Basic Java Library Practice : Loading large JSON

This is a post demoing why we need streams.

package util;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.io.StringWriter;
import java.io.Writer;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.util.TreeSet;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.Set;
import java.util.TreeMap;
import java.util.Map;
import java.util.Scanner;


public class TestSolutions {

    /**
     * Run this <code>
     $("table.table.table-striped tr").each(function() {
     if($(this).find('i.fa.fa-unlock').length == 0) $(this).hide();
     });
     </code> in browser's JS console for filtering out premium only problems
     */

    static Set<Integer> probSet = new TreeSet<>();
    static Map<Integer, String> slugMap = new TreeMap<>();

    public static final String LEETCODE_PROBLEMS_OFFLINE_FILE = "all_probs.json";
    public static final String LEETCODE_PROBLEM_URL = "https://leetcode.com/problems/%s/";
    public static final String LEETCODE_GET_PROBLEMS_URL = "https://leetcode.com/api/problems/algorithms/";

    private static String readAll(Reader rd) throws IOException {
        StringBuilder sb = new StringBuilder();
        int cp;
        while ((cp = rd.read()) != -1) {
            sb.append((char) cp);
        }
        return sb.toString();
    }

    public static JSONObject readJsonFromUrl(String url) throws IOException, JSONException {
        InputStream is = new URL(url).openStream();
        try {
            BufferedReader rd = new BufferedReader(new InputStreamReader(is, StandardCharsets.UTF_8));
            String jsonText = readAll(rd);
            JSONObject json = new JSONObject(jsonText);
            return json;
        } finally {
            is.close();
        }
    }

    public static void removeDuplicates(String inputFile, String outputFile) throws IOException {
        File input = new File(inputFile);
        Scanner sc = new Scanner(input);
        while(sc.hasNext()) {
            String line = sc.nextLine().strip();
            if(!line.isEmpty() && !line.contains("//")) {
                probSet.add(Integer.parseInt(line));
            }
        }
		/*
		FileWriter fileWriter = new FileWriter(outputFile);
		for(Integer s : probSet) {
			fileWriter.append(s.toString() + "\n");
		}
		fileWriter.close();
		*/
    }

    public static void getSlugs(String url) throws JSONException, IOException {
        JSONObject probsJSON = readJsonFromUrl(url);
        JSONArray problems = probsJSON.getJSONArray("stat_status_pairs");

        for(int i = 0; i < problems.length(); i++) {
            Integer id = problems.getJSONObject(i).getJSONObject("stat")
                    .getInt("frontend_question_id");
            String slug = problems.getJSONObject(i).getJSONObject("stat")
                    .getString("question__title_slug");

            slugMap.put(id, slug);
        }
    }

    public static void getSlugs() throws FileNotFoundException {
        InputStreamReader streamReader = new InputStreamReader(
                new FileInputStream(LEETCODE_PROBLEMS_OFFLINE_FILE),
                StandardCharsets.UTF_8);
        Writer writer = new StringWriter();
        char[] buffer = new char[1024];
        try {
            Reader reader = new BufferedReader(streamReader);
            int n;
            while ((n = reader.read(buffer)) != -1) {
                writer.write(buffer, 0, n);
            }
            reader.close();
            writer.close();
        } catch (IOException io) {
            io.printStackTrace();
        }
        JSONObject probsJSON = new JSONObject(writer.toString());
        JSONArray problems = probsJSON.getJSONArray("stat_status_pairs");

        for(int i = 0; i < problems.length(); i++) {
            Integer id = problems.getJSONObject(i).getJSONObject("stat")
                    .getInt("frontend_question_id");
            String slug = problems.getJSONObject(i).getJSONObject("stat")
                    .getString("question__title_slug");

            slugMap.put(id, slug);
        }

    }

    public static void createHTMLFile(String outputFile) throws IOException {

        FileWriter fileWriter = new FileWriter(outputFile);
        fileWriter.append("<html><style>\n" +
                "table {\n" +
                "  font-family: arial, sans-serif;\n" +
                "  border-collapse: collapse;\n" +
                "  width: 100%;\n" +
                "}\n" +
                "\n" +
                "td, th {\n" +
                "  border: 1px solid #dddddd;\n" +
                "  text-align: left;\n" +
                "  padding: 8px;\n" +
                "}\n" +
                "\n" +
                "tr:nth-child(even) {\n" +
                "  background-color: #dddddd;\n" +
                "}\n" +
                "</style>\n" +
                "</head><body><table>");
        fileWriter.append("<tr><th>Need to do Problems Link</th></tr>");
        for(Map.Entry<Integer, String> e : slugMap.entrySet()) {
            fileWriter.append("<tr><td>");
            fileWriter.append("<a href='" + String.format(LEETCODE_PROBLEM_URL, e.getValue()) + "'>" + e.getKey() + "</a>");
            fileWriter.append("</td></tr>");
        }
        fileWriter.append("</table></body></html>");
        fileWriter.close();
    }

    public static void main(String[] a) {

        try {
            removeDuplicates("leet_code_premium.txt", "premiums.txt");
            getSlugs(LEETCODE_GET_PROBLEMS_URL);
            slugMap.keySet().retainAll(probSet);
            createHTMLFile("problems_to_do.html");
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}