[Read file and parse json] #FileIO

App.java

package com.superzjn;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.HashSet;
import java.util.stream.Collectors;

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;


public class App {

    public String readStringFromFile(String path) {

        StringBuilder sb = new StringBuilder();

        File file = new File(path);
        try (BufferedReader br = new BufferedReader(new FileReader(file))) {
            String line;

            while ((line = br.readLine()) != null) {
                sb.append(line).append("\n");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return sb.toString();

    }

    public String readStreamFromFileAsStream(InputStream inputStream) {
        StringBuilder sb = new StringBuilder();

        try (BufferedReader br = new BufferedReader(new InputStreamReader(inputStream))) {
            String line;

            while ((line = br.readLine()) != null) {
                sb.append(line).append("\n");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return sb.toString();
    }

    public String readStringFromFileUsingLines(String path) {

        String str = null;

        File file = new File(path);
        try (BufferedReader br = new BufferedReader(new FileReader(file))) {

            str = br.lines().collect(Collectors.joining(System.lineSeparator()));

        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;

    }

    public void printDuplicateJsonItems(String jsonStr) throws Exception {
        ObjectMapper mapper = new ObjectMapper();

        // Way 1 HashSet and Iterator
        JsonNode nodes = mapper.readTree(jsonStr);
        HashSet<Integer> set = new HashSet<>();
        Iterator<JsonNode> node = nodes.elements();

        while (node.hasNext()) {
            int currValue = Integer.parseInt(node.next().get("id").asText());
            if (!set.contains(currValue)) {
                set.add(currValue);
            } else {
                System.out.println("Dup" + currValue);
            }
        }

        // This way is not going to work unless we move class Node outside of this file
        // class Node {
        // String id;
        // Integer value;
        // }

        // List<Node> nodes = mapper.readValue(jsonStr, new TypeReference<List<Node>>()
        // {
        // });
        // HashSet<Integer> set = new HashSet<>();

        // for (Node node : nodes) {
        // if (!set.contains(node.value)) {
        // set.add(node.value);
        // } else {
        // System.out.println("Dup" + node.value);
        // }
        // }
    }

    public static void main(String[] args) throws Exception {
        App app = new App();

        // 1st Way - Absolute path
        // String str = app.readStringFromFile(
        // "/home/superzjn/Documents/Java_Test/jsontest/src/main/java/com/superzjn/duplicates.json");

        // 2st Way - relative path
        // String str =
        // app.readStringFromFile("src/main/java/com/superzjn/duplicates.json");

        // 3rd Way - Using getResource
        // URL path = App.class.getResource("duplicates.json");
        // String str = app.readStringFromFile(path.getFile());

        // 4th Way - Using getResourceAsStream
        InputStream inputStream = App.class.getResourceAsStream("duplicates.json");
        String str = app.readStreamFromFileAsStream(inputStream);

        // Using lines() to read file
        // String str = app.readStringFromFileUsingLines(path.getFile());

        // System.out.print(str);
        app.printDuplicateJsonItems(str);
    }
}