Constructors and instantiation in java
Class declarations define new reference types and describe how they are implemented (§8.1).
A top level class is a class that is not a nested class.
A nested class is any class whose declaration occurs within the body of another class or interface.
This chapter discusses the common semantics of all classes — top level (§7.6) and nested (including member classes (§8.5, §9.5), local classes (§14.3) and anonymous classes (§15.9.5)). Details that are specific to particular kinds of classes are discussed in the sections dedicated to these constructs.
A named class may be declared abstract (§8.1.1.1) and must be declared abstract if it is incompletely implemented; such a class cannot be instantiated, but can be extended by subclasses. A class may be declared final (§8.1.1.2), in which case it cannot have subclasses. If a class is declared public , then it can be referred to from other packages. Each class except Object is an extension of (that is, a subclass of) a single existing class (§8.1.4) and may implement interfaces (§8.1.5). Classes may be generic (§8.1.2), that is, they may declare type variables whose bindings may differ among different instances of the class.
Classes may be decorated with annotations (§9.7) just like any other kind of declaration.
The body of a class declares members (fields and methods and nested classes and interfaces), instance and static initializers, and constructors (§8.1.6). The scope (§6.3) of a member (§8.2) is the entire body of the declaration of the class to which the member belongs. Field, method, member class, member interface, and constructor declarations may include the access modifiers (§6.6) public , protected , or private . The members of a class include both declared and inherited members (§8.2). Newly declared fields can hide fields declared in a superclass or superinterface. Newly declared class members and interface members can hide class or interface members declared in a superclass or superinterface. Newly declared methods can hide, implement, or override methods declared in a superclass or superinterface.
Field declarations (§8.3) describe class variables, which are incarnated once, and instance variables, which are freshly incarnated for each instance of the class. A field may be declared final (§8.3.1.2), in which case it can be assigned to only once. Any field declaration may include an initializer.
Member class declarations (§8.5) describe nested classes that are members of the surrounding class. Member classes may be static , in which case they have no access to the instance variables of the surrounding class; or they may be inner classes (§8.1.3).
Member interface declarations (§8.5) describe nested interfaces that are members of the surrounding class.
Method declarations (§8.4) describe code that may be invoked by method invocation expressions (§15.12). A class method is invoked relative to the class type; an instance method is invoked with respect to some particular object that is an instance of a class type. A method whose declaration does not indicate how it is implemented must be declared abstract . A method may be declared final (§8.4.3.3), in which case it cannot be hidden or overridden. A method may be implemented by platform-dependent native code (§8.4.3.4). A synchronized method (§8.4.3.6) automatically locks an object before executing its body and automatically unlocks the object on return, as if by use of a synchronized statement (§14.19), thus allowing its activities to be synchronized with those of other threads (§17 (Threads and Locks)).
Method names may be overloaded (§8.4.9).
Instance initializers (§8.6) are blocks of executable code that may be used to help initialize an instance when it is created (§15.9).
Static initializers (§8.7) are blocks of executable code that may be used to help initialize a class.
Constructors (§8.8) are similar to methods, but cannot be invoked directly by a method call; they are used to initialize new class instances. Like methods, they may be overloaded (§8.8.8).
Creating Objects
As you know, a class provides the blueprint for objects; you create an object from a class. Each of the following statements taken from the CreateObjectDemo program creates an object and assigns it to a variable:
Point originOne = new Point(23, 94); Rectangle rectOne = new Rectangle(originOne, 100, 200); Rectangle rectTwo = new Rectangle(50, 100);
The first line creates an object of the Point class, and the second and third lines each create an object of the Rectangle class.
Each of these statements has three parts (discussed in detail below):
- Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
- Instantiation: The new keyword is a Java operator that creates the object.
- Initialization: The new operator is followed by a call to a constructor, which initializes the new object.
Declaring a Variable to Refer to an Object
Previously, you learned that to declare a variable, you write:
This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable.
You can also declare a reference variable on its own line. For example:
If you declare originOne like this, its value will be undetermined until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object. For that, you need to use the new operator, as described in the next section. You must assign an object to originOne before you use it in your code. Otherwise, you will get a compiler error.
A variable in this state, which currently references no object, can be illustrated as follows (the variable name, originOne , plus a reference pointing to nothing):
Instantiating a Class
The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor.
Note: The phrase «instantiating a class» means the same thing as «creating an object.» When you create an object, you are creating an «instance» of a class, therefore «instantiating» a class.
The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.
The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:
Point originOne = new Point(23, 94);
The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:
int height = new Rectangle().height;
This statement will be discussed in the next section.
Initializing an Object
Here’s the code for the Point class:
This class contains a single constructor. You can recognize a constructor because its declaration uses the same name as the class and it has no return type. The constructor in the Point class takes two integer arguments, as declared by the code (int a, int b). The following statement provides 23 and 94 as values for those arguments:
Point originOne = new Point(23, 94);
The result of executing this statement can be illustrated in the next figure:
Here’s the code for the Rectangle class, which contains four constructors:
public class Rectangle < public int width = 0; public int height = 0; public Point origin; // four constructors public Rectangle() < origin = new Point(0, 0); >public Rectangle(Point p) < origin = p; >public Rectangle(int w, int h) < origin = new Point(0, 0); width = w; height = h; >public Rectangle(Point p, int w, int h) < origin = p; width = w; height = h; >// a method for moving the rectangle public void move(int x, int y) < origin.x = x; origin.y = y; >// a method for computing the area of the rectangle public int getArea() < return width * height; >>
Each constructor lets you provide initial values for the rectangle’s origin, width, and height, using both primitive and reference types. If a class has multiple constructors, they must have different signatures. The Java compiler differentiates the constructors based on the number and the type of the arguments. When the Java compiler encounters the following code, it knows to call the constructor in the Rectangle class that requires a Point argument followed by two integer arguments:
Rectangle rectOne = new Rectangle(originOne, 100, 200);
This calls one of Rectangle ‘s constructors that initializes origin to originOne . Also, the constructor sets width to 100 and height to 200. Now there are two references to the same Point objectan object can have multiple references to it, as shown in the next figure:
The following line of code calls the Rectangle constructor that requires two integer arguments, which provide the initial values for width and height. If you inspect the code within the constructor, you will see that it creates a new Point object whose x and y values are initialized to 0:
Rectangle rectTwo = new Rectangle(50, 100);
The Rectangle constructor used in the following statement doesn’t take any arguments, so it’s called a no-argument constructor:
Rectangle rect = new Rectangle();
All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent’s no-argument constructor, or the Object constructor if the class has no other parent. If the parent has no constructor ( Object does have one), the compiler will reject the program.
Constructors and instantiation in java
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