SomeJavaApi.java

import org.jetbrains.annotations.NotNull;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class SomeJavaApi {

    private static final String SUCCESS_RESULT = "SUCCESSFUL_CALL";
    private static final long WORK_DELAY = 1000L;

    private ExecutorService executorService;

    public SomeJavaApi() {
        executorService = Executors.newSingleThreadExecutor();
    }

    public void successRemoteCall(CustomCallback callback) {
        System.out.println("[Thread id: " + Thread.currentThread().getId() + "]\t" +
                "Start api function with successful result.");
        runTask(() -> callback.onSuccess(SUCCESS_RESULT), WORK_DELAY);
    }

    public void failedRemoteCall(CustomCallback callback) {
        System.out.println("[Thread id: " + Thread.currentThread().getId() + "]\t" +
                "Start api function with unsuccessful result.");
        runTask(() -> callback.onFailure(new Exception("Dangerous exception!")), WORK_DELAY);
    }

    public void destroy() {
        executorService.shutdown();
    }

    public interface CustomCallback {

        void onSuccess(@NotNull String data);

        void onFailure(@NotNull Exception exception);

    }

    private void runTask(Runnable task, long delay) {
        executorService.submit(() -> {
            try {
                if(delay > 0) {
                    Thread.sleep(delay);
                }
                task.run();
            } catch (Throwable e) {
                e.printStackTrace();
            }
        });
    }
}