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Author: dellis1972

Documentation/guides/BindingJavaSource.md

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+---
+id:
+title: "Binding Java Source"
+dateupdated: 2021-05-21
+---
+
+# Overview
+
+Bindings can be done on not only .jar and .aar files but also raw
+.java code. This will allow developers of bindings or applications
+to write custom API's for underlying bindings and expose them
+easily to the developer.
+
+Applications already had the ability to include `AndroidJavaSource`
+items in them, these would then be compiled into the final `classes.dex`.
+However this custom java source code was not "bound" and if users wanted
+to call these classes they would need to do that manually.
+
+The `AndroidJavaSource` Item Group now supports a `Bind` attribute. This
+will instruct the build system to not only compile the code but also
+produce a C# API for it.
+
+```xml
+<ItemGroup>
+    <AndroidJavaSource Include="MyClass.java" Bind="True" />
+</ItemGroup>
+```
+
+The `Bind` attribute will default to `true` when in binding projects.
+But for applications it will need to be set manually.
+
+# Example
+
+Consider the following Java code.
+
+```java
+package com.xamarin.test;
+
+class MyClass {
+    public bool IsDave (String name)
+    {
+        return name.equals ("Dave");
+    }
+}
+```
+
+We want to bind this into a C# API in our app project.
+So we can add a new `AndroidJavaSource` items with the `Bind`
+attribute set to `true`.
+
+```xml
+<ItemGroup>
+    <AndroidJavaSource Include="MyClass.java" Bind="True" />
+</ItemGroup>
+```
+
+When we build the app, the java code will be compiled into a
+.jar file which matches the application project name.
+This .jar will then be bound and the generated C# API will end
+up being something like this.
+
+```csharp
+using System;
+
+namespace Com.Xamarin.Test {
+    public class MyClass : Java.Lang.Object {
+        public bool IsDave (string name){
+            // binding code
+        }
+    }
+}
+```
+
+All this will happen before the main C# code is compiled. The binding
+code will be included in the C# compile process, so you can use the
+code directly in your app.
+
+```csharp
+public class MainActivity : Activity {
+    public override void OnCreate()
+    {
+        var c = new MyClass ();
+        c.IsDave ("Bob");
+    }
+}
+```
+
+
+# Limitations
+
+You will be limited to standard java types and any types that
+are available in a .jar or .aar which you reference.
+
+You CANNOT use Java Generics. The Binding process currently does not
+really support Java Generic very well. So stick to primitive types
+or normal classes as much as possible. This is so that you don't need
+to use the metadata.xml to alter the API of the Java class.