Erasure type inference issue in Java

Written on Feb, 15, 2020

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;
    }
}

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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)

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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

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/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…

Updating Tomcat in an EMR cluster

Written on Feb, 08, 2020

This is a short story of fixing Tomcat in an EMR cluster. There is a CVE about Tomcat 8 hole. I’m using EMR 5.12.2. Let’s try installing CVE patch as a bootstrap action with

sudo yum update -y --releasever=latest --cve CVE-2019-0232

sudo shutdown -r now

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sudo yum update -y --releasever=latest --cve CVE-2019-0232

sudo shutdown -r now

It should fix the mentioned hole but after…

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Written on Feb, 01, 2020

This is the fourth part of the JVM Inside Out series. For your convenience you can find other parts in the table of contents in Part 1 — Getting object address Typical locking pattern in Java (and other languages, even outside them JVM…

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Written on Jan, 25, 2020

This is the third part of the JVM Inside Out series. For your convenience you can find other parts in the table of contents in Part 1 — Getting object address Erasure in Java seems pretty easy but sometimes it has unexpected consequences.…

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Written on Jan, 18, 2020

Today we will remove Content Security Policy (CSP) header from pages hosted in web view. Technically it doesn’t matter which web view you use (whether it is IE, dotnetbrowser, CEF or whatever else). It all goes down to the same: we need to…

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Written on Jan, 11, 2020

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Written on Jan, 04, 2020

This is the second part of the JVM Inside Out series. For your convenience you can find other parts in the table of contents in Part 1 — Getting object address Last time we saw how to read object address. We can use…

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Written on Dec, 28, 2019

This is the first part of the JVM Inside Out series. For your convenience you can find other parts using the links below: Part 1 — Getting object address Part 2 — Reading object content Part 3 — Java raw type trickery Part…

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Written on Dec, 21, 2019

This is the fifth part of the Availability Anywhere series. For your convenience you can find other parts in the table of contents in Part 1 – Connecting to SSH tunnel automatically in Windows Today I include some various notes around remote connectivity…

Availability Anywhere Part 4 — Fixing LogonUi hang

Written on Dec, 14, 2019

This is the fourth part of the Availability Anywhere series. For your convenience you can find other parts in the table of contents in Part 1 – Connecting to SSH tunnel automatically in Windows Sometimes when I RDP into other machine and forget…

Availability Anywhere Part 3 — How to enable PowerShell remoting in Windows

Written on Dec, 07, 2019

This is the third part of the Availability Anywhere series. For your convenience you can find other parts in the table of contents in Part 1 – Connecting to SSH tunnel automatically in Windows Today we will enable PowerShell Remoting in Windows. This…

Availability Anywhere Part 2 — Connecting to VPN automatically in Windows

Written on Nov, 30, 2019

This is the second part of the Availability Anywhere series. For your convenience you can find other parts in the table of contents in Part 1 – Connecting to SSH tunnel automatically in Windows Last week we saw how to configure automatic SSH…

Availability Anywhere Part 1 — Connecting to SSH tunnel automatically in Windows

Written on Nov, 23, 2019

This is the first part of the Availability Anywhere series. For your convenience you can find other parts using the links below: Part 1 — Connecting to SSH tunnel automatically in Windows Part 2 — Connecting to VPN automatically in Windows Part 3…

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Written on Nov, 16, 2019

This is the thirteenth part of the .NET Inside Out series. For your convenience you can find other parts in the table of contents in Part 1 – Virtual and non-virtual calls in C# Last time we saw how to modify library on…

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Written on Nov, 09, 2019

This is the twelfth part of the .NET Inside Out series. For your convenience you can find other parts in the table of contents in Part 1 – Virtual and non-virtual calls in C# Imagine that you have a managed library which doesn’t…

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Written on Nov, 02, 2019

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Written on Oct, 26, 2019

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Written on Oct, 19, 2019

This is the eighth part of the Concurrency series. For your convenience you can find other parts in the table of contents in Part 1 – Mutex performance in .NET We know how to use global mutexes to synchronize processes. However, there is…

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Written on Oct, 12, 2019

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Capturing all output in Powershell

Written on Oct, 05, 2019

Today very simple trick to capture whole output from Powershell. Let’s see those files: 1.ps1

echo "1_1 STD OUT"
write-host "1_2 HOST OUT"
write-error "1_3 STD ERR"

.\2.ps1

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echo "1_1 STD OUT"

write-host "1_2 HOST OUT"

write-error "1_3 STD ERR"

.\2.ps1

2.ps1

echo "2_1 STD OUT"
write-host "2_2 HOST OUT"
write-error "2_3 STD ERR"

.\3.bat

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echo "2_1 STD OUT"

write-host "2_2 HOST OUT"

write-error "2_3 STD ERR"

.\3.bat

3.bat

echo "3_1 STD OUT"
echo "3_2 STD ERR" 1>&2
echo "3_2 STD 3" 1>&3

cscript 4.vbs

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echo "3_1 STD OUT"

echo "3_2 STD ERR" 1>&2

echo "3_2 STD 3" 1>&3

cscript 4.vbs

4.vbs

Wscript.Echo "4_1 STD OUT"
Set fso = CreateObject ("Scripting.FileSystemObject")
Set stdout = fso.GetStandardStream (1)
Set stderr = fso.GetStandardStream (2)
stdout.WriteLine "4_2 STD OUT"
stderr.WriteLine "4_3 STD ERR"

