Bugfix for empty string in const pool

tests/empty_string_in_cpool.rs

@@ -0,0 +1,12 @@
+mod utils;
+use utils::get_int;
+use utils::setup;
+
+#[test]
+fn empty_string_in_cpool() {
+    let mut vm = setup();
+    let last_frame_value = vm
+        .run("samples.javacore.strings.trivial.EmptyStringInCPool")
+        .unwrap();
+    assert_eq!(0, get_int(last_frame_value))
+}

tests/test_data/EmptyStringInCPool.java

@@ -0,0 +1,7 @@
+package samples.javacore.strings.trivial;
+
+public class EmptyStringInCPool {
+    public static void main(String[] args) {
+        int result = "".length();
+    }
+}

vm/src/execution_engine/string_pool_helper.rs

@@ -17,6 +17,10 @@ impl StringPoolHelper {
     }
 
     fn create_string(string: &str) -> crate::error::Result<i32> {
+        if string.is_empty() {
+            return Self::create_empty_string();
+        }
+
         let codepoints = Self::string_to_codepoints(&string);
 
         let array_ref =
@@ -50,6 +54,37 @@ impl StringPoolHelper {
         Ok(string_instance_ref)
     }
 
+    // todo: consider creating all CPool strings like this
+    fn create_empty_string() -> crate::error::Result<i32> {
+        let byte_array_ref =
+            with_heap_write_lock(|heap| heap.create_array_with_values("[b", &vec![]));
+        let string_class_name = "java/lang/String".to_string();
+        let instance_with_default_fields = with_method_area(|method_area| {
+            method_area.create_instance_with_default_fields(&string_class_name)
+        });
+
+        let string_instance_ref =
+            with_heap_write_lock(|heap| heap.create_instance(instance_with_default_fields));
+
+        let full_signature = "<init>:([BB)V";
+        let rc = with_method_area(|method_area| method_area.get(string_class_name.as_str()))?;
+        let special_method = rc
+            .methods
+            .method_by_signature
+            .get(full_signature)
+            .ok_or_else(|| Error::new_constant_pool(&format!("Error getting JavaMethod by class name {string_class_name} and full signature {full_signature} invoking special")))?;
+
+        let mut engine = Engine::new();
+        let mut stack_frame = special_method.new_stack_frame()?;
+        stack_frame.set_local(0, string_instance_ref);
+        stack_frame.set_local(1, byte_array_ref);
+        stack_frame.set_local(2, 0); // coder LATIN1
+
+        engine.execute(stack_frame)?;
+
+        Ok(string_instance_ref)
+    }
+
     fn string_to_codepoints(s: &str) -> Vec<i32> {
         s.chars().map(|c| c as i32).collect()
     }