Question

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

Short answer

Classes System and String are part of the `java.lang` package, which is imported by default in all Java programs, eliminating the need for explicit import statements for these classes.

Step-by-step solution

Understanding the Java Default Package

In Java, the `java.lang` package is imported by default. This means that any class that is part of the `java.lang` package does not need to be explicitly imported by the programmer. Since the classes System and String are part of this package, they can be used without an explicit import statement.

Exploring the 'java.lang' Package

Examine the classes contained in the `java.lang` package. This package includes classes that are fundamental to the design of the Java programming language. The creators of Java decided that these classes are so commonly used that requiring an import for them would be unnecessary and burdensome for programmers.

System and String Classes

Identify the roles of the System and String classes. The System class provides facilities for standard input and output, among other utilities, while the String class represents and manipulates character strings. Their frequent use in programs justifies their inclusion in `java.lang`.

Key concepts

java.lang package

When learning Java, one of the first things to note is the convenience provided by the java.lang package. This package, known as the default package, is automatically imported into every Java program you write. It contains a set of classes that are essential to the Java language, acting as the basic building blocks for all Java applications. For instance, it includes primitive wrapper classes like Integer and Double, as well as Math for mathematical operations, and Throwable which is the superclass of all error and exception classes.

Because these classes are so fundamental, Java designers opted to include them by default to simplify the code-writing process. This decision eliminates the repetitive task of importing them manually with each new program you write. It's akin to having an essential tools kit always at hand, freeing you to focus on crafting the unique parts of your application.

System class in Java

In Java, the System class is a key part of the java.lang package and serves as a gateway for interacting with the underlying system. One of its most recognizable features is providing access to the standard input, standard output, and standard error streams through its static fields System.in, System.out, and System.err, respectively. These streams are commonly used for reading input from the console and writing output to the console.

Additionally, the System class hosts an array of useful methods, such as System.currentTimeMillis() which returns the current time in milliseconds since the Unix epoch, and System.getenv() which can retrieve environment variables. The usefulness and frequency of operations like these are why System is included in the auto-imported java.lang package, ensuring that developers have instant access to these essential functions without added import statements in every program.

String class in Java

The String class is another central component of the java.lang package and represents a sequence of characters, commonly known as a string. In Java, strings are immutable, which means that once a String object is created, its content cannot be changed. Instead, any operation that seems to modify a String actually creates a new String object containing the modification.

For example, concatenating two strings together does not alter the original strings but instead produces a new String that combines the content of both. The immutability of strings is beneficial for several reasons: it's simple for the Java Virtual Machine (JVM) to optimize the memory usage since identical strings can be represented by the same String object in memory, and it makes strings thread-safe, as concurrent threads cannot cause corrupt updates to string objects.

Given that strings are a fundamental data type used in virtually every Java application, from console output to network communication, having the String class readily available without the need for explicit import statements streamlines the development process significantly.