11.9. Key Terms

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Chapter Summary

• Abstract classes are like regular classes with data and methods , but you cannot create instances of abstract classes using the new operator.

• An abstract method cannot be contained in a nonabstract class. If a subclass of an abstract superclass does not implement all the inherited abstract methods of the superclass, the subclass must be declared abstract.

• A class that contains abstract methods must be abstract. However, it is possible to declare an abstract class that contains no abstract methods.

• A subclass can be abstract even if its superclass is concrete.

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• An interface is a classlike construct that contains only constants and abstract methods. In many ways, an interface is similar to an abstract class, but an abstract class can contain constants and abstract methods as well as variables and concrete methods.

• An interface is treated like a special class in Java. Each interface is compiled into a separate bytecode file, just like a regular class.

• The java.lang.Comparable interface defines the compareTo method. Many classes in the Java library implement Comparable .

• The java.lang.Cloneable interface is a marker interface. An object of the class that implements the Cloneable interface is cloneable.

• A class can extend only one superclass but can implement one or more interfaces.

• An interface can extend one or more interfaces.

• Many Java methods require the use of objects as arguments. Java offers a convenient way to incorporate , or wrap, a primitive data type into an object (e.g., wrapping int into the Integer class, and wrapping double into the Double class). The corresponding class is called a wrapper class . By using a wrapper object instead of a primitive data type variable, you can take advantage of generic programming.

• The wrapper class for byte is Byte , for short is Short , for int is Integer , for long is Long , for float is Float , for double is Double , for char is Character , and for boolean is Boolean .

• The numeric wrapper classes extend the abstract java.lang.Number class and implement the java.lang.Comparable interface. The Character class also implements java.lang.Comparable .