Java task-deque project: command parsing, endpoint order and rejected work

Java 8+. This is a complete program using JDK classes.

Make each command a small boundary

The command grammar supports add, urgent, next and list. add and urgent require a nonempty task description; next and list take no argument. An unknown command or extra argument is rejected rather than silently interpreted as a task.

Two equal descriptions remain distinct items. This contract has no task identifiers or deduplication. Urgent work can delay earlier normal work indefinitely, so it is an ordering rule rather than a fairness guarantee.

The implementation depends on Deque<String> but stores a LinkedList. Swapping the implementation to ArrayDeque preserves these operations for the validated non-null input. A concurrent or durable queue needs different coordination and storage decisions.

Keep input ownership outside the queue

The demonstration executes a fixed sequence so it can be compiled and checked without waiting for stdin. A CLI loop can pass each complete line to execute and decide how to display rejected inputs. It should not close a shared System.in reader merely because one command completed.

Working program

import java.util.Deque;
import java.util.LinkedList;
public class TaskDequeCommands {
    final Deque<String> pending=new LinkedList<>();
    String execute(String line){
        if(line==null||line.trim().isEmpty())throw new IllegalArgumentException("Missing command");
        String[] parts=line.trim().split("\\s+",2);String command=parts[0];String argument=parts.length==2?parts[1].trim():"";
        if(command.equals("add")||command.equals("urgent")){
            if(argument.isEmpty())throw new IllegalArgumentException("Missing task");
            if(command.equals("add"))pending.addLast(argument);else pending.addFirst(argument);
            return "accepted";
        }
        if(!argument.isEmpty())throw new IllegalArgumentException("Unexpected argument");
        if(command.equals("next")){String task=pending.pollFirst();return task==null?"empty":task;}
        if(command.equals("list"))return pending.toString();
        throw new IllegalArgumentException("Unknown command");
    }
    public static void main(String[] args){
        TaskDequeCommands queue=new TaskDequeCommands();
        queue.execute("add Audit stock");queue.execute("add Print label");queue.execute("urgent Verify address");
        System.out.println(queue.execute("list"));System.out.println(queue.execute("next"));
        System.out.println(queue.execute("list"));
        try{queue.execute("add");}catch(IllegalArgumentException rejected){System.out.println("Empty task rejected");}
    }
}

Output

[Verify address, Audit stock, Print label]
Verify address
[Audit stock, Print label]
Empty task rejected

Costs and boundaries

Endpoint insertion and removal use O(1) linked-node work; parsing visits the command characters and list rendering visits O(n) tasks. The queue retains descriptions until removed. It is in memory only and has no bounded admission policy; the calling CLI must limit admitted work if input is not trusted to stay small.

Common Mistakes

• Do not promise persistence across restart.
• Urgent-at-head ordering can starve normal work.
• A Deque interface does not make a LinkedList thread-safe.

Read next

LinkedList contracts, Bounded work queues.