Develop Reference

How to effectively generate object in for or while

I have this kind of structure of program.
This program have one function named run(), and the other function named solve().
In this program, I want to get some information from run() and solve().
Also, I want to put these information to one list vector (named information_one_iteration in the following code).
Because this vectors get together to form a matrix (named information in the following code).
That's why I define the information_one_iteration as static variable, and in every new iteration new objects is defined newly.
But, I know this kind of way is not effective!
How can I improve it?
package java_test2;
import java.util.ArrayList;
import java.util.ArrayList;
import java.util.Random;
public class staticvarTest {
private Random random = new Random();
static ArrayList<ArrayList<String>> information = new ArrayList<ArrayList<String>>();
static ArrayList<String> information_one_iteration = new ArrayList<String>();
public staticvarTest() {
}
public void run() {
for (int i=0; i<10; i++) {
information_one_iteration.add(String.valueOf(i));
solve(); // add two random number generated in function solve()
information.add(information_one_iteration);
information_one_iteration = new ArrayList<String>();
}
print_information(information);
}
public void solve() {
information_one_iteration.add(String.valueOf(random.nextInt()));
information_one_iteration.add(String.valueOf(random.nextInt()));
}
public static void print_information(ArrayList<ArrayList<String>> information) {
for (ArrayList<String> newLine : information) {
ArrayList<String> list_set = newLine;
System.out.println("");
for(String data: list_set) {
System.out.print(data+" ");
}
System.out.println("");
}
System.out.println("");
}
public static void main(String args[]) {
staticvarTest na = new staticvarTest();
na.run();
}
}

Since you are not using static ArrayList<String> information_one_iteration = new ArrayList<String>(); anywhere apart from the run method you can remove it.
I have changed the random variable since it's okay for multiple objects to share it - the behaviour remains the same therefore no need to initialize every time Main object is created.
Change the solve method to take a ArrayList as a parameter too.
print_information looks okay but there is no need for ArrayList<String> list_set = newLine;
I have also made information as a state variable over a static variable so that each instance of the object can have it's own information therefore allowing you to run multiple instances of this class.
I also added a lambda to print the members to give you an idea about them newLine.forEach(data -> System.out.println(data + " "));.
Finally I have used variable names and class names that adhere to Java coding conventions for better readability. Here is one of the style guides which you may follow https://google.github.io/styleguide/javaguide.html
The cleaned up version of your code is below
package com.company;
import java.util.ArrayList;
import java.util.Random;
public class Main {
private static Random random = new Random();
private ArrayList<ArrayList<String>> information = new ArrayList<>();
private void run() {
for (int i=0; i<10; i++) {
ArrayList<String> informationInIteration = new ArrayList<>();
informationInIteration.add(String.valueOf(i));
solve(informationInIteration); // add two random number generated in function solve()
information.add(informationInIteration);
}
printInformation();
}
private void solve(ArrayList<String> iterationInformation) {
iterationInformation.add(String.valueOf(random.nextInt()));
iterationInformation.add(String.valueOf(random.nextInt()));
}
private void printInformation() {
for (ArrayList<String> newLine : information) {
System.out.println();
newLine.forEach(data -> System.out.println(data + " "));
System.out.println();
}
System.out.println();
}
public static void main(String[] args) {
Main main = new Main();
main.run();
}
}
Based on your comments I am adding a method that creates no object for storing any information.
package com.company;
import java.util.Random;
public class Main {
private Random random = new Random();
private void run() {
for (int i=0; i<10; i++) {
System.out.println(String.valueOf(i));
System.out.println(random.nextInt() + " " + random.nextInt() + "\n");
}
}
public static void main(String[] args) {
Main main = new Main();
main.run();
}
}

This should work for you.
public void run() {
for (int i=0; i<10; i++) {
staticvarTest.information.add(solve(String.valueOf(i)));
}
print_information(information);
}
public String[] solve(String i) {
return {String.valueOf(random.nextInt()),
String.valueOf(random.nextInt()),
i};
}
Or you could maybe concatenate the array returned in solve() and the i String and avoid passing the variable around.
I have not tested the code above btw but apart from accessing and modifying the static variable there shouldn't be a problem.
Hope this works for you.

