Maven工程报Diamond types are not supported at language level '5’或Diamond types are not supported at this language level 的错误，只需要检查以下三个个地方即可：
已IDEA为例：
打开工程设置：

1.Project – Project language level:

2. Modules – Language level：

3.pom.xml

<build>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>2.3.2</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>