[GROOVY-1171] NullPointerException caused by interaction of for loop scope and SwingBuilder

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 1.0-JSR-4
• Fix Version/s: 1.0-JSR-5
• Environment: Ubuntu Linux, 1.0-JSR-4, on JDK 1.5.0
• Assignee: Jochen Theodorou
• Reporter: John Riedl
• Created: 02/Dec/05 13:58
• Updated: 19/Jan/06 18:54
• Resolved: 19/Jan/06 18:54

Description

I get a NullPointerException with this simple program unless I change the name of the variable inside the second for loop. The example is:

import groovy.swing.SwingBuilder

final int N_BUTTONS = 1

for (i in 0..<N_BUTTONS) {
for (j in 0..<N_BUTTONS) {
}
}
//println j

// I get:
// groovy simpleswing.groovy
// Caught: java.lang.NullPointerException
// at simpleswing$_run_closure1.doCall(simpleswing.groovy)
// at simpleswing$_run_closure1.doCall(simpleswing.groovy)
// at simpleswing.run(simpleswing.groovy:21)
// at simpleswing.main(simpleswing.groovy)
// If I uncomment the println j, then I get:
// 29: The current scope does already contain a variable of the name j
// @ line 29, column 7.
// for (j in 1..N_BUTTONS) {

// The problem goes away if I use any variable other than the inner
// loop variable from above as the loop variable below.

// I suspect the bug is related to the weird scope rules of for loop
// variables in Groovy. I recommend this behavior be changed: it
// leads to very confusing and frustrating situations for
// newbies. Much better to have the variable scope only be the for
// loop itself, unless it was declared prior to the for loop.

swing = new SwingBuilder()
gui = swing.frame() {
panel() {
for (j in 1..N_BUTTONS)

{ b = button(text: "foo") println "Creating button." }

}
}

gui.pack();
gui.show();