1. 2: Everything is an Object You Manipulate Objects Using References Primitives Arrays in Java Scoping You Never Destroy Objects Creating New Data Types: class Methods, Arguments, and Return Values Naming Conventions Building a Java Program Name Visibility Your First Java Program Comments & Embedded Documentation

2. 2: Everything is an Object Both C++ and Java are hybrid languages The reason C++ is hybrid is to support backward compatibility with the C language Although it is based on C++, Java is more of a “pure” object-oriented language

3. You manipulate objects with references –Java does not support pointer (Syntax) –But implemented by pointer (perhaps) –String s;// Reference only Example: television / file open You must create all the objects –String s;// s.length() ?? –String s =new String(“ABC”); // String s =“ABC”; Where storage lives –Register –Stack –Heap

4. String s=new String(“ABC”); int i=5; s “ABC” i (5) CODE STACK HEAP

5. Special case: primitive types Primitive type SizeMinimumMaximumWrapper type boolean ——— Boolean char16-bitUnicode 0Unicode 2 16 - 1Character byte8-bit-128127Byte short16-bit -215 +2 15 — 1Short int32-bit -231 +2 31 — 1Integer long64-bit -263 +2 63 — 1Long float32-bitIEEE754 Float double64-bitIEEE754 Double void ——— Void

6. Special case: primitive types –int char float double … –Wrapper classes : Integer Character … char ch=‘x’; Character c= new Character(‘x’);

7. Arrays in Java –Objects (not pointers) –Auto boundary checking C/C++ memory overflow –Creation int[ ] aint=new int[5]; –Array of objects When you create an array of objects, you are really creating an array of references Its elements are initialized to null –String[] arr = new String[10]; –arr.length; –arr[0].length(); //?

8. You never need to destroy an object Scoping (C/C++/Java) { int x = 12; // Only x available { int q = 96; // Both x & q available } // Only x available // q “ out of scope ” }

9. You never need to destroy an object { String s = new String("a string"); } /* end of scope */ ========================================== package test; public class Hello { public Hello() { } public static void main(String[] args) { System.out.println("Hello World"); } garbage collector Who is responsible for releasing memory: garbage collector

10. Creating new data types: class

11. When you define a class, you can put two types of elements in your class: –fields (sometimes called data members / properties), –methods (sometimes called member functions). If the field is a reference to an object, you must initialize that reference to connect it to an actual object –(using new, as seen earlier) in a special method called a constructor. If it is a primitive type, you can initialize it directly by assigning a value.

12. Default values for primitive members This guarantee doesn ’ t apply to “ local ” variables — those that are not fields of a class. Thus, if within a function definition you have: int x; Then x will get some arbitrary value (as in C and C++); it will not automatically be initialized to zero

13. Default values for primitive members class Data { int i; float f; } class Data { int i=10; float f=1.0; String s=new String( “ ABC ” ); }

14. Default values for primitive members class Data { int i; float f; void func(){ int j; int k=2*j; }

15. Creating new data types: class how to refer to a member of an object –objectReference.member –For example: Feedback Feedback DataOnly d=new DataOnly(); d.i = 47; d.f = 1.1f; // ‘ f ’ after number indicates float constant d.b = false; –Objects inside an object myPlane.leftTank.capacity = 100;

16. Methods, arguments, and return values returnType methodName( /* Argument list */ ) { /* Method body */ } Methods in Java can be created only as part of a class. A method can be called only for an object (Except for static method) –objectName.methodName(arg1, arg2, arg3); –Example: int x = a.f(); This act of calling a method is commonly referred to as sending a message to an object. In the preceding example, the message is f( ) and the object is a.

17. Naming Conventions

18. Name visibility/Using other components package com.bruceeckel.utility.foibles ; class AAA {……} ======================= import com.bruceeckel.utility.foibles.* ; import java.util.ArrayList; import java.util.*; AAA a=new AAA(); ======================= com.bruceeckel.utility.foibles.AAA a=new com.bruceeckel.utility.foibles.AAA(); java.lang.* is the exception which is needless to import

19. First Program:Hello world // HelloWorld.java public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World ! "); } // HelloWorld.c int main(int argc,char * argv[]) { printf("Hello World ! \n"); }

20. Compiling and running // HelloDate.java import java.util.*; public class HelloDate { public static void main(String[] args) { System.out.println("Hello, it's: "); System.out.println(new Date()); } –javac HelloDate.java –java HelloDate

21. 2: Everythingis an Object Comments and embedded documentation class AllTheColorsOfTheRainbow { int anIntegerRepresentingColors; void changeTheHueOfTheColor(int newHue) { //... } //... }

22. //: c02:HelloDate.java import java.util.*; /** The first Thinking in Java example program. * Displays a string and today's date. * @author Bruce Eckel * @author www.BruceEckel.com * @version 2.0 */ public class HelloDate { /** Sole entry point to class & application * @param args array of string arguments * @return No return value * @exception exceptions No exceptions thrown */ public static void main(String[] args) { System.out.println("Hello, it's: "); System.out.println(new Date()); } } ///:~

23. Homework Text p77-90 Exercise p104 1.Following the HelloDate.java example in this chapter, create a “hello, world” program that simply prints out that statement. You need only a single method in your class (the “main” one that gets executed when the program starts). Remember to make it static and to include the argument list, even though you don’t use the argument list. Compile the program with javac and run it using java.