build.gradle

/*
 * This build file was generated by the Gradle 'init' task.
 *
 * This generated file contains a commented-out sample Java project to get you started.
 * For more details take a look at the Java Quickstart chapter in the Gradle
 * user guide available at https://docs.gradle.org/4.1/userguide/tutorial_java_projects.html
 */

// Apply the java plugin to add support for Java
apply plugin: 'java'

// In this section you declare where to find the dependencies of your project
repositories {
    // Use 'jcenter' for resolving your dependencies.
    // You can declare any Maven/Ivy/file repository here.
    jcenter()
}

// In this section you declare the dependencies for your production and test code
dependencies {
    // The production code uses the SLF4J logging API at compile time
    compile 'org.slf4j:slf4j-api:1.7.25'
// https://mvnrepository.com/artifact/com.google.code.gson/gson
    compile group: 'com.google.code.gson', name: 'gson', version: '2.8.1'

    // Declare the dependency for your favourite test framework you want to use in your tests.
    // TestNG is also supported by the Gradle Test task. Just change the
    // testCompile dependency to testCompile 'org.testng:testng:6.8.1' and add
    // 'test.useTestNG()' to your build script.
    testCompile 'junit:junit:4.12'
}

first.json

[
  {
    "key_name_1": "val1"
  },
  {
    "key_name_1": "val2"
  },
  {
    "key_name_1": "val3"
  },
  {
    "key_name_1": "val3"
  }
]

output.txt

class java.util.ArrayList
class com.google.gson.internal.LinkedTreeMap
class com.google.gson.internal.LinkedTreeMap
class com.google.gson.internal.LinkedTreeMap
class com.google.gson.internal.LinkedTreeMap
val1
class java.util.ArrayList
class com.google.gson.internal.LinkedTreeMap

ParsingTests.java

package snf;

import com.google.gson.Gson;
import org.junit.Test;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.StringJoiner;

public class ParsingTests {
    String readFromInput(String resourceName) {
        StringJoiner sj = new StringJoiner("\n");
        try (InputStream in =
                     ParsingTests.class.getResourceAsStream(resourceName)) {
            BufferedReader br =
                    new BufferedReader(new InputStreamReader(in, "UTF-8"));
            for (String line = br.readLine();
                 line != null;
                 line = br.readLine()) {
                sj.add(line);
            }
        } catch (IOException e) {
            throw new RuntimeException(resourceName + " cannot be loaded.", e);
        }
        return sj.toString();
    }

    @Test
    public void testParsing() {
        Gson gson = new Gson();
        List data = gson.fromJson(readFromInput("/first.json"), ArrayList.class);
        System.out.println(data.getClass());
        data.stream().map( s -> s.getClass()).forEach(System.out::println);
    }
    @Test
    public void testParsing2() {
        Gson gson = new Gson();
        List data = gson.fromJson(readFromInput("/second.json"), ArrayList.class);Map layer2= (Map) data.get(0); System.out.println(layer2.get("key1"));
        System.out.println(data.getClass());
        data.stream().map( s -> s.getClass()).forEach(System.out::println);

    }
}

second.json

[
  {
    "key_name_1": "val3",
    "key1": "val1",
    "key2": "val2"
  }
]