Counting the number of common elements between two arraylists - Java

Question

I'm trying to see how many common elements two array lists have. This is the code I've come up with but this unfortunately returns: Exception in thread "AWT-EventQueue-0"

Here is the code:

private int score(int correctAnswers){ 
    correctAnswers = 0;

    Collections.addAll(quiz1Answers, "ottawa", "toronto", "tokyo", "northamerica", "seoul", "Asia", "unitedstates", "50");
    Collections.addAll(quiz1UserAnswers, "ottawa", "dfdf", "tokyo", "northamerica", "seoul", "x", "unitedstates", "50");

    for (int i = 0; i <= quiz1Answers.size(); i++)
    {
        if (quiz1Answers.contains(quiz1UserAnswers.get(i)))
        {
            correctAnswers++;

        }
    } 
    return correctAnswers;
}

How would I go about solving this?

EDIT: Here is the full error messsage:

Exception in thread "AWT-EventQueue-0" java.lang.IndexOutOfBoundsException: Index: 8, Size: 8
at java.util.ArrayList.rangeCheck(ArrayList.java:635)
at java.util.ArrayList.get(ArrayList.java:411)
at Quiz1.score(Quiz1.java:309)
at Quiz1.jButton4ActionPerformed(Quiz1.java:364)
at Quiz1.access$400(Quiz1.java:16)
at Quiz1$5.actionPerformed(Quiz1.java:148)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$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.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:289)
at java.awt.Component.processMouseEvent(Component.java:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
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:4687)
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

Java ArrayList is indexable from 0 to size() - 1, when you access the element at index size() you are out of bounds.

for (int i = 0; i < quiz1Answers.size(); i++) // not <=

From the ArrayList.get(int index) JavaDoc (bold added),

Throws:

if the index is out of range (index < 0 || index >= size() )

Also, your if condition appears incorrect and you should test that the index isn't outside second collection's range,

if (quiz1UserAnswers.size() < i && quiz1UserAnswers.get(i).equals(quiz1Answers.get(i)))