11.10. Chapter Summary

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Review Questions

Section 10.2 Abstract Classes

10.1	Which of the following class definitions defines a legal abstract class?
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10.2	The getArea and getPerimeter methods may be removed from the GeometricObject class. What are the benefits of defining getArea and getPerimeter as abstract methods in the GeometricObject class?

Section 10.4 Interfaces

10.3	Which of the following is a correct interface?
10.4	Two max methods are defined in §10.4. Explain why the max with the signature max(Comparable, Comparable) is better than the one with the signature max(Object, Object) . Are these statements correct? String s = Max.max( "abc" , "efg" ); Date date = Max.max( new Date(), new Date());
10.5	You can define the compareTo method in a class without implementing the Comparable interface. What are the benefits of implementing the Comparable interface?
10.6	What would happen if the House class (defined in Listing 10.4) does not override the clone() method or if House does not implement java.lang.Cloneable ?
10.7	Show the printout of the following code: java.util.Date date = new java.util.Date(); java.util.Date date1 = (java.util.Date)(date.clone()); System.out.println(date == date1); System.out.println(date.equals(date1));
10.8	Show the printout of the following code: java.util.ArrayList list = new java.util.ArrayList(); list.add( "New York" ); list.add(new java.util.Date()); java.util.ArrayList list1 = (java.util.ArrayList)(list.clone()); System.out.println(list == list1); System.out.println(list.get( ) == list1.get( )); System.out.println(list.get( 1 ) == list1.get( 1 ));
10.9	What is wrong in the following code? public class Test { public static void main(String[] args) { GeometricObject x = new Circle( 3 ); GeometricObject y = x.clone(); System.out.println(x == y); } }

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Section 10.5 Processing Primitive Data Type Values as Objects

10.10	Can you assign new int[10] , new String[100] , new Object[50] , or new Calendar[20] into a variable of Object[] type?
10.11	Describe primitive-type wrapper classes. Why do you need these wrapper classes?
10.12	Are the following statements correct? Integer i = new Integer( "23" ); Integer i = new Integer( 23 ); Integer i = Integer.valueOf( "23" ); Integer i = Integer.parseInt( "23" , 8 ); Double d = new Double(); Double d = Double.valueOf( "23.45" ); int i = (Integer.valueOf( "23" )).intValue(); double d = (Double.valueOf( "23.4" )).doubleValue(); int i = (Double.valueOf( "23.4" )).intValue(); String s = (Double.valueOf( "23.4" )).toString();
10.13	How do you convert an integer into a string? How do you convert a numeric string into an integer? How do you convert a double number into a string? How do you convert a numeric string into a double value?
10.14	Why do the following two lines of code compile but cause a runtime error? Number numberRef = new Integer( ); Double doubleRef = (Double)numberRef;
10.15	Why do the following two lines of code compile but cause a runtime error? Number[] numberArray = new Integer[ 2 ]; numberArray[ ] = new Double( 1.5 );
10.16	What is wrong in the following code? public class Test { public static void main(String[] args) { Number x = new Integer( 3 ); System.out.println(x.intValue()); System.out.println(x.compareTo(new Integer( 4 ))); } }
10.17	What is wrong in the following code? public class Test { public static void main(String[] args) { Number x = new Integer( 3 ); System.out.println(x.intValue()); System.out.println((Integer)x.compareTo(new Integer( 4 ))); } }
10.18	What is the output of the following code? public class Test { public static void main(String[] args) { java.math.BigInteger x = new java.math.BigInteger( "3" ); java.math.BigInteger y = new java.math.BigInteger( "7" ); x.add(y); System.out.println(x); } }

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Section 10.6 Automatic Conversion Between Primitive Types and Wrapper Classes

10.19

Describe the boxing and unboxing features in JDK 1.5. Are the following statements correct in JDK 1.5?

 Number x =   3   ; Integer x =   3   ; Double x =   3   ; Double x =   3.0   ;   int   x = new Integer(   3   );   int   x = new Integer(   3   ) + new Integer(   4   );   double   y =   3.4   ; y.intValue(); JOptionPane.showMessageDialog(   null   ,   45.5   ); 

Comprehensive

10.20

Define the following terms: abstract classes, interfaces. What are the similarities and differences between abstract classes and interfaces?

10.21

Indicate true or false for the following statements: An abstract class can have instances created using the constructor of the abstract class. An abstract class can be extended. An interface is compiled into a separate bytecode file. A subclass of a nonabstract superclass cannot be abstract. A subclass cannot override a concrete method in a superclass to declare it abstract. An abstract method must be non-static An interface can have static methods. An interface can extend one or more interfaces. An interface can extend an abstract class. An abstract class can extend an interface.