java 无法推断 hashmap<> 的类型参数
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Cannot infer type arguments for hashmap<>
提问by Chaitanya Uttarwar
I am getting
我正进入（状态
Cannot infer type arguments for java.util.HashMap<>
for the following code
对于以下代码
class Test {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(1, "x");
map.put(2, "y");
map.put(3, "x");
map.put(4, "z");
//the following line has error
Map<String, ArrayList<Integer>> reverseMap = new java.util.HashMap<>(map.entrySet().stream()
.collect(Collectors.groupingBy(Map.Entry::getValue)).values().stream()
.collect(Collectors.toMap(item -> item.get(0).getValue(),
item -> new ArrayList<>(item.stream().map(Map.Entry::getKey).collect(Collectors.toList()))));
System.out.println(reverseMap);
}
}
What went wrong and Can anyone Explain me this ? I have checked for proper imports and found out that I am importing java.util.hashmap and none other. Still the pesky error is buging me.
出了什么问题，谁能给我解释一下？我检查了正确的导入，发现我正在导入 java.util.hashmap 而不是其他。讨厌的错误仍然困扰着我。
回答by Eugene
That's a bug in ecj(eclipse compiler), you can work around it and add more type information :
这是ecj（eclipse 编译器）中的一个错误，您可以解决它并添加更多类型信息：
item -> new ArrayList<Integer>(item.stream().map(Entry::getKey)
See how I've added ArrayList<Integer>.
看看我是如何添加的ArrayList<Integer>。
It compiles fine with javac-8 and 9.
它编译得很好javac-8 and 9。
And btw seems like there is a simpler way to do things:
顺便说一句，似乎有一种更简单的做事方法：
map.entrySet()
.stream()
.collect(Collectors.groupingBy(
Entry::getValue,
HashMap::new,
Collectors.mapping(Entry::getKey, Collectors.toList())));
回答by zee
In my case, the error disappeared after adding import java.util.Map;:
就我而言，添加后错误消失了 import java.util.Map;：
import java.util.Date;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.jdbc.core.simple.SimpleJdbcInsert;
import com.fasterxml.Hymanson.databind.ObjectMapper;
public void saveFooOrder(Foo foo, long orderId) {
Map<String, Object> values = new HashMap<>();
/*^^^^ Error was here: Cannot
infer type arguments for HashMap<>*/
values.put("fooOrder", orderId);
values.put("foo", foo.getId());
orderFooInserter.execute(values);
}
回答by ΦXoc? ? Пepeúpa ツ
your code is not completed, and you are missing a )
您的代码未完成，并且您缺少一个 )
try doing:
尝试做：
import java.util.*;
import java.util.stream.Collectors;
public class HelloWorld{
public static void main(String []args){
Map<Integer, String> map = new HashMap<>();
map.put(1, "x");
map.put(2, "y");
map.put(3, "x");
map.put(4, "z");
Map<String, ArrayList<Integer>> reverseMap = new java.util.HashMap<>(map.entrySet().stream()
.collect(Collectors.groupingBy(Map.Entry::getValue)).values().stream()
.collect(Collectors.toMap(item -> item.get(0).getValue(),
item -> new ArrayList<>(item.stream().map(Map.Entry::getKey).collect(Collectors.toList())))));
System.out.println(reverseMap);
}
}
this produce the output
这产生输出
{x=[1, 3], y=[2], z=[4]}
