Java is the general name for the Java object-oriented programming language and Java platform launched by Sun Microsystems in May 1995. It was jointly developed by James Gosling and colleagues and officially launched in 1995.
Java is divided into three systems:
In June 2005, the JavaOne conference was held, and SUN released Java SE 6. By this time, various versions of Java had been renamed to remove the number "2": J2EE was renamed Java EE, J2SE was renamed Java SE, and J2ME was renamed Java ME.
In 2009, Sun was acquired by Oracle.
In 2018, the open source organization Eclipse Foundation announced that JavaEE (Enterprise Edition) would be renamed JakartaEE (Jakarta).
The syntax of Java language is very close to C language and C++ language, making it easy for most programmers to learn and use. On the other hand, Java discards those features of C++ that are rarely used, difficult to understand, and confusing, such as operator overloading, multiple inheritance, and automatic casts. In particular, the Java language does not use pointers, but references. It also provides an automatic memory recycling management mechanism so that programmers do not have to worry about memory management.
The Java language provides primitives such as classes, interfaces and inheritance. For the sake of simplicity, it only supports single inheritance between classes, but supports multiple inheritance between interfaces, and supports the implementation mechanism between classes and interfaces (the keyword is implements) . The Java language fully supports dynamic binding, while the C++ language only uses dynamic binding for virtual functions. In short, the Java language is a pure object-oriented programming language.
Java language supports the development of Internet applications. Among the basic Java application programming interfaces, there is a network application programming interface (java net), which provides class libraries for network application programming, including URL, URLConnection, Socket, ServerSocket, etc. Java's RMI (Remote Method Activation) mechanism is also an important means of developing distributed applications.
Java's strong typing mechanism, exception handling, automatic garbage collection, etc. are important guarantees for the robustness of Java programs. Discarding pointers is a smart choice for Java. Java's security checking mechanism makes Java more robust.
Java is usually used in network environments. For this reason, Java provides a security mechanism to prevent malicious code attacks. In addition to the many security features of the Java language, Java has a security prevention mechanism (class ClassLoader) for classes downloaded through the network, such as allocating different name spaces to prevent replacement of local classes with the same name, byte code inspection, and providing security management Mechanism (class SecurityManager) allows Java applications to set up security sentries.
Java programs (files with the suffix java) are compiled into an architecture-neutral bytecode format (files with the suffix class) on the Java platform, and can then be run on any system that implements this Java platform. This approach is suitable for heterogeneous network environments and software distribution.
This portability comes from architecture neutrality. In addition, Java also strictly regulates the length of each basic data type. The Java system itself is also highly portable. The Java compiler is implemented in Java, and the Java running environment is implemented in ANSI C.
As mentioned before, Java programs are compiled into bytecode format on the Java platform and can then be run on any system that implements this Java platform. At runtime, the Java interpreter in the Java platform interprets and executes these bytecodes, and the classes required during the execution are loaded into the running environment during the connection phase.
Compared with those interpreted high-level scripting languages, Java is indeed high-performance. In fact, with the development of JIT (Just-In-Time) compiler technology, Java's running speed is getting closer and closer to C++.
In the Java language, a thread is a special object that must be created by the Thread class or its descendants (grandchildren). There are usually two ways to create a thread: first, use the constructor of type Thread(Runnable) to wrap an object that implements the Runnable interface into a thread; second, derive a subclass from the Thread class and override run Method, the object created using this subclass is a thread. It is worth noting that the Thread class has implemented the Runnable interface, so any thread has its run method, and the run method contains the code to be run by the thread. The activity of a thread is controlled by a set of methods. The Java language supports the simultaneous execution of multiple threads and provides a synchronization mechanism between multiple threads (the keyword is synchronized).
One of the design goals of the Java language is to adapt to dynamically changing environments. The classes required by Java programs can be dynamically loaded into the running environment, or the required classes can be loaded through the network. This also facilitates software upgrades. In addition, classes in Java have a run-time representation and can perform run-time type checking.
The Java language tries to ensure that the system memory is above 1G. Other tools are as follows:
After installing the above tools, we can output Java's first program "Hello World!"
public class MyFirstJavaProgram { public static void main(String []args) { System.out.println("Hello World"); } }In the next chapter we will introduce how to configure the java development environment.