Every class in Java has a constructor. constructor
is a method that is used to create an instance or object of the class. Every time you create an instance, you must invoke a constructor.
If you do not create a constructor method of your class, the compiler will build a default one. A default constructors is a constructor that accept no argument.
Things to be noted when you declare a constructor:
- Constructor must have the same name as the class in which they are declared.
- Constructor can’t have a return type.
- Constructor can have access modifier.
- Constructor can take arguments.
- Constructor can’t be marked static.
- Constructor can’t be marked final or abstract.
package org.kodejava.basic;
public class ConstructorDemo {
private String arg;
private int x;
private int y;
public ConstructorDemo() {
}
public ConstructorDemo(String arg) {
this.arg = arg;
}
public ConstructorDemo(int x) {
this.x = x;
}
public ConstructorDemo(int x, int y) {
this.y = y;
}
}
class RunConstructor {
public static void main(String[] args) {
// Change the default constructor from private to public in
// the ConstructorDemo class above then call the statement
// below. It will create an instance object cons0 without
// any error.
ConstructorDemo cons0 = new ConstructorDemo();
// Change the default constructor back to private, then call
// the statement below. ConstructorDemo() is not visible
// because it declared as private.
ConstructorDemo cons1 = new ConstructorDemo();
// invoke Constructor(String arg)
ConstructorDemo cons2 = new ConstructorDemo("constructor");
// invoke public Constructor(int x)
ConstructorDemo cons3 = new ConstructorDemo(1);
//invoke Constructor(int x, int y)
ConstructorDemo cons4 = new ConstructorDemo(1, 2);
}
}
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