Создать класс rectangle java

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):

1. Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
2. Instantiation: The new keyword is a Java operator that creates the object.
3. 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 object—an 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.

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Создать класс rectangle java

This class uses 32-bit integers to store its location and dimensions. Frequently operations may produce a result that exceeds the range of a 32-bit integer. The methods will calculate their results in a way that avoids any 32-bit overflow for intermediate results and then choose the best representation to store the final results back into the 32-bit fields which hold the location and dimensions. The location of the result will be stored into the x and y fields by clipping the true result to the nearest 32-bit value. The values stored into the width and height dimension fields will be chosen as the 32-bit values that encompass the largest part of the true result as possible. Generally this means that the dimension will be clipped independently to the range of 32-bit integers except that if the location had to be moved to store it into its pair of 32-bit fields then the dimensions will be adjusted relative to the «best representation» of the location. If the true result had a negative dimension and was therefore non-existant along one or both axes, the stored dimensions will be negative numbers in those axes. If the true result had a location that could be represented within the range of 32-bit integers, but zero dimension along one or both axes, then the stored dimensions will be zero in those axes.

Nested Class Summary

Nested classes/interfaces inherited from class java.awt.geom.Rectangle2D

Field Summary

Fields inherited from class java.awt.geom.Rectangle2D

Constructor Summary

Constructs a new Rectangle whose upper-left corner is at (0, 0) in the coordinate space, and whose width and height are both zero.

Constructs a new Rectangle whose top left corner is (0, 0) and whose width and height are specified by the Dimension argument.

Constructs a new Rectangle whose upper-left corner is at (0, 0) in the coordinate space, and whose width and height are specified by the arguments of the same name.

Constructs a new Rectangle whose upper-left corner is specified as (x,y) and whose width and height are specified by the arguments of the same name.

Constructs a new Rectangle whose upper-left corner is the specified Point , and whose width and height are both zero.

Constructs a new Rectangle whose upper-left corner is specified by the Point argument, and whose width and height are specified by the Dimension argument.

Method Summary

Checks whether this Rectangle entirely contains the Rectangle at the specified location (X,Y) with the specified dimensions (W,H) .

Returns a new Rectangle2D object representing the intersection of this Rectangle2D with the specified Rectangle2D .

Returns a new Rectangle2D object representing the union of this Rectangle2D with the specified Rectangle2D .

Sets the bounds of this Rectangle to the integer bounds which encompass the specified x , y , width , and height .

Translates this Rectangle the indicated distance, to the right along the X coordinate axis, and downward along the Y coordinate axis.

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Class Rectangle

A Rectangle specifies an area in a coordinate space that is enclosed by the Rectangle object’s upper-left point (x,y) in the coordinate space, its width, and its height.

A Rectangle object’s width and height are public fields. The constructors that create a Rectangle , and the methods that can modify one, do not prevent setting a negative value for width or height.

A Rectangle whose width or height is exactly zero has location along those axes with zero dimension, but is otherwise considered empty. The isEmpty() method will return true for such a Rectangle . Methods which test if an empty Rectangle contains or intersects a point or rectangle will always return false if either dimension is zero. Methods which combine such a Rectangle with a point or rectangle will include the location of the Rectangle on that axis in the result as if the add(Point) method were being called.

Methods which affect only the location of a Rectangle will operate on its location regardless of whether or not it has a negative or zero dimension along either axis.

Note that a Rectangle constructed with the default no-argument constructor will have dimensions of 0x0 and therefore be empty. That Rectangle will still have a location of (0,0) and will contribute that location to the union and add operations. Code attempting to accumulate the bounds of a set of points should therefore initially construct the Rectangle with a specifically negative width and height or it should use the first point in the set to construct the Rectangle . For example:

 Rectangle bounds = new Rectangle(0, 0, -1, -1); for (int i = 0; i

 Rectangle bounds = new Rectangle(points[0]); for (int i = 1; i

This class uses 32-bit integers to store its location and dimensions. Frequently operations may produce a result that exceeds the range of a 32-bit integer. The methods will calculate their results in a way that avoids any 32-bit overflow for intermediate results and then choose the best representation to store the final results back into the 32-bit fields which hold the location and dimensions. The location of the result will be stored into the x and y fields by clipping the true result to the nearest 32-bit value. The values stored into the width and height dimension fields will be chosen as the 32-bit values that encompass the largest part of the true result as possible. Generally this means that the dimension will be clipped independently to the range of 32-bit integers except that if the location had to be moved to store it into its pair of 32-bit fields then the dimensions will be adjusted relative to the «best representation» of the location. If the true result had a negative dimension and was therefore non-existent along one or both axes, the stored dimensions will be negative numbers in those axes. If the true result had a location that could be represented within the range of 32-bit integers, but zero dimension along one or both axes, then the stored dimensions will be zero in those axes.

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