Java Tutorial/Class Definition/Constructor
Содержание
Calling a Constructor From a Constructor
<source lang="java">
class Sphere {
int radius = 0;
double xCenter;
double yCenter;
double zCenter;
Sphere() {
radius = 1;
}
Sphere(double x, double y, double z) {
this();
xCenter = x;
yCenter = y;
zCenter = z;
}
Sphere(int theRadius, double x, double y, double z) {
this(x, y, z);
radius = theRadius;
}
}</source>
Class Initializer: during declaration
<source lang="java">
// ClassInitializer2.java class ClassInitializer2 {
static boolean bool = true;
static byte by = 20;
static char ch = "X";
static double d = 8.95;
static float f = 2.1f;
static int i = 63;
static long l = 2L;
static short sh = 200;
static String str = "test";
public static void main(String[] args) {
System.out.println("bool = " + bool);
System.out.println("by = " + by);
System.out.println("ch = " + ch);
System.out.println("d = " + d);
System.out.println("f = " + f);
System.out.println("i = " + i);
System.out.println("l = " + l);
System.out.println("sh = " + sh);
System.out.println("str = " + str);
}
}</source>
Duplicating Objects using a Constructor
<source lang="java">
class Sphere {
int radius = 0;
double xCenter;
double yCenter;
double zCenter;
Sphere() {
radius = 1;
}
Sphere(double x, double y, double z) {
this();
xCenter = x;
yCenter = y;
zCenter = z;
}
Sphere(int theRadius, double x, double y, double z) {
this(x, y, z);
radius = theRadius;
}
// Create a sphere from an existing object
Sphere(final Sphere oldSphere) {
radius = oldSphere.radius;
xCenter = oldSphere.xCenter;
yCenter = oldSphere.yCenter;
zCenter = oldSphere.yCenter;
}
}</source>
Multiple Constructors
<source lang="java">
class Sphere {
int radius = 0;
Sphere() {
radius = 1;
}
Sphere(int radius) {
this.radius = radius;
}
}</source>
Order of constructor calls
<source lang="java">
class Meal {
Meal() {
System.out.println("Meal()");
}
} class Bread {
Bread() {
System.out.println("Bread()");
}
} class Cheese {
Cheese() {
System.out.println("Cheese()");
}
} class Lettuce {
Lettuce() {
System.out.println("Lettuce()");
}
} class Lunch extends Meal {
Lunch() {
System.out.println("Lunch()");
}
} class PortableLunch extends Lunch {
PortableLunch() {
System.out.println("PortableLunch()");
}
} class Sandwich extends PortableLunch {
private Bread b = new Bread();
private Cheese c = new Cheese();
private Lettuce l = new Lettuce();
public Sandwich() {
System.out.println("Sandwich()");
}
} public class MainClass {
public static void main(String[] args) {
new Sandwich();
}
}</source>
Meal() Lunch() PortableLunch() Bread() Cheese() Lettuce() Sandwich()
The Default Constructor
<source lang="java">
public class MainClass {
double radius;
MainClass() {
}
// Class constructor
MainClass(double theRadius) {
radius = theRadius;
}
}</source>
Using Constructors
- Every class must have at least one constructor.
- If there is no constructors for your class, the compiler will supply a default constructor(no-arg constructor).
- A constructor is used to construct an object.
- A constructor looks like a method and is sometimes called a constructor method.
- A constructor never returns a value
- A constructor always has the same name as the class.
- A constructor may have zero argument, in which case it is called a no-argument (or no-arg, for short) constructor.
- Constructor arguments can be used to initialize the fields in the object.
The syntax for a constructor is as follows.
<source lang="java">
constructorName (listOfArguments) {
[constructor body]
}</source>
