A class defines the shared aspects of objects created from it. Classes consist variables and methods which will be associated with a particular object or all objects of a class through their states and behaviours.

Each class relates to a single concept and it's variables and methods should reflect that. For example a class that models a dog should probably not have variables like clothing or methods like meow.

For an object to be created from a class it's constructor needs to be called.

Example

public class Dog {
  int age;

  public Dog(int age) {
    this.age = age;
  }

  public void bark() {
	  System.out.println("Bark");
  }
  
  public int getAgeInDogYears() {
	return this.age * 7;
  }
}

In this example the class has 3 methods, bark, getAgeInDogYears and it's constructor Dog. Additionally it has a variables age.