1. Classes and objects

• What is the class

  • Classes are formed by combining the static properties and dynamic operations of a class of things.
  • Classes are abstract, simulating a class of things;
  • A class is a design template for an object and does not occupy memory. Memory is allocated according to the design of the class when creating an object.

• What is an object

  • An object is an entity of a class;
  • The object is concrete and actually exists;
  • Objects have a life cycle;

For example, a class can be likened to a design drawing of a car, from which cars are produced objects:

2. Class definition and object creation

2.1. How is a class defined?

• Define class useclassKey words;
• Variables defined in a class are called properties of that class;
• The functions defined in a class are called the methods of that class;

For example, define a Car class:

class Car
{
	//Car attributes
	int number;		// Vehicle id
	
	//Car class method
	void go(a)
	{
		// Methods in a class access attributes of the class
		System.out.printf("My number is %d, go go go!", number); }}Copy the code

2.2. How do I create an object?

class CarTest
{
	public static void main(String[] args)
	{
		// Create a dynamic object
		Car testCar = new Car();
		
		// Access the properties of the object and modify them
		testCar.number = 1;

		// Call the method of the objecttestCar.go(); }}Copy the code

javac CarTest
java  CarTest
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3. Class and object memory allocation details (key)

!!!!!!!!! Memory here refers to the memory occupied by the program while it is running, which can be understood as RAM space. !!!!!!!!!

• Write good program source code, executejavac CarTestThe resultingCarTest.classThe executable program is saved on the hard disk;

1. When we executejava CarTestAfter the command,The operating system loads the entire program into memory, ready for operation;
2. The program starts with the main entry function;
3. The program allocates memory during execution. Let’s start parsing step by step:

3.1. How is the memory space allocated for classes?

3.2. How is the object’s memory allocated?

Car testCar = new Car();
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1. Car()This function is called the Car constructor. Although we haven’t defined it, it exists by default and is called automatically when an object of this class is created, as described in Section 4 (P.
2. newThe keyword indicates that memory space is allocated dynamically when this object is created, soAn object that comes out new has memory space in the heap.newThe role of keywords and C languagemallocThe function does the same thing, but is simpler.The system allocates memory in the heap as defined in the class, and returns the head address;
3. The first address of the object is returned and stored in the testCar variable, so in C testCar refers to the Car object in the heap, and in Java testCar refers to the Car object in the heap.
4. TestCar is a variable of type Car and is defined in the main function, so it is a local variable whose memory is allocated on the stack.

3.3. How are properties and methods of objects in the heap accessed?

4. Constructor

4.1. The role of constructors

4.2. Default no-parameter constructor

When creating the object, we call the Car() function, but this function is not in our class definition, so how does this function get called?

Car testCar = new Car();
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Car() is the Car constructor. After we define the Car class, the default constructor Car() is automatically generated.

4.3. Custom constructors

class Car
{
	//Car attributes
	int number;		// Vehicle id

	// No argument constructor
	public Car(a)
	{
		System.out.printf("The no-argument constructor is called.\n");
	}
	// Has a parameter constructor
	public Car(int i)
	{
		number = i;
		System.out.printf("Parameter constructor called.\n");
	}
	
	//Car class method
	void go(a)
	{
		// Methods in a class access attributes of the class
		System.out.printf("My number is %d, go go go! \n", number); }}Copy the code

class CarTest
{
	public static void main(String[] args)
	{
		// Create a dynamic object using the no-argument constructor
		Car testCar1 = new Car();
		
		// Access the properties of the object and modify them
		testCar1.number = 1;

		// Call the method of the object
		testCar1.go();

		// Use the argument constructor to create a dynamic object
		Car testCar2 = new Car(2);

		// Call the method of the objecttestCar2.go(); }}Copy the code

4.4. Assignment of data members in constructors

For the data members of the class (attributes of the class), the system executes the initial values assigned when they are defined, and then the initial values assigned in the constructor.

For example, if I assign the number attribute to 1 and the constructor to 2, the result is 2:

class Car
{
	//Car attributes
	int number = 1;		// Vehicle id

	// No argument constructor
	public Car(a)
	{
		System.out.printf("The no-argument constructor is called.\n");
	}
	// Has a parameter constructor
	public Car(int i)
	{
		number = i;
		System.out.printf("Parameter constructor called.\n");
	}
	
	//Car class method
	void go(a)
	{
		// Methods in a class access attributes of the class
		System.out.printf("My number is %d, go go go! \n", number); }}class CarTest
{
	public static void main(String[] args)
	{
		// Use the argument constructor to create a dynamic object
		Car testCar = new Car(2);

		// Call the method of the objecttestCar.go(); }}Copy the code

Running results:



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