Handle Javac Optimisation of String Concatenations

Description of the bug It appears that when I am trying to concatenate two Strings, when one is from the storage, then the compiler complains with the following message:

Inherited methods that are not specifically implemented are not supported. Implement the method 'toString' without a 'super' call to the class Object to use it.
io.neow3j.compiler.CompilerException: Inherited methods that are not specifically implemented are not supported. Implement the method 'toString' without a 'super' call to the class Object to use it.
        at io.neow3j.compiler.Compiler.throwIfObjectIsOwner(Compiler.java:603)
        at io.neow3j.compiler.Compiler.handleInsn(Compiler.java:584)
        at io.neow3j.compiler.NeoMethod.convert(NeoMethod.java:481)
        at io.neow3j.compiler.converters.MethodsConverter.handleUncachedMethodCall(MethodsConverter.java:224)

To Reproduce Steps to reproduce the behavior:

1. create new smart contract
2. try to concatenate two strings: map.getString(key) + “foo”;
3. or: map.get(key).toString() + “foo”
4. compile

Expected behavior It should be possible to concatenate two strings like this without compiling issues.

Platform and Software

• neow3j 3.17

[ykoitzsch]

I tested it, and it compiles just fine now @mialbu @gsmachado 😃

[mialbu]

@ykoitzsch I could reproduce now. Javac is run with different compile options in VSCode than in Intellij. This includes an optimisation concerning the use of the StringBuilder class. It ultimately breaks the compilation to Neo byte code when an optimised concatenation was produced by the Java compiler.

I’ll add this issue to our backlog for now and we’ll address it in the future. As for now, until we’ve fixed this, please use the gradle plugin to compile when developing with VSCode.

If you’re interested in more details, check out this and this.

Additional Info

The following contract class fails to compile using VSCode:

class StringConcatenation {
        private static final StorageContext ctx = Storage.getStorageContext();
        public static String getSomething(String key) {
            return Storage.getString(ctx, key) + "foo";
        }
}

In the following I’ve listed the pattern of the compiled asm class when using either Intellij or VSCode.

Parts of the asmClass instructions based on compilation using Intellij IDE

TypeInsnNode StringBuilder 187
InsnNode 89
MethodInsnNode StringBuilder <init> 183
FieldInsnNode StorageContext ctx 178
VarInsnNode 25
MethodInsnNode Storage getString 184
MethodInsnNode StringBuilder append 182
LdcInsnNode 18
MethodInsnNode StringBuilder append 182
MethodInsnNode StringBuilder toString 182
InsnNode 176

Parts of the asmClass instructions based on compilation using VSCode IDE with RedHat Java extension

TypeInsnNode StringBuilder 187
InsnNode 89
FielInsnNode StorageContext ctx 178
VarInsnNode 25
MethodInsnNode Storage getString 184
MethodInsnNode String valueOf 184
MethodInsnNode String <init>
LdcInsnNode 18
MethodInsnNode StringBuilder append 182
MethodInsnNode StringBuilder toString 182
InsnNode 176