design of a bank, the “bank teller” class needs to relate to the “customer” class. These relationships are called associations.
We will see that, when software is packaged as classes, these classes can be reused in future software systems. Groups of related classes are often packaged as reusable components. Just as
real-estate brokers tell their clients that the three most important factors affecting the price of real estate are “location, location and location,” many people in the software community believe
that the three most important factors affecting the future of software development are “reuse, reuse and reuse.”
Indeed, with object technology, we can build much of the software we will need by combining “standardized, interchangeable parts” called classes. This book teaches you how to “craft
valuable classes” for reuse. Each new class you create will have the potential to become a valuable software asset that you and other programmers can use to speed and enhance the quality of
future software-development efforts—an exciting possibility.
Relation of Java with C, C++, & C#:
From C Java derives its syntax and from C++ it derives object oriented features. It is not an enhanced version of C++. Java is neither upwardly nor downwardly compatible with C++. Java
was not designed to replace C++. C# another language developed by Microsoft to support the .NET Framework, C# is closely related to Java because both share C++ & C style syntax,
support distributed programming, and utilize the same object model.
Primary Objective of Java is to achieve:
1.
Security:
There is no threat of virus infection when we use Java compatible Web Browser. Also there is no threat of malicious programs that can gather private information, such as
credit card numbers, bank account balances and passwords from local machine. Java provides a firewall between a networked application and our computer.
2.
Portability:
Java programs are portable from one computer to another computer running different types of operating systems and having different hardware.
Java Bytecode:
The output of a Java compiler is bytecode not the machine code (“.class” file). Bytecode is a highly optimized set of instructions designed to be executed by the Java runtime system, which is called as
JVM (Java Virtual Machine). JVM is the interpreter which interprets the bytecode. Compiled program runs faster but still Java uses interpreter to achieve portability so Java programs runs a little slower.
Now a program compiled through a Java compiler can run in any environment but JVM needs to be implemented for each platform. Java programs are interpreted. This also helps to make it secure
because the execution of every Java program is under the control of JVM.
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