1.7 Application Program Interface(API) and Libraries

1. Exam Points

• Identify attributes (instance variables) and behaviors(methods).
• Identify the correct data types for instance variables.
• Identify a class and an instance.

2. Knowledge Points

(1) Class and Object

• A class defines common attributes and behaviors of objects.
• An object is a specific instance of a class with defined attributes.
• Example:
  • Student is a class
  • Xiaoming is an object/instance of the Student class
• Attributes refer to the data related to the class and are stored in variables.
• Behaviors refer to what instances of the class can do (or what can be done with them) and are defined by methods.
• Example:
  

  public class Dog{
      private String name;
      private int age;
      
      public void bark(){}
      public void bite(){}
  }
  
  // here is a code segment in another class
  Dog d1 = new Dog();
  Dog d2 = new Dog();
  

  • In this example:
    • Dog is a class
    • d1, d2 are two objects/instances of the Dog class.
    • name and age are instance variables(实例变量), representing attributes of Dog objects.
    • bark and bite are methods (方法), representing behaviors of Dog objects.

(2) Libraries and APIs

• Libraries are collections of classes.
• Classes are grouped into packages.
• An application programming interface (API) specification informs the programmer how to use those classes.
• Documentation found in API specifications is essential to understanding the attributes and behaviors of a class defined by the API.

3. Exercises

• Practice
• Solution