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Java Constructors

Constructor in Java is a special method used to get things started. A constructor is called when a class object is built. Can be used to set initial values for object attributes:

Example

Create a constructor:

// Create a Main class
            public class Main {
              int x;  // Create a class attribute
            
              // Create a class constructor for the Main class
              public Main() {
                x = 5;  // Set the initial value for the class attribute x
              }
            
              public static void main(String[] args) {
                Main myObj = new Main(); // Create an object of class Main (This will call the constructor)
                System.out.println(myObj.x); // Print the value of x
              }
            }
            
            // Outputs 5

Note that the constructor name must match the class name, and may not have a return type (such as void).

Also note that the constructor is named when the object is created.

All classes have builders by default: if you do not generate a class constructor yourself, Java creates one for you. However, you cannot set the initial values of the object attribute.

Constructor Parameters

constructor can also take parameters, which are used to launch attributes.

The following example adds an int y parameter to a constructor. Inside the constructor we place x to y (x = y). When we call a constructor, we pass the parameter to the constructor (5), which will set the value of x to 5:

Example

public class Main {
            int x;
          
            public Main(int y) {
              x = y;
            }
          
            public static void main(String[] args) {
              Main myObj = new Main(5);
              System.out.println(myObj.x);
            }
          }
          
          // Outputs 5

You can have as many parameters as you want:

Example

public class Main {
            int modelYear;
            String modelName;
          
            public Main(int year, String name) {
              modelYear = year;
              modelName = name;
            }
          
            public static void main(String[] args) {
              Main myCar = new Main(1969, "Mustang");
              System.out.println(myCar.modelYear + " " + myCar.modelName);
            }
          }
          
          // Outputs 1969 Mustang

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Java Constructors

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