When we create a java program, do we need to write a class for our program?

When you write a java program it is just a .java file. Once you successfully compile your .java file the jvm will automatically create a .class file for you. That class file is actually the byte code that the computer uses to run your program. To put it in easier terms: The Java Virtual Machine (JVM) takes the code that you understand and compiles it into code that the computer understands (byte code) and the end result is the .class file.

Yes just write the Java code then make sure that it gets compiled before you try running it: javac className.java

Then run your code:

java className

This will automatically produce your class file; and if you ever lose your class file or it gets corrupted then just compile your Java code again and it will produce another class file and that's all there is to it.

You're welcome, after you compile your next program check the same directory as your .java file and your .class should be there too. Unless you changed the file path for your .class file but I wouldn't do that until you are more experienced.