ABSTRACT CLASSES AND INTERFACES - Introduction to Java Programming: Comprehensive Version - page 509

13.16

Show the error in the following code:

interface A {
void m1();

}

class B implements A {
void m1() {
System.out.println( "m1" );
}
}

13.6 The Comparable Interface
The Comparable interface defines the compareTo method for comparing objects.

Key
Point

Suppose you want to design a generic method to find the larger of two objects of the same
type, such as two students, two dates, two circles, two rectangles, or two squares. In order to
accomplish this, the two objects must be comparable, so the common behavior for the objects
must be comparable. Java provides the Comparable interface for this purpose. The interface
is defined as follows:

// Interface for comparing objects, defined in java.lang
package java.lang;

java.lang.Comparable

public interface Comparable<E> {
public int compareTo(E o);

}

The compareTo method determines the order of this object with the specified object o and
returns a negative integer, zero, or a positive integer if this object is less than, equal to, or
greater than o .
The Comparable interface is a generic interface. The generic type E is replaced by a
concrete type when implementing this interface. Many classes in the Java library implement
Comparable to define a natural order for objects. The classes Byte , Short , Integer , Long ,
Float , Double , Character , BigInteger , BigDecimal , Calendar , String , and Date
all implement the Comparable interface. For example, the Integer , BigInteger , String ,
and Date classes are defined as follows in the Java API:

public class Integer extends Number
implements Comparable<Integer> {
// class body omitted

public class BigInteger extends Number
implements Comparable<BigInteger> {
// class body omitted

@Override
public int compareTo(Integer o) {
// Implementation omitted

@Override
public int compareTo(BigInteger o) {
// Implementation omitted

}
}

}
}

public class String extends Object
implements Comparable<String> {
// class body omitted

public class Date extends Object
implements Comparable<Date> {
// class body omitted

@Override
public int compareTo(String o) {
// Implementation omitted

@Override
public int compareTo(Date o) {
// Implementation omitted

}
}

}
}