Objects and classes of Java learning notes (1/3)

Java Object-Oriented Programming (OOP) and how to write your own classes. We know that Java is completely object-oriented, and we must be familiar with OOP to write better Java programs. This is very different from the "structured" and "procedural" programming in the past 1970s S.

I. Category

A class is a blueprint or template for constructing objects. The construct object is the process of creating an instance of the class.

Encapsulation is an important concept related to classes. Formally, encapsulation combines data and behavior in a package, and hides the data implementation method from the object user. The data of an object is called an instance field, and the process of operating data is called a method ).

Inheritance allows one class in Java to be extended by another class. In fact, all classes in Java inherit the most primitive superclass-Object. The inherited new class has all the attributes and methods of the base class.

Ii. Objects

Three main features of an object:

1. behavior-What operations or methods are applied to objects?
2. state-how does the object instance domain change when a method is applied?
3. identity-how to identify different objects with the same behavior and status?
Iii. Relationships between classes

There are three common types:

1. Dependency (uses-a) -- a class method needs to manipulate the object of another class;
2. Aggregation (has-a) -- an object of a class contains an object of another class;
3. Inheritance (is-a) -- all attributes and methods of another class can be extended and additional functions can be added.
Iv. Custom classes

1. class definition form

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Class example { Constructor1 Constructor2 ... Method1 Method2 ... Field1 Field2 ... }

For example:

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Class Student { // Constructor Public Student (String n, int num, int year, int month, int day) { Name = n; Number = num; GregorianCalendar calendar = new GregorianCalendar (year, month-1, day ); Birthday = calendar. getTime (); } // Method Public String getName () { Return name; } Public int getNumber () { Return number; } Public Date getBirthday () { Return birthday; } // Domain Private String name; Private int number; Private Date birthday; }

2. implicit and explicit parameters

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Public void setNumber (int n) { Number = n; } Student student = new Student ("Jiankun Lei", 136524,1991, 1,1 ); Student. setNumber (1000 );

// Student is an implicit parameter, and 1000 is an explicit parameter, which is an implicit (implicit) parameter. Class Object before the method name; explicit) parameters are parameters in the brackets after the method name.

3. class-based Access Permissions

Remember: methods can access the private data of the called object. That is to say, a class method can access the private data of all objects in the class.

For example, you can define a method to determine the equals of two students:

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Boolean equals (Student other) { Return number. equals (other. number); // number is the private data of the Student class. }

4. final Instance domain

Define the instance domain in the class as final. When constructing an object, you must initialize such a domain. After the object is constructed, it cannot be modified.

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Class Student { ... Private final String name; }

5. Static domain and static method

Static domain: defines a domain as static. Each class can have only one such domain. It is a class and does not belong to any independent object.

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Class Employee { ... Private int id; Private static int nextId; ... } Public void setId () { Id = nextId; NextId ++; }

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