MySQL 入门：CRUD、约束与事务

import java.util.*;
import java.math.BigInteger;
public class Main {
    record Transfer(String from,String to,long amount) {}
    static Map<String,Long> apply(Map<String,Long> original,List<Transfer> transfers) {
        Map<String,Long> work=new HashMap<>(original);
        for (long balance:work.values())
            if (balance<0) throw new IllegalArgumentException("negative initial");
        for (Transfer t:transfers) {
            if (t.amount()<=0||t.from().equals(t.to())||
                    !work.containsKey(t.from())||!work.containsKey(t.to()))
                throw new IllegalArgumentException("transfer");
            long source=work.get(t.from()), target=work.get(t.to());
            if (source<t.amount()) throw new IllegalArgumentException("insufficient");
            // 先完成所有计算，目标溢出时不修改工作状态。
            long newTarget=Math.addExact(target,t.amount());
            work.put(t.from(),source-t.amount());
            work.put(t.to(),newTarget);
        }
        // 整批成功才发布新快照，调用方原始状态始终未动。
        return Map.copyOf(work);
    }
    static BigInteger total(Map<String,Long> balances) {
        BigInteger sum=BigInteger.ZERO;
        // 校验总额用大整数，避免测试断言自身溢出。
        for (long x:balances.values()) sum=sum.add(BigInteger.valueOf(x));
        return sum;
    }
    static void eq(Object expected,Object actual) {
        if (!Objects.equals(expected,actual)) throw new AssertionError(actual);
    }
    static void reject(Runnable r) {
        try { r.run(); throw new AssertionError("accepted"); }
        catch (IllegalArgumentException | ArithmeticException expected) {}
    }
    public static void main(String[] args) {
        Map<String,Long> initial=Map.of("A",100L,"B",0L);
        Map<String,Long> result=apply(initial,List.of(new Transfer("A","B",30)));
        eq(Map.of("A",70L,"B",30L),result);
        eq(total(initial),total(result));
        reject(()->apply(initial,List.of(new Transfer("A","B",30),new Transfer("A","B",100))));
        eq(Map.of("A",100L,"B",0L),initial);
        reject(()->apply(initial,List.of(new Transfer("A","C",1))));
        reject(()->apply(Map.of("A",1L,"B",Long.MAX_VALUE),List.of(new Transfer("A","B",1))));
        eq(initial,apply(initial,List.of()));
        reject(()->apply(initial,List.of(new Transfer("A","A",1))));
        System.out.println("PASS 8 transaction invariants");
    }
}