• Abstract Data Type (ADT) - a language has support for ADTs if it offers support for encapsulation and information hiding.
  
  
• Access Control Keywords - allows us to restrict access to information.
  
  
• Classpath - list of directories in which the compiler and interpreter will search for classes. If we use the statement
  
  import a.b;
  in our program, then the compiler will search through each directory listed in the class path looking for a subdirectory called a. Inside that subdirectory it will look for a file named b.class.
  
  
• Encapsulation - the concept of binding data to the code that is to operate on it.
  
  
• Information Hiding - the ability to limit access to information.
  
  
• Default package - when classes are generated within the same directory they are assumed to be part of the same package.
  
  
• Deprecated features - Java is an evolving language and some features are are removed.
  
  
• finalize() - a method similar to a constructor but called when an object is destroyed.
  
  
• inner class - another element that can be placed inside a class definition.
  
  
• static initializer - another element that can be placed inside a class definition. This code has the keyword static in front of it and nothing else. The static initializer is executed when the class is loaded into main memory. The static initializer is used to initialize class variables just like a constructor is used to initialize instance variables.
  
  
• StreamTokenizer - a class available in the JDK. It allows us to get tokens one at a time from a Reader. The StreamTokenizer gives us a token return type for each token that is read. The important token types are
  
  
  1. TT_EOF (Token Type End of File)
  2. TT_NUMBER (Token Type Number)
  3. TT_WORD (Token Type Word)

import java.io.*;
import USA.USA;

class TestStream {

   public static void main(String[] args) throws IOException {
      StreamTokenizer       fileParser;
      String                inputFileName;
      FileReader            inputFile;
      int                   tokenType;
      int                   numberCount = 0;
      int                   wordCount = 0;
      int                   otherCount = 0;

      inputFileName = USA.getNameOfExistingFile("Enter name of input file: ");
      inputFile     = new FileReader(inputFileName);
      fileParser    = new StreamTokenizer(inputFile);

      tokenType     = fileParser.nextToken();
      while (tokenType != StreakTokenizer.TT_EOF) {
         switch (tokenType) {
            case StreamTokenizer.TT_NUMBER : {
               numberCount++;
               System.out.println("Number: " + fileParser.nval);
               break;
            }
            case StreamTokenizer.TT_WORD : {
               wordCount++;
               System.out.println("Word : " + fileParser.sval);
               break;
            }
            default : {
               System.out.println("Other token type : " + tokenType);
               otherCount++;
            }
         }
         if ((wordCount + numberCount + otherCount) % 20 == 0)
            USA.pause();
         tokenType = fileParser.nextToken();
      }

      System.out.println("In the file '" + inputFileName + "' we found: ");
      System.out.println("   " + wordCount + " words, ");
      System.out.println("   " + numberCount + " numbers, and ");
      System.out.println("   " + otherCount + " other tokens.");
   }

}

package sub;

public class Abc {

   String name;

   public Abc (String name) {
      this.name = name;
   }

   public String toString() {
      return("(Abc): " + name);
   }

}

package sub;

public class Def {

   Abc myAbc;
   int age;

   public Def (String name, int age) {
      this.age = age;
      myAbc = new Abc(name);
   }

   public String toString() {
      return("I am a Def " + age + " years old and is called " + myAbc);
   }

   public static void main(String args[]) {
      Abc anAbc = new Abc("John");
      Def aDef = new Def("David", 21);
      System.out.println("Abc object is : " + anAbc);
      System.out.prnitln("Def object is : " + aDef);
   }

}

import Sub.Def;

public class UseSub {

   public static void main(String[] args) {
      Def aDef = new Def("Mary", 19);
      System.out.println("My Def: " + aDef);
   }

}