Android Question If List1 = List2 doesn't work

Tim Chapman · Dec 29, 2025

I want to quickly compare two lists to see if they contain the same information.
Why does this code say the lists are equal when they are not?
The log shows List1 = List2.

List1=List2:

#Region  Project Attributes
    #ApplicationLabel: B4A Example
    #VersionCode: 1
    #VersionName:
    'SupportedOrientations possible values: unspecified, landscape or portrait.
    #SupportedOrientations: unspecified
    #CanInstallToExternalStorage: False
#End Region

#Region  Activity Attributes
    #FullScreen: False
    #IncludeTitle: True
#End Region

Sub Process_Globals
    'These global variables will be declared once when the application starts.
    'These variables can be accessed from all modules.
End Sub

Sub Globals
    'These global variables will be redeclared each time the activity is created.
End Sub

Sub Activity_Create(FirstTime As Boolean)
    Activity.LoadLayout("Layout")
    Dim List1 As List
    List1.Initialize
    Dim List2 As List
    List2.Initialize
   
    List1.AddAll(Array As String("value1", "value2"))
    List2.AddAll(Array As String(""))
   
    If List1 = List2 Then
        Log("List1 = List2")
    End If
End Sub

Sub Activity_Resume

End Sub

Sub Activity_Pause (UserClosed As Boolean)

End Sub

Star-Dust · Dec 29, 2025

Lists are complex objects and are managed by reference, not by value. You're comparing references, which will never be correct. Loop through the list and compare the individual values. But I'd start by comparing the length of the lists.

LucaMs · Dec 29, 2025

I haven't tried with B4A but with B4J the log doesn't appear.

LucaMs · Dec 29, 2025

Star-Dust said:
Lists are complex objects and are managed by reference, not by value. You're comparing references, which will never be equal. Loop through the list and compare the individual values. But I'd start by comparing the length of the lists.

I was thinking the same thing, @Star-Dust , but then I read the question carefully.

Star-Dust · Dec 29, 2025

The answer doesn't change, the comparison will never give a correct result. The content values must be compared with loop.

LucaMs · Dec 29, 2025

Star-Dust said:
The answer doesn't change, the comparison will never give a correct result. The content values must be compared with loop.

The question is: why is the log "List1 = List2" written even though they are two different objects (and with different content, but that's not the point)

Il log non dovrebbe avvenire, dato IF List1 = List2 Then Log... (ma non per il contenuto, per i puntatori diversi, ovviamente).

LucaMs · Dec 29, 2025

LucaMs said:
I haven't tried with B4A but with B4J the log doesn't appear.

Tried (B4A): no log!

Tim Chapman · Dec 29, 2025

Thank you both for your replies. I am not sure why LucaMs is getting no log, but that's OK.

Sagenut · Dec 29, 2025

Could something like this work correctly?

B4X:

Dim List1 As List
Dim List2 As List
Dim List3 As List
List1.Initialize
List2.Initialize
List3.Initialize
List1.AddAll(Array As String("One", "Two", "Three", "Four"))
List2.AddAll(Array As String("Four", "Three", "Two", "One"))
List3.AddAll(Array As String("One", "Two", "Three", "Four"))
Dim s As B4XSerializator
Dim bc As ByteConverter
Dim b1 As String = bc.StringFromBytes(s.ConvertObjectToBytes(List1), "UTF8")
Dim b2 As String = bc.StringFromBytes(s.ConvertObjectToBytes(List2), "UTF8")
Dim b3 As String = bc.StringFromBytes(s.ConvertObjectToBytes(List3), "UTF8")
If b1 = b3 Then
    Log("List1 is equal to List3")
End If
If b1 = b2 Then
Else
    Log("List1 is NOT equal to List2")
End If

Tested in B4J.
Flagged libraries:
RandomAccessFile
ByteConverter

aeric · Dec 29, 2025

LucaMs said:
Tried (B4A): no log!

No log because it is false.

B4X:

If List1 = List2 Then
    Log("List1 = List2")
Else
    Log("List1 <> List2")
End If

I tried in B4A and B4J, it shows "List1 <> List2"

hatzisn · Dec 29, 2025

Maybe a sub that compares initially the lengths and if they are both the same, then sort both lists and compare each individual value in X position in both lists, would do the trick.

