A Class is like a blue print, it describes how the instantiated object will behave in the running program and at what state it will resemble in memory.
private String variableName="Ismail";
public void setVariableName(String variableName){this.variableName=variableName;}
Classes live inside files with the .java extension which are called java files, a Java file can contain an unlimited number of Package private classes, however, it can only contain a single Public class whose name must conform to the name of the java file, otherwise, the java file won't compile.
Package private and Public are java key words known as access modifiers, which we'll see them in detail in a future section.