1. Java fundamentals Todor Kolev

2. Java architecture Java's architecture arises out of four distinct but interrelated technologies: -the Java programming language -the Java class files -the Java Application Programming Interface -the Java virtual machine

3. Java architecture When you write and run a Java program, you are tapping the power of these four technologies. You express the program in source files written in the Java programming language, compile the source to Java class files, and run the class files on a Java virtual machine. Java API is actually a huge collection of library routines that performs basic programming tasks. The combination of the Java virtual machine and Java API is called the Java Platform See the figure below...

4. Java architecture The Java programming environment.

5. Java architecture Java Virtual Machine(JVM)

6. Java architecture Java Virtual Machine(JVM) The Method area The method area is where the bytecodes reside. The program counter always points to (contains the address of) some byte in the method area. The program counter is used to keep track of the thread of execution. After a bytecode instruction has been executed, the program counter will contain the address of the next instruction to execute. After execution of an instruction, the JVM sets the program counter to the address of the instruction that immediately follows the previous one, unless the previous one specifically demanded a jump.

7. Java architecture Java Virtual Machine(JVM) The Java stack The Java stack is used to store the results of the bytecode instructions, to pass parameters to and return values from methods as well as to keep the state of each method invocation. The state of each method is called its Stack frame. For example, if we have method that calls other m If our program is calling the A method, the A method will automatically call B, C and D, because of its state. Stack frame of A

8. - The JVM's heap stores all objects instantiated by an executing Java program. Objects are created by Java's "new" operator, and memory for new objects is allocated on the heap at run time. Garbage collection What is the Java heap memory? - Garbage collection is the process of automatically freeing objects that are no longer referenced by the program. This frees the programmer from having to keep track of when to free allocated memory, thereby preventing many potential bugs and headaches. Java architecture Java Virtual Machine(JVM)

9. Java references and parameters passing Java, unlike C++, stores all objects on the heap, and requires thenewoperator to create the object. Variables are still stored on the stack, but they hold a pointer to the object, not the object itself (and of course, to confuse C++ programmers more, these pointers are called "references").

10. Java references and parameters passing In C++, objects may be created on the stack using "automatic" allocation. The following is legal C++: Integer foo = Integer(1); it creates a newIntegerobject on the stack. A Java compiler, however, will reject it as a syntax error, because we havent allocated heap memory by new operator.

11. Java references and parameters passing Truth #1: The values of variables in Java are always primitives or references, never objects. public static void foo(String bar) { Integer baz = new Integer(bar); }

12. Java references and parameters passing Truth #2: Everything in Java is passed by value. Objects, however, are never passedat all. 1. public void foo(Dog d) { 2. d.name == "Max"; // true 3. d = new Dog("Fifi"); 4. d.name == "Fifi"; // true 5. } 6. 7. public static void main(String [] args){ 9. Dog aDog = new Dog("Max"); 10. foo(aDog); 11. aDog.name == "Max"; // true 12. }

13. Java references and parameters passing Java references vs. C++ pointers Pointers in C++ must be referenced and dereferenced using the *, whereas the referencing and dereferencing of objects is handled for you automatically by Java. C++ int *num = new int(1); std::cout num = n um ; } }; void main(){ Number * ptr = new Number(1); // ptr==00345948 std::cout