您的方法中存在一些问题：
你只要把钱还给我就行了 tasks 什么时候 task.getDependsOn() 是空的，忽略剩余的 task 独立，你应该打电话 continue 相反
你打电话的时候没有复印 addDependencies(task.getDependsOn()) ，这会产生意外的副作用（改变 task.getDependsOn() ).
对于这个问题，我更喜欢使用stream with flatmap，因为它不会产生副作用，并且我们不需要创建列表的多个副本。

public static Stream<Task> flattenDependencies(List<Task> tasks) {
    return tasks.stream().flatMap(task -> {
        Stream<Task> flattened = flattenDependencies(task.getDependsOn());
        return Stream.concat(Stream.of(task), flattened);
    });
}

完整示例
主类

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class FlattenTask {
    public static void main(String[] args) {
        Task a = new Task("a");
        Task b = new Task("b");
        Task c = new Task("c");
        Task d = new Task("d");
        Task e = new Task("e");
        Task f = new Task("f");
        a.setDependsOn(List.of(b, c));
        b.setDependsOn(List.of(d, e));
        List<Task> tasks = List.of(a, f);
        System.out.println(flattenDependencies(tasks).collect(Collectors.toList()).toString());
    }

    public static Stream<Task> flattenDependencies(List<Task> tasks) {
        return tasks.stream().flatMap(task -> {
            Stream<Task> flattened = flattenDependencies(task.getDependsOn());
            return Stream.concat(Stream.of(task), flattened);
        });
    }
}

任务

import java.util.Collections;
import java.util.List;

public class Task {

    private final String code;
    private List<Task> dependsOn = Collections.emptyList();

    public Task(String code) {
        this.code = code;
    }

    public List<Task> getDependsOn() {
        return dependsOn;
    }

    public void setDependsOn(List<Task> dependsOn) {
        this.dependsOn = dependsOn;
    }

    @Override
    public String toString() {
        return code;
    }
}

我认为您的异常正在发生，因为您试图在迭代列表时同时追加列表。如果我明白你想做什么，也许像这样的事情会奏效

public List<Task> addDependencies(List<Task> tasks) {
  List<Task> subTaskList = new ArrayList<Task>;
  for (Task task: tasks) {
   if (!task.getDependsOn().isEmpty())
    subTaskList.addAll(addDependencies(task.getDependsOn()));
  }
  tasks.addAll(subTaskList);
  return tasks;
}