正在执行操作的Java无法添加到arrayList，结果为“ AWT-EventQueue-0” java.lang.UnsupportedOperationException

Question

Thanks! This problem is solved by new ArrayList. phrases = new ArrayList(Arrays.asList(s));

First, thank-you stackoverflow.com, for having such good information and help! Below is my minimum failing case. I am running java jdk 1.7.0_60 from a cygwin window on Windows 7. The problem is line 60 inside actionPerformed where I try to do: phrases.add("QUIT;"); where phrases is an array list. Although work-around is to append QUIT to JTextArea , I would like to understand why the phrases.add() does not work.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.lang.*;
import java.io.*;
import java.util.List;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.Collections;

class MFC extends JFrame implements ActionListener {
    JTextArea myTextArea = new JTextArea(12, 30);

    //same result if public or private
    private List<String> phrases = new ArrayList<String>(Arrays.asList(
            "Type or paste your SQL here","quit;"));

    MFC() {
        super("List<String>");
        setSize(370, 300);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        FlowLayout flow = new FlowLayout(FlowLayout.RIGHT);
        setLayout(flow);
        JPanel row1 = new JPanel();

        myTextArea.setLineWrap(true);
        myTextArea.setWrapStyleWord(true);

        // initialize the text area from initial phrases
        for(int i=0; i < phrases.size(); i++) {
           myTextArea.append(phrases.get(i) + "\n");
        }
        //http://stackoverflow.com/questions/23701113/keeping-the-text-in-a-java-textfield-always-selected
        //Make sure the new text is visible, even if there
        //was a selection in the text area.
        myTextArea.setCaretPosition(myTextArea.getDocument().getLength());

        JScrollPane scroll = new JScrollPane(myTextArea,
           JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
           JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
        row1.add(scroll);
        add(row1);
        JPanel row2 = new JPanel();
        JButton execute = new JButton("Execute");
        execute.addActionListener(this);
        row2.add(execute);
        add(row2);
        setVisible(true);
    } // end MFC()

    public void actionPerformed(ActionEvent evt) {
        myTextArea.append("QUIT;");                      //this works
        String s[] = myTextArea.getText().split("\\n");  //this works
        phrases = Arrays.asList(s);                      //this works

        dump();

        /*** THIS IS THE STATEMENT NOT WORKING ***/
        phrases.add("QUIT;");                     // NOT WORKING

       dump();

    }//end actionPerformed()

    public static void main(String[] arguments) {
        MFC mfc = new MFC();
    }

    void dump() {
        System.out.println("phrases.size() is " + phrases.size());
        Iterator iterator1 = phrases.iterator();   // single use
        while (iterator1.hasNext()) {
            System.out.println("Phrase " + iterator1.next());  // works
        }
    }
}//end MFC class

Here is the result:

$ java MFC
phrases.size() is 5
Phrase abc
Phrase def
Phrase 123
Phrase 456
Phrase QUIT;
Exception in thread "AWT-EventQueue-0" java.lang.UnsupportedOperationException
        at java.util.AbstractList.add(AbstractList.java:148)
        at java.util.AbstractList.add(AbstractList.java:108)
        at MFC.actionPerformed(MFC.java:60)
        at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
        at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
        at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
        at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
        at java.awt.Component.processMouseEvent(Component.java:6516)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
        at java.awt.Component.processEvent(Component.java:6281)
        at java.awt.Container.processEvent(Container.java:2229)
        at java.awt.Component.dispatchEventImpl(Component.java:4872)
        at java.awt.Container.dispatchEventImpl(Container.java:2287)
        at java.awt.Component.dispatchEvent(Component.java:4698)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
        at java.awt.Container.dispatchEventImpl(Container.java:2273)
        at java.awt.Window.dispatchEventImpl(Window.java:2719)
        at java.awt.Component.dispatchEvent(Component.java:4698)
        at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
        at java.awt.EventQueue.access$200(EventQueue.java:103)
        at java.awt.EventQueue$3.run(EventQueue.java:694)
        at java.awt.EventQueue$3.run(EventQueue.java:692)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
        at java.awt.EventQueue$4.run(EventQueue.java:708)
        at java.awt.EventQueue$4.run(EventQueue.java:706)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Answer 1

You've assigned your List variable phrases to Arrays.asList which returns a fixed size List

phrases = Arrays.asList(s);

Since elements can be added to an ArrayList you could just use

phrases = new ArrayList<>(Arrays.asList(s));

as you did originally.

Answer 2

Arrays.asList() doesn't create a new list, really. It gives you an object that acts like a List , but really just accesses the array. Since you can't make an array larger by adding elements to it, you can't append to this kind of list, either.

Instead of

phrases = Arrays.asList(s); 

try

phrases = new ArrayList<String>(Arrays.asList(s));

which does create a new list, and initializes it by copying the elements of the array.

Answer 3

Read the javadoc :

public static List asList(T... a)

Returns a fixed-size list backed by the specified array.

(emphasis mine)