从 Arraylist 转换<double>到原始类型双数组</double>

Question

Hey I am trying to get my ArrayList from DBhelper class and convert it to a double from Doublearraylist since I want to be able to calculate the value from another variable.

My dbhelperclass contains this code for getting the arraylist

public ArrayList<Double> getLastEntry(){
    ArrayList<Double> data = new ArrayList<Double>();
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.query(TABLE_NAME, new String[]{COL2},null, null, null, null, null);
    Double add = null;
    while(cursor.moveToNext()){
        if(cursor.moveToLast()){
            add=cursor.getDouble(0);
            data.add(add);
        }
    }
    cursor.close();
    return data;
}

And so far I tried two things to convert the data but in my error message I get found java.lang.double need a string to check if it can be logged in logcat but I just get all these symbols logged instead of a string so I guess I converted it wrongly.

Code for the converter

First method i tried

ArrayList<Double> data = mDatabaseHelper.getLastEntry();
double []arrX =  new double [data.size()];
for(int j=0; j<data.size();j++){
    double convert = data.get(j);
    arrX[j]=convert;
}
Log.d(TAG, arrX.toString());

Second method I tried was from a book with newer java i think. but it says cannot infer arguments inbetween the new Arraylist

code:

data = new ArrayList<>(Arrays.asList(data));
double[] arr = data.stream().mapToDouble(Double::doubleValue).toArray();

Answer 1

Regarding the second way - i think there is a mistake:

data = new ArrayList<>(Arrays.asList(data)); -> here you make an ArrayList<Double> into an ArrayList<ArrayList<Double>> .

You probably might want to write:

ArrayList<Double> data = mDatabaseHelper.getLastEntry();
double[] arr = data.stream().mapToDouble(Double::doubleValue).toArray();