• Constructors have same name as its class.
• In a manner of speak, constructors are like methods, but does not return values.
• They are automatically called every time an instance of its class is created.
• There's no problem in having an empty constructor or no constructor at all.

Just to add to GoToLoop's answer.

In the second listing there is no constructor because the method at line 6 returns a value, it returns void.

A class can have multiple constructors, they all have the exactly same name as the class,  but must differ in the number and/or types of parameters. This is called method overloading. For instance the class below has 2 constructors.

1. class MyClass() {
2.       
3.       // Declare, define properties
4.       float property1;
5.       float property2;
6.       float property3;
7. 
   
8.             MyClass(){
9.                         property1 = -1;
10.                         property2 = 0;
11.                         property3 = 1;
12.             }
13. 
    
14.       // Pass properties into the constructor
15.       MyClass(float temp1, float temp2, float temp3) {
16.             property1 = temp1;
17.             property2 = temp2;
18.             property3 = temp3;
19. 
    
20.       // Define methods
21.       void init() {
22.       }
23.       
24.       void update() {
25.       }
26.       
27.       void render() {
28.       }
29. }

In the second listing there is no constructor because the method at line 6 returns a value, it returns void.

As quarks warned about, even putting void in front of a wanna-be constructor voids it.
To be considered a constructor, it must have same name as its class and not have any return declared, not even void!