Memory allocation in Java

When a Java program is running, it needs to allocate space in memory. In order to improve the efficiency of operation, the space is divided into different regions, because each region has a specific way of data processing and memory management.

• Stack store local variable
  • The variables of local variables in method definition or method declaration are called local variables, which disappear immediately after use
• Heap to store new things

  • Each entity has a first address value

  • Data within each entity has a default value

    byte,short,int,long 0
    
    float,double 0.0
    
    char '\u0000'
    
    boolean false
    
    Reference type: null

  • After use, it will be recycled when the garbage collector is idle.

• Class information, constants, static constants, etc. loaded by virtual machine in method area.
• Local method area (system related)
• Register (for CPU)

class Phone{
	//brand
	String brand;
	//Price
	int price;
	//colour
	String color;
	
	//Phone
	public void call(String name){
		System.out.println("to"+name+"Phone");
	}
	
	//Send message
	public void sendMessage(){
		System.out.println("send message...");
	}
	
	//Play a game
	public void playGame(){
		System.out.println("Glory of Kings carry Medium.");
	}
}

class PhoneDemo{
	public static void main(String[] args){
		//Objects need to be created before use
		//Class name object name = new class name ();
		Phone p = new Phone();
		
		System.out.println(p.brand+"==="+p.price+"==="+p.color);
		
		//Assign a value to a member
		p.brand = "iPhone X";
		p.price = 6888;
		p.color = "black";
		System.out.println(p.brand+"==="+p.price+"==="+p.color);
		
		//Calling method
		p.call("Stay in the bell");
		p.sendMessage();
		p.playGame();
		
		System.out.println("------------------------");
		
		Phone p2 = new Phone();
		
		System.out.println(p2.brand+"==="+p2.price+"==="+p2.color);
		
		//Assign a value to a member
		p2.brand = "Hammer";
		p2.price = 2299;
		p2.color = "Red";
		System.out.println(p2.brand+"==="+p2.price+"==="+p2.color);
		
		//Calling method
		p2.call("Di Ali Gerba");
		p2.sendMessage();
		p2.playGame();
		System.out.println("------------------------");
		Phone p3 = p;
		System.out.println(p3.brand+"==="+p3.price+"==="+p3.color);
		
		//Assign a value to a member
		p3.brand = "HUAWEI";
		p3.price = 1999;
		p3.color = "blue";
		System.out.println(p3.brand+"==="+p3.price+"==="+p3.color);
		System.out.println(p.brand+"==="+p.price+"==="+p.color);
		
	}
}

Memory graph of an object

Memory map of two objects

Memory graph of three objects

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