引用JTabbedPane的属性时发生NullPointerException

Question

What is wrong with the logic behind creating a JTabbedPane and getting the tab count of 11, then using a setter?

I try to reference the tabCount of the SetObjects.currentJTabbedPane and it throws NullPointerException .

create

jtp.setSelectedIndex(0);

int jtpCount=jtp.getTabCount();
System.out.println(C+M+AND+jtpCount+": jtpCount of The JTabbedPane created: \n");

setCurrentJTabbedPane(jtp);
int currentCount=SetObjects.currentJTabbedPane.getTabCount();
System.out.println(C+M+AND+currentCount+": currentCount for
    SetObjects.currentJTabbedPane.getTabCount() : \n");

setter

public static void setCurrentJTabbedPane(JTabbedPane currentJTabbedPane) {
    String M =(" --> in setCurrentJTabbedPane() var: <-- \n");
    System.out.println(M);

    SetObjects.currentJTabbedPane = currentJTabbedPane;
    System.out.println(C+M+AND+SetObjects.currentJTabbedPane.getTabCount()+":
        SetObjects.currentJTabbedPane.getTabCount() : \n");
}

output

CLASS
SetObjects:
--> JTabbedPane obj() var: xxxxxxxxx : xxxxxxxx<--
: 11: jtpCount of The JTabbedPane created:

 --> in setCurrentJTabbedPane() var: <-- 

Exception in thread "main" java.lang.NullPointerException
at view.SetObjects.setCurrentJTabbedPane(SetObjects.java:385)
at view.SetObjects.obj(SetObjects.java:217)
at view.SchoolJDesktopPane.createInnerFrame(SchoolJDesktopPane.java:119)
at view.SchoolJDesktopPane.(SchoolJDesktopPane.java:97)
at view.Main.createJDesktopPane(Main.java:60)
at view.Main.main(Main.java:54)

Answer 1

Are you passing null to setCurrentJTabbedPane ? Try adding more System.out.println statements to inspect each variable that might be the problem. Also, make your debugging statements simpler, ie replace System.out.println(A + B + C) with three different statements.

EDIT: The reason for splitting up debug statements is that if System.out.println(object1.foo() + ", " + object2.bar()) throws a NullPointerException then you don't know which object was null. Splitting it into two statements means the stack trace tells you exactly where the problem was.