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12.3 Constructors and Instantiation

Constructors

Now that we know how to declare a class, how do we create a new object? That’s where a special method called a constructor comes in. The constructor of a class is always public and has the exact same name as the class. It can have no parameters (the default constructor), or one or more parameters depending on how much of the class you want to customize. It also has no return type.

It’s generally good practice to have at least 2 constructors: the default constructor and a constructor with enough parameters to initialize all of the class’s instance variables (non-static fields). Here is an example of 2 constructors for the Person class:

/** Constructs a default Person object */
public Person() {
	name = "";
	age = 0;
	address = "";
	hungry = false;
}

/**
	* Constructs a Person object with a given name,
  * age, address, and hunger status
  * @param name  name of person
  * @param age  how old the person is, in years
  * @param address  address of person
  * @param hungry  whether the person is hungry or not
  */
public Person(String name, int age, String address, boolean hungry) {
	this.name = name;
	this.age = age;
  this.address = address;
  this.hungry = hungry;
}
View this code on GitHub

this keyword - what does it mean? The this keyword is an abstract way of referring to an object. It can be used to call an instance’s fields or methods. In the case of a constructor, it’s usually used to differentiate between the parameter names and the field itself. For example, in the code above, the value that is passed into the parameter name will be assigned to the name variable of this object. Without using the this keyword, you’d simply be typing name = name, which doesn’t make sense because you’re just assigning a variable to itself.

Above each of the constructors is what is called a Javadoc comment, which is a special multi-line comment that is used to generate documentation for your code. It is completely optional, but is good practice to have. For more on Javadocs, see this website: https://www.oracle.com/technical-resources/articles/java/javadoc-tool.html

Instantiation

To call a constructor to create a new object, you need to use the new keyword. The process of creating a new object is called instantiation, because you’re creating an instance (synonymous with object) of the class.

Person myPerson = new Person(); // call constructor

Note how like any variable, when you declare a variable you need to declare the type. The type is simply the class name, whether it’s PersonString, or something else.

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