ArrayList class separate from Java main file

I have two separate files, one named WonderfulArrayList, and the other named ArrayListMain (I'm experimenting with ArrayLists, and I'm not quite sure what to do) and so I have a method in the WonderfulArrayList file, but the main file cannot see the method, which I have named booladdData, which would return true once the data is added to the array list. My WonderfulArrayList file is the following:
import java.util.*;
import java.io.*;
public class WonderfulArrayList{ //implement WonderfulArrayList
public static int ADDNums;
public static int index;
public static int HEADNums;
public static ArrayList<Integer> arr = new ArrayList<Integer>(15);
public static boolean booladdData(ArrayList<Integer>arr){
arr.add(ADDNums);
return true;
}
}
As you can see, I have booladdData instantiated with the ArrayList, named arr. Now, if you look at my main file:
public class ArrayListMain{
//public ArrayList<Integer> arr = new ArrayList<Integer>(15);
public static void main(String[]args){
ArrayList<Integer> arr = new ArrayList<Integer>(15);
int MenuNum = 0;
int ADDNums = 0;
Object Obj = new Object();
Scanner scanner1 = new Scanner(System.in); //set up scanner for user input
while(MenuNum != 7){ //menu loop
Menu(MenuNum);
MenuNum = scanner1.nextInt();
if(MenuNum == 1){
arr.booladdData();
}
For some reason, even though I know that booladdData is created as public and they're both in the same folder, the main file doesn't have the scope to be able to see booladdData in the separate file.
Any idea what I'm doing wrong?

You should be calling WonderfulArrayList.booladdData(arr) instead of arr.booladdData(). The method booladdData() is defined as a class method of your WonderfulArrayList class. It's not an instance method of Java's ArrayList.
You also might want to read into object-oriented programming. Everything in your code is static.

You need to create your type instead of ArrayList
package com.jbirdvegas.test;
import java.util.ArrayList;
public class MainClazz {
public static void main(String[] args) {
// notice I'm creating my type `MyArrayList` instead of `ArrayList` type
MyArrayList myArrayList = new MyArrayList();
myArrayList.add("blah");
System.out.println("My message:" + myArrayList.getSomething());
}
}
class MyArrayList extends ArrayList {
public String getSomething() {
return "something";
}
}
Prints:
My message: something

Sort() "cannot find symbol"

Okay i got some issue with the Collection.sort(myintarray); line... it says cannot find symbol trying to make it sort the list so the lowest number in array come first.
package uppgift.pkg1;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.Arrays;
/**
* #author Fredrik
*
*/
public class Uppgift1 {
public static void main(String[] args) {
//Anrop av metod
skapaArray();
}
public static void skapaArray() {
List<Integer> myintarray = new ArrayList<Integer>();
int ints = 0;
int size = 1;
while (ints < size) {
myintarray.add(ints);
ints++;
}
Collection.sort(myintarray);
System.out.println(myintarray.size());
}
}

You should use Collections:
java.util.Collections;
and:
Collections.sort(myintarray);

It should be
Collections.sort(myintarray);

Sorting a collection in Java is easy, just use the Collections.sort(Collection) to sort your values.
You can do
Collections.sort(myintarray);
instead of
Collection.sort(myintarray);

Reversing ArrayList<String>

All im trying to do is reverse the ArrayList. Is there a way I could do it through the toString as well or should I just create a method like I did. Im so close, any answers will help! Thanks!
package edu.purse.test;
java.util.ArrayList;
import java.util.Collections;
public class Purse
{
ArrayList<String> coins = new ArrayList<String>();
public Purse()
{
}
public void addCoin(String coinName)
{
coins.add(coinName);
}
public String toString()
{
return "Purse" + coins.toString();
}
public ArrayList<String> getReversed(ArrayList<String> coins)
{
ArrayList<String> copy = new ArrayList<String>(coins);
Collections.reverse(copy);
return copy;
}
}
TESTERCLASS
package edu.purse.test;
import java.util.Collections;
import java.util.List;
public class PurseTester {
public static void main(String[] args) {
Purse p = new Purse();
p.addCoin("Quarter");
p.addCoin("Dime");
p.addCoin("Nickel");
p.addCoin("Penny");
System.out.println(p.toString());
p.getReversed(coins);
}
}

