What are the differences between static, instance, and global methods in Apex?

Apex methods have a class scope and a visibility scope.

Class scope:

• Instance method — bound to an object instance. Access via myObj.method(). Can use instance variables.
• Static method — class-level. Access via MyClass.method(). No this. Used for utilities and stateless logic.

Visibility scope:

• `private` — visible only within the class (default).
• `protected` — visible to the class and its subclasses.
• `public` — visible to any class in the same namespace.
• `global` — visible to any code, including code in other namespaces (other Salesforce orgs / managed packages). Required for web service methods, batch start/execute/finish, and methods called from outside your namespace.

When to use each:

• `private` + instance — most internal class methods.
• `public` + static — utility methods callable from anywhere in your namespace.
• `global` — only when truly necessary. global methods are part of your public API and changing their signature breaks consumers — they're effectively eternal commitments. Most code shouldn't be global.

Common mistake: making everything global "just in case". This locks the API permanently. Use public until you have a concrete reason to expose to other namespaces.