Scala Eclipse IDE compiler giving errors until "clean" is run

Question

I am writing a mixed Java/Scala project and my Eclipse compiler is giving errors for all of the Scala code. It seems to try to compile it as Java instead of Scala. However, when I run a clean, all of the errors go away. Also, building this using maven works completely fine.

I am building this project in Maven. Originally this project was only Java, so I added the necessary plugins/dependencies into my pom.xml.

Any idea why Eclipse is treating my Scala files as Java?

Here's my pom.xml:

<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>org.antlrfun</groupId>
    <artifactId>antlrfun</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>Antlr4 Spark/Solr</name>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <antlr4.visitor>true</antlr4.visitor>
        <antlr4.listener>true</antlr4.listener>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.antlr</groupId>
            <artifactId>antlr4-runtime</artifactId>
            <version>4.5</version>
        </dependency>
        <dependency>
        <groupId>junit</groupId>     <!-- NOT org.junit here -->
        <artifactId>junit</artifactId>
        <version>4.12</version>
    </dependency>
    <dependency>
        <groupId>org.scala-lang</groupId>
        <artifactId>scala-library</artifactId>
        <version>2.10.5</version>
        </dependency>
        <dependency>
            <groupId>org.apache.spark</groupId>
            <artifactId>spark-core_2.10</artifactId>
            <version>1.3.1</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.2.1</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>java</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <mainClass>org.antlrfun.Hello</mainClass>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>build-helper-maven-plugin</artifactId>
                <version>1.7</version>
                <executions>
                    <execution>
                        <id>add-source</id>
                        <phase>generate-sources</phase>
                        <goals>
                            <goal>add-source</goal>
                        </goals>
                        <configuration>
                            <sources>
                                <source>src/main/generated-sources</source>
                            </sources>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.1</version>
                <configuration>
                    <source>1.7</source>
                    <target>1.7</target>
                    <showWarnings>true</showWarnings>
                    <showDeprecation>true</showDeprecation>
                    <compilerArguments>
                        <Xlint/>
                    </compilerArguments>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.antlr</groupId>
                <artifactId>antlr4-maven-plugin</artifactId>
                <version>4.1</version>
                <configuration>
                    <sourceDirectory>src/main/java</sourceDirectory>
                    <outputDirectory>src/main/generated-sources</outputDirectory>
                </configuration>
                <executions>
                    <execution>
                        <goals>
                            <goal>antlr4</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>

        <plugin>
                <groupId>org.scala-tools</groupId>
                <artifactId>maven-scala-plugin</artifactId>
                <version>2.15.2</version>
                <executions>
                    <!-- Run scala compiler in the process-resources phase, so that dependencies on
                         scala classes can be resolved later in the (Java) compile phase -->
                    <execution>
                        <id>scala-compile-first</id>
                        <phase>process-resources</phase>                        
                        <goals>
                            <goal>compile</goal>
                        </goals>
                        </execution>

                        <!-- Run scala compiler in the process-test-resources phase, so that dependencies on
                             scala classes can be resolved later in the (Java) test-compile phase -->                    
                        <execution>
                        <id>scala-test-compile</id>
                        <phase>process-test-resources</phase>
                        <goals>
                            <goal>testCompile</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>

            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>build-helper-maven-plugin</artifactId>
                <executions>
                    <!-- Add src/main/scala to source path of Eclipse -->
                    <execution>
                        <id>add-source</id>
                        <phase>generate-sources</phase>
                        <goals>
                            <goal>add-source</goal>
                        </goals>
                        <configuration>
                            <sources>
                                <source>src/main/scala</source>
                            </sources>
                        </configuration>
                    </execution>

                    <!-- Add src/test/scala to test source path of Eclipse -->
                    <execution>
                        <id>add-test-source</id>
                        <phase>generate-test-sources</phase>
                        <goals>
                            <goal>add-test-source</goal>
                        </goals>
                        <configuration>
                            <sources>
                                <source>src/test/scala</source>
                            </sources>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

            <!-- to generate Eclipse artifacts for projects mixing Scala and Java -->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-eclipse-plugin</artifactId>
                <version>2.8</version>
                <configuration>
                    <downloadSources>true</downloadSources>
                    <downloadJavadocs>true</downloadJavadocs>
                    <projectnatures>
                        <projectnature>org.scala-ide.sdt.core.scalanature</projectnature>
                        <projectnature>org.eclipse.jdt.core.javanature</projectnature>
                    </projectnatures>
                    <buildcommands>
                        <buildcommand>org.scala-ide.sdt.core.scalabuilder</buildcommand>
                    </buildcommands>
                    <classpathContainers>
                        <classpathContainer>org.scala-ide.sdt.launching.SCALA_CONTAINER</classpathContainer>
                        <classpathContainer>org.eclipse.jdt.launching.JRE_CONTAINER</classpathContainer>
                    </classpathContainers>
                    <excludes>
                        <!-- in Eclipse, use scala-library, scala-compiler from the SCALA_CONTAINER rather than POM <dependency> -->
                        <exclude>org.scala-lang:scala-library</exclude>
                        <exclude>org.scala-lang:scala-compiler</exclude>
                    </excludes>
                    <sourceIncludes>
                        <sourceInclude>**/*.scala</sourceInclude>
                        <sourceInclude>**/*.java</sourceInclude>
                    </sourceIncludes>
                </configuration>
            </plugin>


            <!-- When run tests in the test phase, include .java and .scala source files -->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>2.8.1</version>
                <configuration>
                    <includes>
                        <include>**/*.java</include>
                        <include>**/*.scala</include>
                    </includes>
                </configuration>
            </plugin>

        </plugins>
    </build>
</project>

Answer 1

Clear the Scala IDE project settings by removing and adding back Scala Nature