B4J Tutorial Mini-Tutorial Creating a B4J Jar Library in Java

Overview
This repository is a mini-tutorial that shows how to create a simple B4J library (jMathLib) written in Java and use it inside a B4J application.

It’s not meant as a full-blown library project, but rather as a step-by-step guide for hobbyists (like myself) who haven’t touched Java/B4J library development for a while and need a quick refresher.

You’ll learn how to:

1. Write a small Java class (MathLib.java).
2. Compile it into the B4J library jMathLib using the Simple Library Compiler (SLC).
3. Add it to your B4J Additional Libraries folder.
4. Reference and use it in a B4J B4XPages test app.
5. (Optional) Generate Javadoc documentation.

Everything is kept simple and self-contained.
If you just want the ready-to-use library and demo, download the archive containing the latest source codes B4JHowToCreateJarLibrary.zip.

Note
This tutorial uses

• JDK 8 because the SLC requires it. Paths in the examples assume installation under `C:\Prog\jdk8`, but you can adapt them to your setup.
• JDK21 for thr Java library `MathLib.java`.

---

Quick Start

1. Download the ZIP
Download the attached archive `B4JHowToCreateJarLibrary.zip` and unzip it to a folder of choice.

2. Copy the JAR
Place `jMathLib.jar` into your B4J `Additional Libraries` folder.

3. Refresh libraries
In B4J, click Windows > Library Manager > Right click Refresh to make the library show up.

4. Use in your project
Select the library `jMathLib` from the Library Manager.

Dim ml As MathLib
ml.Initialize
Log(ml.add(2,3))   ' Prints 5

Folder Structure
Project Folder: c:\Daten\b4\b4j\b4jhowto\B4JHowToCreateJarLibrary
with sub-folders (Notes)
Java Library
+-Java
+--bin (will be created during the build)
+--bin\classes\de\rwbl\mathlib\MathLib.class
+--libs (put any additional libraries that should be referenced during compilation)
+--src
+--src\de\rwbl\mathlib\MathLib.java (Library Java source)
+--src\docs (JavaDoc documentation)
+--src\genjavadoc.bat (Generate Javadoc)

B4J B4XPages Test Application
+-B4J (Main folder which has several sub-folders as given by the B4XPages project)

Batch
+-slc.bat (Simple Library Compiler to create the jMathLib.jar & .xml)

---

Step 1 – Write Java class
File: MathLib.java
Notes

• Initialize currently does nothing. It is only included to follow common B4J library patterns and can be safely omitted for static-only classes.
• The source code example below shows version 0.1, but the source code in the archive has the latest like version 0.5 or higher.

/**
 * File: MathLib.java
 * Version: 0.1.0
 */
package de.rwbl.mathlib;

// Anywhere Software
import anywheresoftware.b4a.BA;
import anywheresoftware.b4a.BA.ActivityObject;
import anywheresoftware.b4a.BA.DependsOn;
import anywheresoftware.b4a.BA.Permissions;
import anywheresoftware.b4a.BA.ShortName;
import anywheresoftware.b4a.BA.Version;

@Version(0.1f)
@ShortName("MathLib")
@ActivityObject

public class MathLib {

    @BA.Hide
    private BA ba;   

    /** Common math constant PI */
    public static final double PI = 3.141592653589793;

    /** Common math constant E */
    public static final double E = 2.718281828459045;

    /**
     * Init new instance
     */
    public void Initialize() {
        // Does nothing (yet).
    }

    /**
     * Add two double values.
     * @param a first operand
     * @param b second operand
     * @return sum of a and b
     */
    public static double add(double a, double b) {
        return a + b;
    }

    /**
     * Subtract one double value from another.
     * @param a first operand
     * @param b second operand
     * @return result of a - b
     */
    public static double subtract(double a, double b) {
        return a - b;
    }

    /**
     * Multiply two double values.
     * @param a first operand
     * @param b second operand
     * @return product of a and b
     */
    public static double multiply(double a, double b) {
        return a * b;
    }

    /**
     * Divide one double value by another safely.
     * @param a numerator
     * @param b denominator
     * @return quotient of a / b, or Double.NaN if b == 0
     */
    public static double divide(double a, double b) {
        if (b == 0) {
            System.err.println("MathLib.divide: Division by zero detected!");
            return Double.NaN; // safer than returning 0
        }
        return a / b;
    }

}

---

Step 2 – Compile with SLC
Windows .bat `slc.bat` compiles the Java sources and generates the library Jar & XML in the B4J additional libraries folder.
Requires JDK 8.

