How to do a Select in a query

I'm wondering how to do a SELECT in my database.

Here are the classes I use:

DBHELPER

    public class DBHelper extends SQLiteOpenHelper {

    private static String NAME = "sqlitejuh.db";
    private static int VERSION = 1;

    public DBHelper (Context context){
        super(context, NAME, null, VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {

        db.execSQL(
            "CREATE TABLE [exercicio] (\n" +
            "[codigo] INTEGER  NOT NULL PRIMARY KEY AUTOINCREMENT,\n" +
            "[nome] VARCHAR(60)  NOT NULL,\n" +
            "[categoria] VARCHAR(60)  NOT NULL,\n" +
            "[nivel] VARCHAR(60)  NOT NULL,\n" +
            "[descricao] VARCHAR(256)  NOT NULL\n" +
            ")"
        );

    }

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

    }
}

Adapter

    public class ExerciseAdapter extends ArrayAdapter {

    private ArrayList<Exercise> exercises;

    public ExerciseAdapter(@NonNull Context context, @NonNull ArrayList<Exercise> exercises) {
        super(context, 0, exercises);
        this.exercises = exercises;
    }

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNullViewGroup parent) {
        Exercise exercise = exercises.get(position);

        convertView = LayoutInflater.from(getContext()).inflate(R.layout.item_edit_list_exercises, null);

        TextView tvName = convertView.findViewById(R.id.item_text_view_name);
        TextView tvCategory = convertView.findViewById(R.id.item_text_view_category);
        TextView tvNivel = convertView.findViewById(R.id.item_text_view_nivel);

        tvName.setText(exercise.getNome());
        tvCategory.setText(exercise.getCategoria());
        tvNivel.setText(exercise.getNivel());

        return convertView;

    }

Modal

public class Exercise {

private int codigo;
private String nome;
private String categoria;
private String nivel;
private String descricao;
private boolean excluir;
private Context context;

public Exercise(Context context){
    this.context = context;
    codigo = -1;
}

public int getCodigo() {
    return codigo;
}

public String getNome() {
    return nome;
}

public void setNome(String nome) {
    this.nome = nome;
}

public String getCategoria() {
    return categoria;
}

public void setCategoria(String categoria) {
    this.categoria = categoria;
}

public String getNivel() {
    return nivel;
}

public void setNivel(String nivel) {
    this.nivel = nivel;
}

public String getDescricao() {
    return descricao;
}

public void setDescricao(String descricao) {
    this.descricao = descricao;
}

public boolean isExcluir() {
    return excluir;
}

public void setExcluir(boolean excluir) {
    this.excluir = excluir;
}

public ArrayList<Exercise> getExercises() {
    DBHelper dbHelper = null;
    SQLiteDatabase sqLiteDatabase = null;
    Cursor cursor = null;
    ArrayList<Exercise> exercises = new ArrayList<>();
    try{
        dbHelper = new DBHelper(context);
        sqLiteDatabase = dbHelper.getReadableDatabase();
        cursor = sqLiteDatabase.query("exercicio", null, null,
                null,null,null,null);
        while(cursor.moveToNext()){
            Exercise exercise = new Exercise(context);
            exercise.codigo = cursor.getInt(cursor.getColumnIndex("codigo"));
            exercise.nome = cursor.getString(cursor.getColumnIndex("nome"));
            exercise.categoria = cursor.getString(cursor.getColumnIndex("categoria"));
            exercise.nivel = cursor.getString(cursor.getColumnIndex("nivel"));
            exercise.descricao = cursor.getString(cursor.getColumnIndex("descricao"));
            exercises.add(exercise);
        }

    }catch(Exception e){
        e.printStackTrace();
    }finally {
        if ((cursor != null) && (!cursor.isClosed()))
            cursor.close();
        if (sqLiteDatabase != null)
            sqLiteDatabase.close();
        if (dbHelper != null)
            dbHelper.close();
    }

    return exercises;
}

public ArrayList<Exercise> getExercisesByCategory(){
    DBHelper dbHelper = null;
    SQLiteDatabase sqLiteDatabase = null;
    Cursor cursor = null;
    ArrayList<Exercise> exercises = new ArrayList<>();
    try{
        dbHelper = new DBHelper(context);
        sqLiteDatabase = dbHelper.getReadableDatabase();
        cursor = sqLiteDatabase.query("exercicio",
                new String[]{"categoria"},
                "categoria = ?",
                new String[]{String.valueOf(categoria)},
                null,
                null,
                null);
        while(cursor.moveToNext()){
            Exercise exercise = new Exercise(context);
            exercise.codigo = cursor.getInt(cursor.getColumnIndex("codigo"));
            exercise.nome = cursor.getString(cursor.getColumnIndex("nome"));
            exercise.categoria = cursor.getString(cursor.getColumnIndex("categoria"));
            exercise.nivel = cursor.getString(cursor.getColumnIndex("nivel"));
            exercise.descricao = cursor.getString(cursor.getColumnIndex("descricao"));
            exercises.add(exercise);
        }

    }catch(Exception e){
        e.printStackTrace();
    }finally {
        if ((cursor != null) && (!cursor.isClosed()))
            cursor.close();
        if (sqLiteDatabase != null)
            sqLiteDatabase.close();
        if (dbHelper != null)
            dbHelper.close();
    }

    return exercises;
}

public boolean save(){
    DBHelper dbHelper = null;
    SQLiteDatabase sqLiteDatabase = null;

    try{
        dbHelper = new DBHelper(context);
        sqLiteDatabase = dbHelper.getWritableDatabase();
        String sql = "";
        if (codigo == -1){
            sql = "INSERT INTO exercicio (nome,categoria,nivel,descricao) VALUES (?,?,?,?)";
        }else{
            sql = "UPDATE exercicio SET nome = ?, categoria = ?, nivel = ?, descricao = ? WHERE codigo = ?";
        }
        sqLiteDatabase.beginTransaction();

        SQLiteStatement sqLiteStatement = sqLiteDatabase.compileStatement(sql);
        sqLiteStatement.clearBindings();

        sqLiteStatement.bindString(1, nome);
        sqLiteStatement.bindString(2, categoria);
        sqLiteStatement.bindString(3, nivel);
        sqLiteStatement.bindString(4, descricao);

        if (codigo != -1)  sqLiteStatement.bindString(5, String.valueOf(codigo));

        sqLiteStatement.executeInsert();

        sqLiteDatabase.setTransactionSuccessful();
        sqLiteDatabase.endTransaction();
        return true;

    }catch(Exception e){
        e.printStackTrace();
        assert sqLiteDatabase != null;
        sqLiteDatabase.endTransaction();
        return false;
    }finally {
        if (sqLiteDatabase != null)
            sqLiteDatabase.close();
        if (dbHelper != null)
            dbHelper.close();
    }
}

}

And where do I get the items: public class ConeExercisesActivity extends Activity implements AdapterView.OnItemClickListener {

ListView listViewCones;
ExerciseAdapter exerciseAdapter;
private ArrayList<Exercise> exercises;
private AlertDialog alerta;
private Exercise exerciseEdicao;


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

    listViewCones = findViewById(R.id.list_view_cone);

    listViewCones.setOnItemClickListener(this);

    exercises = new Exercise(this).getExercisesByCategory();
    exerciseAdapter = new ExerciseAdapter(this, exercises);
    listViewCones.setAdapter(exerciseAdapter);
}

@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

    Toast.makeText(listViewCones.getContext(),
            "Posição Selecionada:" + position, Toast.LENGTH_LONG)
            .show();
}
}

Finally my question is how do I get the data from the category column only. I tried it that way but I could not ... the list was returning empty.