The method
p.getReversed(coins);
returns a reversed list. You can just print it out
System.out.println(p.getReversed(coins));
Note that you are getting a copy of your instance's list, reversing that, and then returning it. If you want to preserve the change, simple call Collections.reverse() on the original, coins.

copying array values from one class to another

Im stuck with the following problem,
I've two classes, the first is readFromFile and the second class is newClass
readFromFile.java -
This reads a text file
Parses the lines of text into seperate strings
The values of these strings are stored in a String [ ] called dArray
For testing I've printed all values out and it works
newClass.java
This class is intended to copy the value of the string [ ] dArray into a new string and from there use the values ( for simplicity all I've included in the newClass is the code relating to copying the array)
What I'm doing wrong is that I'm returning dArray but its returning an array with nothing stored in it, so I either need a way to call main method from readFromFile.class / help creating a method in readFromFile that would do the same which I call from main
please help
import java.util.Scanner;
import java.io.*;
public class readFromFile
{
static String[] dArray = new String [30];
public static void main (String[] args) throws IOException
{
String part;
Scanner fileScan, partScan;
int i = 0;
int x = 0;
fileScan = new Scanner (new File("C:\\stuff.txt"));
// Read and process each line of the file
while (fileScan.hasNext())
{
part = fileScan.nextLine();
partScan = new Scanner (part);
partScan.useDelimiter(":");
while ( partScan.hasNext()){
dArray[i] = partScan.next();
i++;
}
}
for (x = 0;x<i;x++)
{ System.out.println(dArray[x]);
}
}
public String[] getArray()
{
return dArray;
}}
newClass.java
public class newClass {
readFromFile results = new readFromFile();// creating object from class readFromFile
public void copyArray() {
String[] dArray = results.getArray(); // Trying to return the values of String [] dArray from rr classs
//Method getArray in rr class is
// public String[] getArray()
// { return dArray; }
String[] arrayCopy = new String[dArray.length];
System.arraycopy(dArray, 0, arrayCopy, 0, dArray.length);
for (int i = 0; i < arrayCopy.length; i++)
System.out.println(arrayCopy[i]);
}
public static void main(String[] args) {
newClass.copyArray();
}
}

Your results generation is in readFromFile.main(), but you're expecting to call it in your readFromFile(). You need to make a constructor for readFromFile, and call that in your main method, as well.

The problem is that both classes have a main method. Only the class that you intend to run should have a main method, the other classes need only constructors. Assuming you want to run a unshown class it would be written like this.
public class ThirdClass{
public static void main(String[] args) {
readFromFile reader = new ReadFromFile();
newClass copy = new newClass();
reader.readFromFile();
String[] strings = reader.getArray();
copy.copyArray(strings)
}
For this to work you need to put all of the code in the main of readFromFile in a method called "readFromFile". and you need a method in newClass that accepts a string array as an argument. Or a constructor that accepts a string array.
Make sure that neither of them have main methods or it won't work.

Remove the static keyword before your dArray variable
Change public static void main(String[] args) throws IOException in your first class to public readFromFile() throws IOException. Keep the code inside it the same.
Change the line newClass.copyArray(); in your second class to (new newClass()).copyArray();
Move the line in your second class readFromFile results = new readFromFile(); into the public void copyArray() method.
Change public void copyArray() in your second class to public void copyArray() throws IOException
Put a try..catch block around your code in the second class's main method. i.e. change (new newClass()).copyArray(); to something like try { (new newClass()).copyArray(); } catch (IOException e) { e.printStackTrace(); }
The above should get your thing working, but a friendly note would be to experiment with the code (once it works) since it's an excellent example to understand how static keywords are used, how Exceptions are handled or thrown, and how IO is used. ;)