REM slc.bat
REM B4J Simple Library Compiler (SLC) for project MathLib.
REM Date 2025-09-26
REM
REM Documentation SLC:
REM https://www.b4x.com/android/forum/threads/tool-simple-library-compiler-build-libraries-without-eclipse.29918/#content
REM
REM Settings SLC:
REM Java8 Compiler = C:\Prog\jdk8\bin\javac.exe
REM Project Folder = C:\Daten\b4\b4j\b4jhowto\B4JHowToCreateJarLibrary
REM Library Name = jMathLib

REM Remove bin & classes
if exist bin\classes rd /s /q bin\classes
if exist bin rd /s /q bin

REM Path to the SLC
set SLC_COMPILER=c:\Daten\b4\b4j\tools\B4JSimpleLibraryCompiler

REM Start the SLC
%SLC_COMPILER%\b4j_librarycompiler.exe

with output example:

SLC (B4J)
Starting step: Compiling Java code.
Completed successfully.
Starting step: Creating jar file.
Completed successfully.
Starting step: Creating XML file.
Loading source file C:\Daten\b4\b4j\b4jhowto\B4JHowToCreateJarLibrary\Java\src\rwbl\mathlib\MathLib.java...
Constructing Javadoc information...
[-doclet, BADoclet]
[-docletpath, c:\Daten\b4\b4j\tools\B4JSimpleLibraryCompiler]
[-doclet, BADoclet]
[-docletpath, c:\Daten\b4\b4j\tools\B4JSimpleLibraryCompiler]
[-classpath, C:\Prog\B4J\B4J.exe\../libraries\jFX.jar;C:\Prog\B4J\B4J.exe\../libraries\jCore.jar;C:\Prog\jdk8\bin\..\jre\lib\jfxrt.jar;]
[-sourcepath, src]
[-b4atarget, C:\Daten\b4\b4j\libraries\jMathLib.xml]
starting....
Working with class: de.rwbl.mathlib.MathLib
finish: C:\Daten\b4\b4j\libraries\jMathLib.xml

Completed successfully.
*** Don't forget to refresh the libraries list in the IDE (right click and choose Refresh) ***

The files jMathLib.jar and jMathLib.xml are created in the B4J additional libraries folder: c:\Daten\b4\b4j\libraries\

---

Step 3 – Use in B4J app
This test application uses the library jMathLib.

Sub Class_Globals
    Private Root As B4XView
    Private xui As XUI
   
    Private TextFieldParamA As B4XView
    Private TextFieldParamB As B4XView
    Private LabelResult As B4XView
    Private ChoiceBoxMethod As ChoiceBox
   
    Private MathLib As MathLib
End Sub

Public Sub Initialize
    B4XPages.GetManager.LogEvents = True
End Sub

'This event will be called once, before the page becomes visible.
Private Sub B4XPage_Created (Root1 As B4XView)
    Root = Root1
    Root.LoadLayout("MainPage")
   
    B4XPages.SetTitle(Me, "jMathLib Example")
   
    ' Populate ChoiceBoxMethod
    ChoiceBoxMethod.Items.AddAll(Array As String("Add", "Subtract", "Multiply", "Divide"))
    ChoiceBoxMethod.SelectedIndex = 0

    ' Init the MathLib instance
    MathLib.Initialize

    ' Log constants & operations
    OperationsDemo   

    ' UI
    ' Set defaults
    TextFieldParamA.Text = 2
    TextFieldParamB.Text = 3
    LabelResult.Text = "Result"
End Sub

' Log all operations and constants.
Private Sub OperationsDemo
    ' Log constants
    Log("PI = " & MathLib.PI)
    Log("E  = " & MathLib.E)
    ' Operations
    Log("2 + 3 = " & MathLib.add(2, 3))
    Log("2 - 3 = " & MathLib.subtract(2, 3))
    Log("2 * 3 = " & MathLib.multiply(2, 3))
    Log("6 / 3 = " & MathLib.divide(6, 3))
End Sub

' Calculate result based on operation selected.
Private Sub ButtonCalculate_Click
    Try
        Dim a As Double = TextFieldParamA.Text
        Dim b As Double = TextFieldParamB.Text

