Parameterized constructor
- Java course
- Intermediate
- Lesson 8/116
- 5 min
Learn constructors that take parameters.
Parameterized constructor
A constructor can take parameters. Parameters are declared in parentheses and used in the body. This way, values are passed from outside when the instance is created; no manual assignment is needed.
Example: class Book { String title; Book(String t) { title = t; } }. Here Book(String t) takes a String parameter. When new Book("Java") is called, t becomes "Java" and is assigned to title. This approach finishes value assignment in one step; it is cleaner and safer.
Instantiation with parameters
When a parameterized constructor is called, values are written inside the parentheses. The value types must match the parameters. Multiple parameters are separated by commas and given in the same order.
Example: Book b = new Book("Java", 300);. Here two parameters exist: a string and an integer. The constructor must be declared as Book(String t, int p). Parameter order matters; the string comes first, then the integer. Wrong order causes a compile error.
Practice Add a parameterized constructor to Book
Define a class named Book. Add a String title field. Write a Book(String t) constructor; its body assigns title = t;. In main create an instance with new Book("Java") and print the title field.
Starter code
// Define the Book class
// Add a parameterized constructor
// Instantiate and print in mainSample solution
class Book {
String title;
Book(String t) {
title = t;
}
public static void main(String[] args) {
Book b = new Book("Java");
System.out.println(b.title);
}
}Expected output
Java
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the parameterized constructor parts
| Term | Meaning |
|---|---|
Book |
Constructor name |
String t |
Parameter |
title = t |
Assignment |
new Book("Java") |
Call |
2 The constructor Book(String t) exists. Which call is valid?
- new Book();
- new Book("Java"); Correct answer
- new Book(300);
- Book("Java");
Explanation: New Book("Java"); — the parameter must be a String.
3 Complete the parameterized constructor body
class Book {
String title;
Book(String t) {
title = ____;
}
}class Book {
String title;
Book(String t) {
title = t;
}
}Explanation: The parameter t is assigned to the title field.