抽象生成器

Question

所以我有一个抽象的Transaction类，它有多个实现(Payment，File(。

我想要一个事务(抽象(的构建器+实现器。

我做到了：

public abstract class TransactionBuilder
{
protected final Transaction transaction;

public TransactionBuilder(Transaction transaction)
{
this.transaction = transaction;
}

public TransactionBuilder setSignature(byte[] signature)
{
this.transaction.setSignature(signature);
return this;
}

public TransactionBuilder setPreviousHash(String previousHash)
{
this.transaction.setPreviousHash(previousHash);
return this;
}

public abstract Transaction build();
}

PaymentBuilder类的示例：

public class PaymentBuilder extends TransactionBuilder
{
public PaymentBuilder(String from, String to, double amount)
{
super(new Payment(from, to, amount));
}

public PaymentBuilder addAmount(double amount)
{
((Payment) this.transaction).amount += amount;
}

@Override
public Payment build()
{
return (Payment) this.transaction;
}
}

每个字段都有一个getter/setter，Transaction:

public abstract class Transaction
{
//Used for serialization
private String type;

private String previousTransactionHash;
private String hash;
private String signature;

private String fromAddress;
private String toAddress;

private Instant timeStamp;

public Transaction(String type, String from, String to)
{
this.type = type;
this.fromAddress = from;
this.toAddress = to;
this.timeStamp = Instant.now();
setHash();
}

如何使用：

Payment payment = new PaymentBuilder(from, to, amount)
.setPreviousHash(previousHash)
.build();

但是当我调用setSignature((时；类型不匹配：无法从交易转换为支付"所以我需要将其转换为付款，我如何避免这种情况？我可以吗？

Answer 1

您可以使抽象生成器具有泛型，泛型类型是它生成的事务类型。

public abstract class TransactionBuilder<T extends Transaction> {
...
public abstract T build();
}
public class PaymentBuilder extends TransactionBuilder<Payment> {
@Override
public Payment build() {
...
}
}

Answer 2

感谢@khelwood，我最终使我的抽象生成器通用化：

TransactionBuilder:

public abstract class TransactionBuilder<T extends Transaction>
{
protected final T transaction;

public TransactionBuilder(T transaction)
{
this.transaction = transaction;
}

public TransactionBuilder<T> setSignature(byte[] signature)
{
this.transaction.setSignature(signature);
return this;
}

public TransactionBuilder<T> setPreviousHash(String previousHash)
{
this.transaction.setPreviousHash(previousHash);
return this;
}

public abstract T build();
}

PaymentBuilder：

public class PaymentBuilder extends TransactionBuilder<Payment>
{
public PaymentBuilder(String from, String to, double amount)
{
super(new Payment(from, to, amount));
}
public PaymentBuilder addAmount(double amount)
{
this.transaction.amount += amount;
}

@Override
public Payment build()
{
return this.transaction;
}
}