泛型为什么实现接口的类与Java中的接口类型不同？

Question

我有

out.load(output, transactions, columnHeaders, dataFormat);

其中load定义为：

public boolean load(String outputfile, List<Transaction> transactions, List<String> columnHeaders, String dataFormat);

及

String output = "";
String dataFormat = "";
ArrayList<ARTransaction> transactions = new ArrayList<ARTransaction>();
List<String> columnHeaders = null;

在哪里

ARTransaction implements Transaction

为什么transactions的类型有问题

Answer 1

因为它可能不满足Liskov substitution principle

Answer 2

它很难将ArrayList<ARTransaction>的逆变类型转换为List<Transaction>

试试List<? extends Transaction>

Answer 3

public boolean load(String outputfile, List<? extends Transaction> transactions, List<String> columnHeaders, String dataFormat);

或者只是将事务声明为List<Transaction>

下面是一个常见的例子，说明为什么你显然无法做到这一点：

List<String> list = new ArrayList<String>();
List<Object> objList = list; //if this were possible
objList.add(Integer.valueOf(5));
String val = list.get(0);  //ClassCastException here
System.out.println(val);