Add java/lang/StringBuffer::charAt(I)C

Author: dlunch

java_runtime/src/classes/java/lang/string_buffer.rs

@@ -231,4 +231,13 @@ impl StringBuffer {
 
         Ok(count)
     }
+
+    async fn char_at(jvm: &Jvm, _: &mut RuntimeContext, this: ClassInstanceRef<Self>, index: i32) -> Result<JavaChar> {
+        tracing::debug!("java.lang.StringBuffer::charAt({:?}, {:?})", &this, index);
+
+        let java_value: ClassInstanceRef<Array<JavaChar>> = jvm.get_field(&this, "value", "[C").await?;
+        let char_at: JavaChar = jvm.load_array(&java_value, index as _, 1).await?.into_iter().next().unwrap();
+
+        Ok(char_at)
+    }
 }

java_runtime/tests/classes/java/lang/test_string_buffer.rs

@@ -24,6 +24,9 @@ async fn test_string_buffer() -> Result<()> {
     let length: i32 = jvm.invoke_virtual(&string_buffer, "length", "()I", ()).await?;
     assert_eq!(length, 12);
 
+    let char: JavaChar = jvm.invoke_virtual(&string_buffer, "charAt", "(I)C", (7,)).await?;
+    assert_eq!(char, '4' as JavaChar);
+
     let result = jvm.invoke_virtual(&string_buffer, "toString", "()Ljava/lang/String;", ()).await?;
     let result = JavaLangString::to_rust_string(&jvm, &result).await?;