为什么实现接口的类与 Java 中的接口不属于同一类型？

I have

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

Where load定义为：

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

and

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

where

ARTransaction implements Transaction

为什么类型有问题transactions?

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

或者只是将交易声明为List<Transaction>.

这是为什么你显然不能这样做的常见示例：

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);