Erasure type inference issue in Java

Recently I was working with the following code:

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Test {
    public static void main(String[] args) {
        List<String> strings = new ArrayList<>();
        List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());
    }
}

class Item {
    public Map<String, String> metadata;

    Item(Map<String, String> metadata) {
        this.metadata = metadata;
    }
}

import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

public class Test {

public static void main(String[] args) {

List<String> strings = new ArrayList<>();

List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());

}

class Item {

public Map<String, String> metadata;

Item(Map<String, String> metadata) {

this.metadata = metadata;

}

I was compiling it with

java version "1.8.0_221"
Java(TM) SE Runtime Environment (build 1.8.0_221-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.221-b11, mixed mode)

java version "1.8.0_221"

Java(TM) SE Runtime Environment (build 1.8.0_221-b11)

Java HotSpot(TM) 64-Bit Server VM (build 25.221-b11, mixed mode)

on Windows 10 x64. It wasn’t working because of the following:

/tmp/java_fR1LWz/Test.java:10: warning: [unchecked] unchecked method invocation: constructor <init> in class Item is applied to given types
        List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());
                                                            ^
  required: Map<String,String>
  found: HashMap
/tmp/java_fR1LWz/Test.java:10: warning: [unchecked] unchecked conversion
        List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());
                                                                     ^
  required: Map<String,String>
  found:    HashMap
/tmp/java_fR1LWz/Test.java:10: warning: [unchecked] unchecked method invocation: method map in interface Stream is applied to given types
        List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());
                                               ^
  required: Function<? super T,? extends R>
  found: Function<String,Item>
  where T,R are type-variables:
    T extends Object declared in interface Stream
    R extends Object declared in method <R>map(Function<? super T,? extends R>)
/tmp/java_fR1LWz/Test.java:10: warning: [unchecked] unchecked call to <R,A>collect(Collector<? super T,A,R>) as a member of the raw type Stream
        List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());
                                                                                            ^
  where R,A,T are type-variables:
    R extends Object declared in method <R,A>collect(Collector<? super T,A,R>)
    A extends Object declared in method <R,A>collect(Collector<? super T,A,R>)
    T extends Object declared in interface Stream
/tmp/java_fR1LWz/Test.java:10: error: incompatible types: Object cannot be converted to List<Item>
        List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());
                                                                                            ^
1 error
4 warnings

/tmp/java_fR1LWz/Test.java:10: warning: [unchecked] unchecked method invocation: constructor <init> in class Item is applied to given types

List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());

required: Map<String,String>

found: HashMap

/tmp/java_fR1LWz/Test.java:10: warning: [unchecked] unchecked conversion

List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());

required: Map<String,String>

found: HashMap

/tmp/java_fR1LWz/Test.java:10: warning: [unchecked] unchecked method invocation: method map in interface Stream is applied to given types

List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());

required: Function<? super T,? extends R>

found: Function<String,Item>

where T,R are type-variables:

T extends Object declared in interface Stream

R extends Object declared in method <R>map(Function<? super T,? extends R>)

/tmp/java_fR1LWz/Test.java:10: warning: [unchecked] unchecked call to <R,A>collect(Collector<? super T,A,R>) as a member of the raw type Stream

List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());

where R,A,T are type-variables:

R extends Object declared in method <R,A>collect(Collector<? super T,A,R>)

A extends Object declared in method <R,A>collect(Collector<? super T,A,R>)

T extends Object declared in interface Stream

/tmp/java_fR1LWz/Test.java:10: error: incompatible types: Object cannot be converted to List<Item>

List<Item> items = strings.stream().map(item -> new Item(new HashMap())).collect(Collectors.toList());

1 error

4 warnings

You can try reproducing the issue at compilejava.net, it throws the error at the moment.

I was almost sure that it was a bug in javac, especially that the code was working fine in Java 12 as indicated by Ideone.

Fortunately, with some help from 4programmers.net community I was finally pointed out in the right direction. It is a bug and Oracle knows about that. You can see details at Oracle page.

Takeaway is: most of the time the problem is on the user side, compiler/platform/OS/library/CPU works well. However, sometimes one just hits a bug.