Most classes need to be imported before they can be used in an application. Why is every application allowed to use classes System and String without first importing them?

Java offers a rich library of predefined classes that serve as the building blocks for developing various applications. These classes provide a wide array of functionalities, from basic data manipulation to complex networking operations.
Among these classes, the java.lang package holds a special place. It includes essential classes like Object, System, String, and wrappers for primitive types such as Integer and Boolean. These classes are so fundamental to Java programs that the Java compiler implicitly imports the java.lang package. Thus, there's no need to write an import statement for them at the beginning of your code.

Why Do We Have Predefined Classes?

Predefined classes make it easier for developers to perform common tasks without reinventing the wheel. They also ensure a standard way of handling operations across different programs and help to reduce the potential for errors in code.

The System class in Java stands out as a critical component of the java.lang package. It cannot be instantiated because all its members and functions are static, meaning they belong to the class itself rather than an instance of the class.

Usage of the System Class

The System class provides useful fields and methods for performing system-level operations. For instance, System.out, System.in, and System.err are fields that represent the standard input, output, and error output streams of the system, respectively.
Additionally, the System class offers methods such as System.currentTimeMillis() for fetching the current time in milliseconds since the Unix epoch, and System.getenv() to retrieve environment variables. It's a powerful tool for interacting with the system on which the application is running, and that is why its functionalities are readily accessible without imports.

The String class is perhaps one of the most used classes in Java. It represents a sequence of characters and provides many methods for string manipulation. Because of its frequency of use, the String class is part of the java.lang package and is automatically available in any Java program.

Features of the String Class

Strings are immutable in Java, which means once a string object is created, its content cannot be altered. This characteristic leads to a performance boost, particularly in multi-threaded applications, because strings can be shared without the need for synchronization.
To perform operations on strings, the String class provides a variety of methods. For example, length(), charAt(), substring(), indexOf(), and toLowerCase() are frequently used in programs for basic string processing tasks such as getting the length of a string, extracting parts of a string, finding characters or substrings, and converting to lowercase, to name a few.