java-me-guard

name	java-me-guard
description	Enforces Java ME constraints when writing code in signer/. Prevents Java 5+ language features, validates CLDC 1.1 and MIDP 2.0 API usage, and monitors JAR size. Use whenever editing or creating Java files under signer/src/ or signer/test/.

Java ME Constraint Enforcer

You are working in an extremely constrained Java ME environment targeting the Nokia C1-01 feature phone. Every line of code must be compatible with CLDC 1.1 / MIDP 2.0 and compile with javac -source 1.4 -target 1.4.

Hard Constraints — Never Violate These

Language Level: Java 1.4 Only

FORBIDDEN Java 5+ features (will cause compilation failure):

Feature	Java Version	What to Write Instead
Generics List<String>	5	Vector / raw types
Enhanced for-loop for (X x : list)	5	for (int i = 0; i < v.size(); i++)
Autoboxing Integer i = 5	5	Integer i = new Integer(5)
Varargs foo(String... args)	5	foo(String[] args)
Enums enum	5	public static final int constants
Annotations @Override	5	Remove annotation
Static imports	5	Use fully qualified names
StringBuilder	5	StringBuffer
String.format()	5	Manual concatenation
Arrays.copyOf()	6	System.arraycopy()
Try-with-resources	7	Explicit finally blocks
Diamond operator <>	7	N/A (no generics)
Lambda expressions	8	Anonymous inner classes
Optional	8	Null checks

Collections Available

Only these are available under CLDC 1.1:

• java.util.Vector (not ArrayList)
• java.util.Hashtable (not HashMap)
• java.util.Stack
• java.util.Enumeration (not Iterator)

String Operations

• No String.isEmpty() — use s.length() == 0
• No String.contains() — use s.indexOf(sub) >= 0
• No String.split() — write manual tokenizer
• No StringBuilder — use StringBuffer
• No String.format() — use concatenation

I/O and Streams

• No java.io.Reader/Writer (limited subset)
• No java.nio.* (does not exist)
• No BufferedReader for text — use InputStream with byte arrays
• Storage: javax.microedition.rms.RecordStore only

Numeric Types

• BigInteger is available (we include rt.jar for this)
• No BigDecimal
• Be careful with long — some CLDC implementations have quirks

Crypto

• Bouncy Castle bcprov-jdk14 only (injected via ProGuard, not as library jar)
• Use org.bouncycastle.crypto.* API, not JCE providers
• No javax.crypto.* (not in CLDC)

JAR Size Budget

• Hard limit: 1,048,576 bytes (1 MB)
• Current size: ~286 KB
• Budget remaining: ~738 KB
• Every class you add costs bytes. Prefer extending existing classes over creating new ones.
• Run make size-check after any structural changes.

Test Code Exceptions

Files under signer/test/ compile with -source 1.8 and run on desktop JDK 8. Tests MAY use:

• @Override annotation
• Enhanced for-loops
• Generics
• String.format()
• assertEquals, assertTrue, etc. (JUnit 4)

Tests MUST NOT use:

• Lambdas (keep tests readable for Java ME developers)
• Streams API
• Try-with-resources (keep test patterns close to production code)

Build Verification

After writing signer code, always verify:

# Compile + run tests (153 tests must pass)
cd signer && JAVA_HOME=$SIGNER_JAVA ant test

# Check JAR size
make size-check

Common Mistakes to Avoid

1. Forgetting finally blocks — CLDC has no try-with-resources
2. Using System.out.println — Not available on device. Use custom logging or remove
3. Assuming UTF-8 — Use explicit encoding: new String(bytes, "UTF-8")
4. Large string literals — The 2048-word BIP39 wordlist is already in Bip39Wordlist.java; don't duplicate
5. Thread safety — MIDP is single-threaded for UI; background tasks need Thread + callbacks