Sams Teach Yourself C in 21 Days

9: System.out.println(“The circle’s radius is “

10: + Double.toString(c1.radius));

11: }

12: }

The circle’s radius is 1.25

The first listing defines a class named circle. Line 1 is the class definition, and

line 3 declares a property named radius. The constructor is in lines 5 to 7. You

can see that the constructor is declared to take one type doubleargument. When the con-

structor is called, the value passed in the argument ris assigned to the property radius.

The second listing shows how the constructor is used. Lines 1 to 5 should be familiar to

you. Line 7 declares a variable of type circle. Line 8 uses the newkeyword to create an

instance of the class circle. When the instance is created, the constructor is called and is

passed the argument 1.25that is specified. Lines 9 and 10 demonstrates that the value is

indeed stored in the radiusproperty by displaying it on the screen.

Java is very careful about matching the arguments passed to a constructor and the para-

meters declared as part of the constructor. If they do not match in number and type, the

program will not compile. If a class has no constructor at all or has a constructor defined

with no parameters, use empty parentheses when creating an instance of the class:

MyObject = new ClassName();

Like any method, a class constructor can be overloaded. This can be useful when there

are different ways of initializing the object. Remember, overloaded methods have the

same name, but you can identify them by their parameters. To demonstrate, modify the

previous example so that the circleclass has another property,name, used to hold a text

description of the object. You’ll add two more constructors to the class to permit three

ways of initializing it:

• Pass no arguments. Radiusis set to 0 , andnameis set to “Unnamed”.


• Pass one numeric argument. Radius is set to the argument value, and name is set to
  “Unnamed”.


• Pass one numeric and one string argument. Both radiusandnameare initialized.
  Listings B5.9 and B5.10 show the source code for the new class and demonstration pro-
  gram.

734 Bonus Day 5

LISTINGB5.8 continued

OUTPUT

ANALYSIS

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