Set WshShell = CreateObject("WScript.Shell")
Set WshShellExec = WshShell.Exec("5.exe")
WScript.StdOut.Write WshShellExec.StdOut.ReadAll
WScript.StdErr.Write WshShellExec.StdErr.ReadAll

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Wscript.Echo "4_1 STD OUT"

Set fso = CreateObject ("Scripting.FileSystemObject")

Set stdout = fso.GetStandardStream (1)

Set stderr = fso.GetStandardStream (2)

stdout.WriteLine "4_2 STD OUT"

stderr.WriteLine "4_3 STD ERR"

Set WshShell = CreateObject("WScript.Shell")

Set WshShellExec = WshShell.Exec("5.exe")

WScript.StdOut.Write WshShellExec.StdOut.ReadAll

WScript.StdErr.Write WshShellExec.StdErr.ReadAll

5.exe

#include <iostream>
#include <cstdlib>

int main()
{
    std::cout << "5_1 STD OUT" << "\n";
    std::cerr << "5_2 STD ERR" << "\n";

    system("6.exe");
}

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#include <iostream>

#include <cstdlib>

int main()

{

std::cout << "5_1 STD OUT" << "\n";

std::cerr << "5_2 STD ERR" << "\n";

system("6.exe");

}

6.exe

#include <iostream>
#include <cstdlib>

int main()
{
    std::cout << "6_1 STD OUT" << "\n";
    std::cerr << "6_2 STD ERR" << "\n";
}

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#include <iostream>

#include <cstdlib>

int main()

{

std::cout << "6_1 STD OUT" << "\n";

std::cerr << "6_2 STD ERR" << "\n";

}

Let’s now run this with powershell:

PS C:\Afish\Playground> .\1.ps1
1_1 STD OUT
1_2 HOST OUT
C:\Afish\Playground\1.ps1 : 1_3 STD ERR
At line:1 char:1
+ .\1.ps1
+ ~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,1.ps1

2_1 STD OUT
2_2 HOST OUT
C:\Afish\Playground\2.ps1 : 2_3 STD ERR
At C:\Afish\Playground\1.ps1:5 char:1
+ .\2.ps1
+ ~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,2.ps1


C:\Afish\Playground>echo "3_1 STD OUT"
"3_1 STD OUT"

C:\Afish\Playground>echo "3_2 STD ERR"  1>&2
"3_2 STD ERR"

C:\Afish\Playground>echo "3_2 STD 3"  1>&3
"3_2 STD 3"

C:\Afish\Playground>cscript 4.vbs
Microsoft (R) Windows Script Host Version 5.812
Copyright (C) Microsoft Corporation. All rights reserved.

4_1 STD OUT
4_2 STD OUT
4_3 STD ERR
5_1 STD OUT
6_1 STD OUT
5_2 STD ERR
6_2 STD ERR

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PS C:\Afish\Playground> .\1.ps1

1_1 STD OUT

1_2 HOST OUT

C:\Afish\Playground\1.ps1 : 1_3 STD ERR

At line:1 char:1

+ .\1.ps1

+ ~~~~~~~

+ CategoryInfo : NotSpecified: (:) [Write-Error], WriteErrorException

+ FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,1.ps1

2_1 STD OUT

2_2 HOST OUT

C:\Afish\Playground\2.ps1 : 2_3 STD ERR

At C:\Afish\Playground\1.ps1:5 char:1

+ .\2.ps1

+ ~~~~~~~

+ CategoryInfo : NotSpecified: (:) [Write-Error], WriteErrorException

+ FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,2.ps1

C:\Afish\Playground>echo "3_1 STD OUT"

"3_1 STD OUT"

C:\Afish\Playground>echo "3_2 STD ERR" 1>&2

"3_2 STD ERR"

C:\Afish\Playground>echo "3_2 STD 3" 1>&3

"3_2 STD 3"

C:\Afish\Playground>cscript 4.vbs

Microsoft (R) Windows Script Host Version 5.812

4_1 STD OUT

4_2 STD OUT

4_3 STD ERR

5_1 STD OUT

6_1 STD OUT

5_2 STD ERR

6_2 STD ERR

Okay, we have all STD OUT and STD ERR streams.…

Erasure type inference issue in Java

Updating Tomcat in an EMR cluster

JVM Inside Out Part 4 — Locks and out of band exceptions

JVM Inside Out Part 3 — Java raw type trickery

Removing Content Security Policy header from pages hosted in web view

.NET Inside Out Part 14 — Calling virtual method without dynamic dispatch

JVM Inside Out Part 2 — Reading object content

JVM Inside Out Part 1 — Getting object address

Availability Anywhere Part 5 — Various remote connection things

Availability Anywhere Part 4 — Fixing LogonUi hang

Availability Anywhere Part 3 — How to enable PowerShell remoting in Windows

Availability Anywhere Part 2 — Connecting to VPN automatically in Windows

Availability Anywhere Part 1 — Connecting to SSH tunnel automatically in Windows

.NET Inside Out Part 13 — Bypassing license checks

.NET Inside Out Part 12 — Modifying managed library on an IL level

Concurrency Part 10 — Reentrant mutex

Concurrency Part 9 — Semaphores with custom locks

Concurrency Part 8 — Tracking mutex owner

Concurrency Part 7 — Semaphores trickery

Capturing all output in Powershell

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