Daestrum · Dec 29, 2025

Maybe

B4X:

Log (list1.As(JavaObject).RunMethod("equals",Array(list2)))

Tim Chapman · Dec 29, 2025

The log always shows when I run it. I don't know why some are seeing otherwise.
I converted the relevant contents of the lists to one string for the entire list, then compared the strings to see if they are equal. This is a definitive test to see if they same.

aeric · Dec 29, 2025

You may also try:

B4X:

If List1.As(JSON) = List2.As(JSON) Then
    Log("List1 is equal to List2")
Else
    Log("List1 is not equal to List2")
End If

Tim Chapman · Dec 29, 2025

aeric said:
If List1.As(JSON) = List2.As(JSON) Then Log("List1 is equal to List2") Else Log("List1 is not equal to List2") End If

This works.

Also, this time when I ran the code my original code shows they are not equal as it should. Go figure!

Sagenut · Dec 30, 2025

aeric said:

You may also try:

B4X:

If List1.As(JSON) = List2.As(JSON) Then
    Log("List1 is equal to List2")
Else
    Log("List1 is not equal to List2")
End If

Wow!
I never thought to such solution.
Nice find.
Thanks

aeric · Dec 30, 2025

Sagenut said:
Wow!
I never thought to such solution.
Nice find.
Thanks

B4X is amazing!

Alessandro71 · Dec 30, 2025

syntactically elegant, but probably not optimized: you'll need to convert the whole list to string before comparing, while an iterative item-by-item comparison would stop at the first difference.
for really short lists it may does not really matter at all.

LucaMs · Dec 30, 2025

Alessandro71 said:
syntactically elegant, but probably not optimized: you'll need to convert the whole list to string before comparing, while an iterative item-by-item comparison would stop at the first difference.
for really short lists it may does not really matter at all.

...and it's not certain that it would work with custom types/classes.

Filippo · Dec 30, 2025

Tim Chapman said:

I want to quickly compare two lists to see if they contain the same information.
Why does this code say the lists are equal when they are not?
The log shows List1 = List2.

List1=List2:

#Region  Project Attributes
    #ApplicationLabel: B4A Example
    #VersionCode: 1
    #VersionName:
    'SupportedOrientations possible values: unspecified, landscape or portrait.
    #SupportedOrientations: unspecified
    #CanInstallToExternalStorage: False
#End Region

#Region  Activity Attributes
    #FullScreen: False
    #IncludeTitle: True
#End Region

Sub Process_Globals
    'These global variables will be declared once when the application starts.
    'These variables can be accessed from all modules.
End Sub

Sub Globals
    'These global variables will be redeclared each time the activity is created.
End Sub

Sub Activity_Create(FirstTime As Boolean)
    Activity.LoadLayout("Layout")
    Dim List1 As List
    List1.Initialize
    Dim List2 As List
    List2.Initialize
  
    List1.AddAll(Array As String("value1", "value2"))
    List2.AddAll(Array As String(""))
  
    If List1 = List2 Then
        Log("List1 = List2")
    End If
End Sub

Sub Activity_Resume

End Sub

Sub Activity_Pause (UserClosed As Boolean)

End Sub

If the lists contain simple strings, then the following procedure can help:

B4X:

Sub Activity_Create(FirstTime As Boolean)
    Activity.LoadLayout("1")

    Dim List1 As List
    Dim List2 As List
    List1.Initialize
    List2.Initialize
    
    List1.AddAll(Array As String("One", "Two", "Three", "Four"))
    List2.AddAll(Array As String("Four", "Three", "Two", "One"))

    Wait For (CompareList(List1, List2)) complete(result As Boolean)
    Log("Result=" & result)

End Sub

Private Sub CompareList(lst1 As List, lst2 As List) As ResumableSub
    If lst1.Size <> lst2.Size Then Return False
    For i = 0 To lst2.Size -1
        If lst1.IndexOf(lst2.Get(i)) = -1 Then Return False
    Next
    Return True
End Sub

Android Question If List1 = List2 doesn't work

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