错误链表 peek() 不为 null 和 size() =0

Question

import java.util.LinkedList;

public LinkedList<String> mylinkedlist;

/*...*/

System.out.println("STRING: "+ mylinkedlist.peek() +" SIZE: "+ mylinkedlist.size());

output

STRING: blablabla SIZE: 0

it is a debugging question : How can peek() return a value and size of the list be 0 ?

EDIT FEB 25 - Bug appear again

The error :

java.util.NoSuchElementException
    at java.util.LinkedList.removeFirst(LinkedList.java:270)
    at java.util.LinkedList.pop(LinkedList.java:801)
    at GrblCommunicator.streamCommands(GrblCommunicator.java:164)
    at GrblCommunicator.responseMessage(GrblCommunicator.java:234)
    at ResponseMessageHandler.handleResponse(ResponseMessageHandler.java:33)
    at SerialConnectionJserial.serialEvent(SerialConnectionJserial.java:114)
    at com.fazecast.jSerialComm.SerialPort$SerialPortEventListener.waitForSerialEvent(SerialPort.java:1387)
    at com.fazecast.jSerialComm.SerialPort$SerialPortEventListener$1.run(SerialPort.java:1300)
    at java.lang.Thread.run(Thread.java:748)

The code :

import java.util.LinkedList;

/*...*/

public volatile LinkedList<String> commandBuffer;     


public void streamCommands() {

        if (this.commandBuffer.size() == 0) {
            // NO-OP
            return;
        }

        if (this.sendPaused) {
            // Another NO-OP
            return;
        }


        while (CommUtils.checkRoomInBuffer(this.sentBufferSize, this.commandBuffer.peek())
                && allowMoreCommands()) {

            if(this.commandBuffer.size()==0) {
                System.out.println("ERROR DE COMMAND BUFFER SIZE GrblComm 155: ");
                return;
            }
            else
            {
                 String commandString = this.commandBuffer.pop();

LINE 164 is : this.commandBuffer.pop();

I do not understand this bug because there is a this.commandBuffer.size() test !!!!!

please, explain me !!!

EDIT FEB 25

Im gonna try to replace LinkedList by ConcurrentLinkedDeque

EDIT FEB 26

Same error :

java.util.NoSuchElementException
    at java.util.concurrent.ConcurrentLinkedDeque.screenNullResult(ConcurrentLinkedDeque.java:810)
    at java.util.concurrent.ConcurrentLinkedDeque.removeFirst(ConcurrentLinkedDeque.java:990)
    at java.util.concurrent.ConcurrentLinkedDeque.pop(ConcurrentLinkedDeque.java:1036)
    at GrblCommunicator.streamCommands(GrblCommunicator.java:167)
    at GrblCommunicator.responseMessage(GrblCommunicator.java:240)
    at ResponseMessageHandler.handleResponse(ResponseMessageHandler.java:33)
    at SerialConnectionJserial.serialEvent(SerialConnectionJserial.java:114)
    at com.fazecast.jSerialComm.SerialPort$SerialPortEventListener.waitForSerialEvent(SerialPort.java:1391)
    at com.fazecast.jSerialComm.SerialPort$SerialPortEventListener$1.run(SerialPort.java:1295)

Answer 1

It is not possible for you to peek an item in a LinkedList AND have the size of it be zero. Think about what you're saying - "How can I look at this value and have the list have no values?". The reason for this is that peek() retrieves the first item in the list but does NOT remove it. However , you may be looking for pop() instead. Using pop() you can retrieve AND remove the first item at the same time. Per the documentation here "Pops an element from the stack represented by this list. In other words, removes and returns the first element of this list".

LinkedList<String> myList = new LinkedList<>();
myList.add("blablabla");
System.out.println("String: " + myList.pop() + ", Size: " + myList.size());

This prints out:

String: blablabla, Size: 0

Answer 2

For now, the only explantation is in this thread

"A LinkedList is not thread-safe"

Thread safety issue

so i will use : LinkedBlockingQueue

Answer 3

If you absolutely need to return a value when the size is 0, you have a few options.

1. Create your own LinkedList implementation
2. Catch the exception that will be thrown currently, and then return the value that you find appropriate for this scenario
3. Check the size of the linked list before calling .peek() and return the appropriate value instead of calling .peek() if the size is 0. If it is not 0, go ahead and make the call.

Answer 4

Implementation of peak() method for java.util.LinkedList says that "Retrieves, but does not remove, the head (first element) of this list."

So with java.util.LinkedList it's not possible.

You may have imported LinkedList from some other library other that "java.util.LinkedList"