        Select ChoiceBoxMethod.SelectedIndex
            Case 0
                Dim result As Double = MathLib.add(a, b)
            Case 1
                Dim result As Double = MathLib.subtract(a, b)
            Case 2
                Dim result As Double = MathLib.multiply(a, b)
            Case 3
                Dim result As Double = MathLib.divide(a, b)
        End Select
        LabelResult.Text = result
    Catch
        LabelResult.Text = "Invalid input"
    End Try
    Log($"${ChoiceBoxMethod.Items.Get(ChoiceBoxMethod.SelectedIndex)}: ${TextFieldParamA.Text}, ${TextFieldParamB.Text} = ${result}"$)
End Sub

---

Step 4 – Run the demo
Run the B4J demo application.

---

Step 5 - JavaDoc
Generate Javadoc documentation in the docs folder.
Create a dummy BA.java which is required for Javadoc generation, and should not be used by SLC.

package anywheresoftware.b4a;

/**
 * Dummy class for Javadoc generation only.
 * Contains empty definitions of B4J annotations.
 */
public class BA {

    /** Dummy Hide annotation */
    public @interface Hide {}

    /** Dummy ShortName annotation */
    public @interface ShortName {
        String value();
    }

    /** Dummy Version annotation */
    public @interface Version {
        float value();
    }
}

Goto folder: c:\Daten\b4\b4j\b4jhowto\B4JHowToCreateJarLibrary\Java\src>

Run Javadoc command with parameters

javadoc -d docs -sourcepath . -subpackages de.rwbl.mathlib -locale en_US -encoding UTF-8 -charset UTF-8 -J-Duser.language=en -J-Duser.country=US

Parameter explanation:

* `-d docs` → output folder for the generated documentation.
* `-sourcepath .` → location of your source files (here: current folder).
* `-subpackages de.rwbl.mathlib` → generate docs for this package and its subpackages.
* `-locale en_US` → sets locale to U.S. English (affects some date/number formats).
* `-encoding UTF-8` → specifies source file encoding.
* `-charset UTF-8` → specifies HTML charset in the generated pages.
* `-J-Duser.language=en` → forces JVM to use English language resources.
* `-J-Duser.country=US` → forces JVM to use U.S. country settings.

The documentation is created in the folder: c:\Daten\b4\b4j\b4jhowto\B4JHowToCreateJarLibrary\Java\src\docs\

Open the documentation with index.html

Important
Rename BA.jave to BA.javadoc to avoid being compiled by the SLC.
When generating new Javadoc documentation, rename back to BA.java.

Here is a Windows batch file `genjavadoc.bat` to create the Javadoc.
Run from the src folder.

@echo off
REM =============================================================
REM Generate Javadoc documentation for MathLib
REM Project: jMathLib (B4J Library)
REM Note: BA.javadoc is a stub needed for Javadoc only.
REM It is renamed temporarily to BA.java for Javadoc generation,
REM then restored so it won’t interfere with B4J SLC compilation.
REM =============================================================

echo Creating Javadoc for project MathLib.
echo =====================================

set SRC=.
set SRC_FOLDER=de\rwbl\mathlib
set DOCS=docs
set PKG=de.rwbl.mathlib

REM --- Remove docs folder ---
if exist %DOCS% rmdir /s /q %DOCS%

REM --- Ensure docs folder exists ---
if not exist %DOCS% mkdir %DOCS%

REM --- Temporarily rename BA.javadoc -> BA.java ---
if exist %SRC%\%SRC_FOLDER%\BA.javadoc ren %SRC%\%SRC_FOLDER%\BA.javadoc BA.java

REM --- Run javadoc with English output ---
javadoc -d %DOCS% -sourcepath %SRC% -subpackages %PKG% -locale en_US -encoding UTF-8 -charset UTF-8 -J-Duser.language=en -J-Duser.country=US

REM --- Rename BA.java back -> BA.javadoc ---
if exist %SRC%\%SRC_FOLDER%\BA.java ren %SRC%\%SRC_FOLDER%\BA.java BA.javadoc

REM --- Check if index.html is there ---
if exist %DOCS%\index.html  (
  echo.
  echo Javadoc generated. Open "%DOCS%\index.html".
  echo ===========================================
) else (
  echo.
  echo Javadoc not created. Check console for details.
  echo ===============================================
)

pause

---

 

[Johan Schoeman]

Nice tutorial