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Java Constructor

2026-03-27 22:44

Tags: #java

Author: Duke Hsu

Key concept

  • Constructor is A special method to initialize objects
  • You can pass arguments to a constructor and set up initial values

Code Example

Main class

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package family;

public class Main {

    public static void main(String[] args) {

            //create a new family member
            //if only 3 arrgrment in object , output error
            FamilyMember member1 = new FamilyMember("John",25,18.5,false);

            // member1.name = "John"
            // member1.age = 25
            // member1.bmi = 18.5
            // member1.ismarried = false

            //print member1 name
            System.out.println(member1.name); //output John

    }
}

FamilyMember class

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package family;
public class FamilyMember {

//FamilyMember attributes
String name;
int age;
double bmi;
boolean ismarried;

        //Constructors - initialize objects
        public FamilyMember(String name, int age, double bmi, boolean ismarried ){ 
                //accept four parameter to set object of Class FamilyMember

                this.name = name; //assign name value to object attribute "name"
                this.age = age; //assign age value to object attribute "age"
                this.bmi = bmi; //assign bmi value to object attribute "bmi"
                this.ismarried = ismarried;
        }
}

Constructor in Java - Key Points

The 3 Most Important Things

Priority What to Remember Why It Matters
Most Important Constructor name MUST match the class name exactly Same spelling, same capital letters. If you get it wrong, Java won't recognize it as a constructor
Very Important Initialize all properties Make sure every object property gets a value. Don't leave anything empty or with default values
Important Use thisto refer to object properties It makes clear which is the property and which is the parameter. Makes your code easier to read

Before you submit your code, ask yourself these questions:

  • [ ] Does the constructor name match the class name exactly (same capital letters)?
  • [ ] Did I set a value for every single property?
  • [ ] Did I use "this" when assigning values to properties?
  • [ ] Did I write "public" in front of the constructor?
  • [ ] Are my parameter names clear and easy to understand?

References

Learn CONSTRUCTORS in 10 minutes! - https://www.youtube.com/watch?v=ZD7CB6wKg8A