Classes and objects

Relationships between classes and objects

The understanding of the class

A class is an abstraction of a class of things in real life that share common properties and behavior

A class is the data type of an object. A class is a collection of objects with the same properties and behavior

Simple understanding: a class is a description of something real

The composition of the class

Attribute: Refers to the characteristics of something, such as a mobile phone (brand, price, size)

Behavior: The actions that something can perform, e.g., something on a phone (making a call, sending a text message)

Relationships between classes and objects

A class is a description of a thing, and an object is a concrete thing

The definition of a class

2. To represent (a variable outside a method in a class) by a member variable in a class.

Behavior: Represented by member methods in a class (remove the static keyword compared to the previous method)

public class Student { String name; // int age; Public void study(){system.out.println (" learn "); } // Member methods}Copy the code

Object creation and use

Create the format of the object

Class name Object name = new class name ();

Student stu = new Student();
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The format of the call member

Object name. Member variable

Object name. Member method ();

System.out.println(stu.name); // null System.out.println(stu.age); / / 0Copy the code

case

Demo1.java

package com.gonghr.demo1; public class Demo1 { public static void main(String[] args) { Demo2 Demo=new Demo2(); Demo.show(); }}Copy the code

Demo2.java

package com.gonghr.demo1; public class Demo2 { public void show() { System.out.println("haha"); }}Copy the code

Object memory map

When multiple object references refer to the same memory space (the address values recorded by variables are the same)

Any object that modifies the data in memory is the modified data, regardless of which object is subsequently used to retrieve the data

Member variables and local variables

encapsulation

Private key

A member modified by private can only be accessed in this class. If a member variable modified by private needs to be used by other classes, the corresponding operation is provided

Provides a “get variable name ()” method to get the value of a member variable, decorated with public

Provides a “set variable name (parameter)” method that sets the value of a member variable. The method is public

private String name; 
private int age;
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This keyword

The this modifier is used to refer to member variables. Its main function is to distinguish local variables from member variables.

Is a reference to the current object (an object)

public void setName(String name) { 
    this.name = name; 
}
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Packaging ideas

1. Encapsulate the overview

Is one of the three main characteristics of object orientation (encapsulation, inheritance, polymorphism)

Object oriented programming language is the simulation of the objective world, in the objective world member variables are hidden inside the object, the outside world can not be directly operated

2. Encapsulation principle

Some information of a class is hidden inside the class, and external programs are not allowed to access it directly. Instead, methods provided by the class are used to operate and access the hidden information

The member variable private provides the corresponding getXxx()/setXxx() methods

3. Encapsulate benefits

Methods to control the operation of member variables improve code security

The method encapsulates the code and improves the reusability of the code

A constructor

Constructor format and execution timing

Format notice

The method name must be the same as the class name and case must be the same

There is no return value type, not even void

No concrete return value (cannot be returned by retrun)

Execution time

Called when an object is created, and the constructor is executed each time an object is created

Constructors cannot be called manually

class Student { private String name; private int age; Public Student() {system.out.println (" no argument constructor "); } public void show() { System.out.println(name + "," + age); }}Copy the code

public class StudentDemo {

public static void main(String[] args) {

// Create an object

Student s = new Student();

s.show();

}

}

The role of constructors

Used to initialize the data (attributes) of an object

public Student(String name, int age){ this.name = name; this.age = age; System.out.println(" I am the constructor of the Student class "); }Copy the code

Stu1 = new Student("张三", 20); stu1 = new Student("张三", 20); stu1.show();Copy the code

Constructor considerations

Constructor creation

If no constructor is defined, a default no-argument constructor is given

If a constructor is defined, the system will no longer provide the default constructor

Constructor creation

If no constructor is defined, a default no-argument constructor is given. If a constructor is defined,

The system will no longer provide the default constructor

Recommended usage

Both no-argument constructors and parameterized constructors are written manually, whether used or not

JavaBean Class Quick Constructor (IDEA)

Javabeans are mainly used to transfer data, that is, to combine a group of data into a JavaBean for easy transmission.

The characteristics of

Provides a default constructor and a parameter constructor

Provide get and set functions \ for all private members of a class

Specific operation

Write the basic framework

public class Demo{ private int age; private String name; / /... All private members}Copy the code

Right-click Generate or use Alt + Insert

Constructor: You can customize both no-parameter constructors and parameter constructors

Getter and Setter: You can create custom get and set functions on private data members

The corresponding function is then automatically generated