Your mentors were wrong. Java is a great language and it’s used all over the place.
It gets a bad rap because it’s not a native language like C++, but it can be very fast. Also, it’s the easiest way to build an android app, since it’s the default language. Android has ways to use C/C++ code in your app, but interfacing with the android api, is done with java.
Also, Java was horrendously slow when it came out, but has been improved, a lot over the last 20 years (I’m almost 22, and have only been programming since 2015, so that isn’t form personal experience, but I’ve read a lot of smart people saying that).
Java isn’t going to be as performant as C++, but it performs pretty close in a lot of cases. I doubt you’re doing anything heavy duty enough to where you’d need the C++ speed boost. You also have the problem with C++ taking longer to develop with. Java code is syntactically similar, but better imo. I understand pointers and references, but it’s easier not needing to use them.
Now, if people are telling you to run away from Java because it’s syntax is old, well they may have a point. Java and C++ can both be a bit verbose. But, last year Google, made Kotlin, a primary language for android development. Kotlin is a nice language that runs on the JVM, meaning you can use it anywhere you’d use java code. And, it is interoperable with Java. Meaning you can have a project with 3 source files, write 2 in Java, and one source file in Kotlin, and everything will work just fine. So you can start your project in Java or Kotlin, and start changing files from Java to Kotlin(or vice versa), as you go. Kotlin compiles to java bytecode, so there’s no performance gains, but it’s got a really modern syntax, and for the last year, if I’ve needed to write java code, I’ve written Kotlin code, 98% of the time. It’s great.