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🔸 TL;DR
Web3j is a Java/Android library that lets you talk to Ethereum-compatible blockchains via JSON-RPC. You can read on-chain data, sign & send transactions, and generate type-safe smart contract wrappers (ABI + bytecode) so contracts feel like normal Java APIs.
🔸 TAKEAWAYS
▪️ Connect to nodes (Geth, Besu, Erigon, Quorum…) using HTTP / WebSocket / IPC
▪️ Read the chain: blockNumber, balances, transactions, logs/events
▪️ Write to the chain: build → sign → send → wait for receipt (hash ≠ confirmed)
▪️ Separate read vs write: reads are RPC calls, writes need gas + nonce + signing
▪️ Don’t mess up security: never hardcode private keys (use proper key management)
▪️ Treat RPC like any remote dependency: timeouts, retries, provider quirks
▪️ Prefer wrappers for events/logs: topics + indexed params can be tricky
#Web3j #Java #Ethereum #EVM #Web3 #Blockchain #SmartContracts #Solidity #BackendEngineering #DeFi
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