java - 更新Map值的有效方法

Question

我们有一个包含超过千万条记录的交易明细的数组

Key1    Key2    Value
-----------------------
A       B       <value>
B       C       <value>
D       A       <value>
...
...

要求是对每个记录的 Key1 中的“值”执行基本算术运算(加/减)，并将结果值添加到 Key2(在单个事务中)。应当维护交易秩序。生成的 map 应具有累积交易值。

Key     Result
A       <result>
B       <result>
C       <result>
...
...

请针对这种情况提出有效的解决方案。

编辑

很抱歉没有在之前的问题中明确说明这一点。

Sample data:
------------

Row1 -> A,B,Add,10.0
Row2 -> C,D,Subtract,20.0
Row3 -> D,B,Add,50.0
Row4 -> B,X,Subtract,30.0

Initial Map:
------------
A     1000
B     1000
C     1000
D     1000
X     1000

Row 1 => 10.0 to be subtracted from B and added to A (B:990 - A:1010)
Row 2 => 20.0 to be subtracted from C and added to D (C:980 - D:1020)
Row 3 => 50.0 to be subtracted from B and added to D (B:940 - D:1070)
Row 4 => 30.0 to be subtracted from B and added to X (B:910 - X:1030)

Resulting Map:
--------------
A    1010
B     910
C     980
D    1070
X    1030

Answer 1

您有一个交易列表。让我们用自己的类来表示一项交易

public class Transaction {

    private String target;
    private String source;
    private String operation;
    private int amount;

    public Transaction(String target, String source, String operation, int amount) {
        this.target = target;
        this.source = source;
        this.operation = operation;
        this.amount = amount;
    }

    // + getters

}

source将代表交易的来源和 target目标。在 "Add" 的情况下操作，将由 amount 推导出来源并且目标将增加金额。如果操作是"Subtract" ，交易被撤销。

然后，给定 Map<String, Integer> map保存初始值，我们可以循环这些交易并进行计算。在每个过程中，我们只需从源中减去金额并将其添加到目标中(在 "Subtract" 的情况下，金额为负，因此交易被有效逆转)。

public static void main(String[] args) {
    // set-up sample data
    Map<String, Integer> map = new HashMap<>();
    for (String s : Arrays.asList("A", "B", "C", "D", "X")) {
        map.put(s, 1000);
    }
    List<Transaction> transactions = Arrays.asList(
        new Transaction("A","B","Add",10),
        new Transaction("C","D","Subtract",20),
        new Transaction("D","B","Add",50),
        new Transaction("B","X","Subtract",30)
    );

    // implement the transactions
    for (Transaction t : transactions) {
        final int amount = t.getOperation().equals("Add") ? t.getAmount() : -t.getAmount();
        map.put(t.getSource(), map.get(t.getSource()) - amount);
        map.put(t.getTarget(), map.get(t.getTarget()) + amount);
    }

    System.out.println(map); // prints "{A=1010, B=910, C=980, D=1070, X=1030}"
}