseLogin extends SQLiteOpenHelper {

        static String name = "database";
        static int version = 1;

        String createTableUser = "CREATE TABLE if not exists \"User\" (\n" +    //per DB Browseri sukurem lentele
                "\t\"ID\"\tINTEGER,\n" +
                "\t\"Username\"\tTEXT,\n" +
                "\t\"Password\"\tTEXT,\n" +
                "\t\"Email\"\tTEXT,\n" +
                "\t\"Country\"\tTEXT,\n" +
                "\t\"Dob\"\tTEXT,\n" +
                "\t\"Gender\"\tTEXT,\n" +
                "\tPRIMARY KEY(\"ID\" AUTOINCREMENT)\n" +
                ")";

    public DatabaseLogin(@Nullable Context context) {
        super(context, name, null, version);

        getWritableDatabase().execSQL(createTableUser);

    }

    public void  insertUser(ContentValues contentValues){

        getWritableDatabase().insert("user", "", contentValues);

    }

    public boolean isLoginValid(String username, String password){

        String sql= "Select count (*) from user where username=' " +username+ "'and password'" +password+"'";
        SQLiteStatement statment = getReadableDatabase().compileStatement(sql);
        long l = statment.simpleQueryForLong();
        statment.close();

        if (l==1) {
            return true;
        }
        else{
            return false;
        }

    }

    @Override
    public void onCreate(SQLiteDatabase db) {

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }
}

Answer 1

此问题是您添加了错误的引用。这里的R.id.username需要改为R.id.Username [findViewById(R.id.username)]