There's a difference between becoming a Java Developer and getting a job as a Java Developer.
To become a Java Developer, you will good problem-solving and programming skills. Should be proficient in Java programming. Along with basics like Data types, loops, branches, etc., concepts like Data Structures, Applets, servlets, etc are important to be understood.
Now, to get a job as a Java Developer, you need all of the above skills, plus a Bachelor's degree.