Flash cards
Review the key moves
What is the main idea behind Java Constructors?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
// Create a Main ___ public class Main { int x; // Create a class attribute // Create a class constructorPut the learning moves in the order that makes the concept easiest to apply.
A constructor in Java is a special method that is used to initialize objects.
The constructor is called when an object of a class is created.
It can be used to set initial values for object attributes:
Example
// 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 5Note that the constructor name must match the class name , and it cannot have a return type (like void ).
Also note that the constructor is called when the object is created.
All classes have constructors by default: if you do not create a class constructor yourself, Java creates one for you. However, then you are not able to set initial values for object attributes.
Constructor Parameters
Constructors can also take parameters, which is used to initialize attributes.
The following example adds an int y parameter to the constructor. Inside the constructor we set x to y (x=y). When we call the constructor, we pass a 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 5You 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