I simply installed JDK 10 on my mac machine. Checked for the version:-
localhost:java-10 vinayprajapati$ java -version
java version "10" 2018-03-20
Java(TM) SE Runtime Environment 18.3 (build 10+46)
Java HotSpot(TM) 64-Bit Server VM 18.3 (build 10+46, mixed mode)
Just to make sure that compiler used is also same version, I ran below:-
localhost:java-10 vinayprajapati$ javac -version
javac 10
I created a simple Maven project with following structure:-
localhost:java-10 vinayprajapati$ tree
.
├── pom.xml
├── src
│ ├── main
│ │ └── java
│ │ └── com
│ │ └── practice
│ │ └── java_10
│ │ └── App.java
│ └── test
│ └── java
│ └── com
│ └── practice
│ └── java_10
│ └── AppTest.java
└── target
My pom.xml is:-
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.practice</groupId>
<artifactId>java-10</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>
<name>java-10</name>
<url>http://maven.apache.org</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.7.0</version>
<configuration>
<release>10</release>
</configuration>
<dependencies>
<dependency>
<groupId>org.ow2.asm</groupId>
<artifactId>asm</artifactId>
<version>6.1</version> <!-- Use newer version of ASM -->
</dependency>
</dependencies>
</plugin>
</plugins>
</build>
</project>
My App.java file is:-
package com.practice.java_10;
import java.util.ArrayList;
public class App {
public static void main(String[] args) {
var list = new ArrayList<String>();
System.out.println("Hello Java 10! Shall I welcome you?");
}
}
I ran mvn compile
and mvn install
and both worked successfully. Proof below:-
localhost:java-10 vinayprajapati$ mvn compile
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------< com.practice:java-10 >------------------------
[INFO] Building java-10 0.0.1-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ java-10 ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory /Users/vinayprajapati/Desktop/project/java-10/java-10/src/main/resources
[INFO]
[INFO] --- maven-compiler-plugin:3.7.0:compile (default-compile) @ java-10 ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to /Users/vinayprajapati/Desktop/project/java-10/java-10/target/classes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.823 s
[INFO] Finished at: 2018-03-23T01:49:13+05:30
[INFO] ------------------------------------------------------------------------
localhost:java-10 vinayprajapati$ mvn install
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------< com.practice:java-10 >------------------------
[INFO] Building java-10 0.0.1-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ java-10 ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory /Users/vinayprajapati/Desktop/project/java-10/java-10/src/main/resources
[INFO]
[INFO] --- maven-compiler-plugin:3.7.0:compile (default-compile) @ java-10 ---
[INFO] Nothing to compile - all classes are up to date
[INFO]
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ java-10 ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory /Users/vinayprajapati/Desktop/project/java-10/java-10/src/test/resources
[INFO]
[INFO] --- maven-compiler-plugin:3.7.0:testCompile (default-testCompile) @ java-10 ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to /Users/vinayprajapati/Desktop/project/java-10/java-10/target/test-classes
[INFO]
[INFO] --- maven-surefire-plugin:2.12.4:test (default-test) @ java-10 ---
[INFO] Surefire report directory: /Users/vinayprajapati/Desktop/project/java-10/java-10/target/surefire-reports
-------------------------------------------------------
T E S T S
-------------------------------------------------------
Running com.practice.java_10.AppTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.029 sec
Results :
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] --- maven-jar-plugin:2.4:jar (default-jar) @ java-10 ---
[INFO] Building jar: /Users/vinayprajapati/Desktop/project/java-10/java-10/target/java-10-0.0.1-SNAPSHOT.jar
[INFO] META-INF/maven/com.practice/java-10/pom.xml already added, skipping
[INFO] META-INF/maven/com.practice/java-10/pom.properties already added, skipping
[INFO]
[INFO] --- maven-install-plugin:2.4:install (default-install) @ java-10 ---
[INFO] Installing /Users/vinayprajapati/Desktop/project/java-10/java-10/target/java-10-0.0.1-SNAPSHOT.jar to /Users/vinayprajapati/.m2/repository/com/practice/java-10/0.0.1-SNAPSHOT/java-10-0.0.1-SNAPSHOT.jar
[INFO] Installing /Users/vinayprajapati/Desktop/project/java-10/java-10/pom.xml to /Users/vinayprajapati/.m2/repository/com/practice/java-10/0.0.1-SNAPSHOT/java-10-0.0.1-SNAPSHOT.pom
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3.157 s
[INFO] Finished at: 2018-03-23T01:49:22+05:30
[INFO] ------------------------------------------------------------------------
When ran mvn exec:java -Dexec.mainClass="com.practice.java_10.App"
, I got error as below :-
localhost:java-10 vinayprajapati$ mvn exec:java -Dexec.mainClass="com.practice.java_10.App"
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------< com.practice:java-10 >------------------------
[INFO] Building java-10 0.0.1-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- exec-maven-plugin:1.6.0:java (default-cli) @ java-10 ---
[WARNING]
java.lang.Error: Unresolved compilation problem:
var cannot be resolved to a type
at com.practice.java_10.App.main (App.java:11)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke (Method.java:564)
at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:282)
at java.lang.Thread.run (Thread.java:844)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.019 s
[INFO] Finished at: 2018-03-23T01:51:02+05:30
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.6.0:java (default-cli) on project java-10: An exception occured while executing the Java class. Unresolved compilation problem:
[ERROR] var cannot be resolved to a type
[ERROR]
[ERROR] -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
I am not sure why am I getting this error. Any thoughts are welcome. Meanwhile I try searching for root cause.
"cannot be resolved to a type" means that the compiler has decided that, according to the syntax of the language, what it found at this place in the code has to be type, which means either a class, an interface, or a primitive tpye, but cannot find the definition of any type with that name.
Remember, that the only purpose of using var , according to the design of the language (since JDK 10+) is to beautify the code and make it more readable. var only servers the cleaner-code and better readability purpose, it doesn't amend the Type System of the Java language.
You have to set the runtime plugin to Java 10 too or the root configuration. Right now it's just the compiler.
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