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Access to classes and their elements are controlled in Java by Access Modifiers. These range from private through to public with the latter being the least secure and the former the most. If the modifier is omitted then the default level which is package is applied. Package allows access from other classes within the same package the next level up towards public is protected which allows access to classes within the same package and related classes through inheritance.
public
protected
package
private
The video step you through these access levels.