区块链平台的比较和选择：Ethereum、Hyperledger和Corda的优缺点和适用场景

// 示例代码：一个简单的智能合约，实现存储和检索数据功能
pragma solidity ^0.8.0;

contract SimpleStorage {
    uint256 storedData;

    function set(uint256 x) public {
        storedData = x;
    }

    function get() public view returns (uint256) {
        return storedData;
    }
}

// 示例代码：使用Hyperledger Fabric编写智能合约
package main

import (
    "fmt"
    "github.com/hyperledger/fabric-contract-api-go/contractapi"
)

type SimpleChaincode struct {
    contractapi.Contract
}

func (s *SimpleChaincode) Set(ctx contractapi.TransactionContextInterface, key string, value string) error {
    return ctx.GetStub().PutState(key, []byte(value))
}

func (s *SimpleChaincode) Get(ctx contractapi.TransactionContextInterface, key string) (string, error) {
    value, err := ctx.GetStub().GetState(key)
    if err != nil {
        return "", fmt.Errorf("failed to read from world state: %v", err)
    }
    if value == nil {
        return "", fmt.Errorf("the asset %s does not exist", key)
    }
    return string(value), nil
}

// 示例代码：使用Corda编写简单的智能合约
import net.corda.core.contracts.CommandData
import net.corda.core.contracts.Contract
import net.corda.core.contracts.ContractState
import net.corda.core.contracts.requireThat
import net.corda.core.transactions.LedgerTransaction

// 定义智能合约
class SimpleContract : Contract {
    companion object {
        const val ID = "com.example.SimpleContract"
    }

    override fun verify(tx: LedgerTransaction) {
        val output = tx.outputsOfType<SimpleState>().single()
        requireThat {
            "The data field cannot be empty." using (output.data.isNotEmpty())
        }
    }
}

// 定义智能合约状态
data class SimpleState(val data: String) : ContractState {
    override val participants get() = emptyList<Party>()
}

// 定义智能合约命令
interface Commands : CommandData {
    class Create : Commands
}