Question

I am a noob in Java and am someone who is learning Java after python.
Anyways, I am having a hard time figuring this out.
Suppose I have the class

class Bicycle{
      ....
 }

and

 public class Bicycle{
    ....}

what is the difference.
And what about

  public static class Bicycle{
              // if this can be a valid class def in first place
   }

and then, after this.. lets talk about variables.

    class Bicycle{
     int Gear or public int Gear // whats the difference
    }

When to use which one?

Answer 1

These keywords (or lack of them) are known as access modifiers – in short they control the accessibility of classes or members.

Classes have the following modifiers:

• public – accessible anywhere
• (no modifier) – only accessible in same package

Class members have more possibilities:

• public – accessible anywhere
• protected – only accessible in same package or in an extending class
• (no modifier) – only accessible in same package
• private – only accessible in same class file^*

^{*Note that nested classes can access their outer class’s private members and vice-versa.}

More information on access modifiers can be found here. Also see this helpful article for the basics.

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Edit: I missed your middle example, with public static class Bicycle – the static here must mean that Bicycle is a nested class. See this page (which I had already linked in my subscript) for an explanation of nested classes, which break down into static classes and non-static, aka inner, classes.