Java.lang classes in a software package

The Java.lang package is a central part of the Java language that provides the underlying classes in Java.
Java.lang.Object, this is the root class of Java.lang, and also the superclass of all Java classes. When you construct an instance of a Java class, the default constructor method in object is called first.

Class
Java.lang.class<t> is also a very important class. Instances of class classes are used to represent classes and interfaces in a running Java application. The class object is automatically constructed by the DefineClass method in the class loader called by the JVM when it is loaded.
Java.lang.ClassLoader is responsible for loading classes.
The Java.lang.CompilerCompiler class can be used as a placeholder for the compiler, and it does nothing to support Java-to-native code compilers and related services.

Numbers and characters
Java.lang contains a set of encapsulation classes for simple types. Includes Boolean, Character, Byte, short, Integer, Long, Float, Double, String. These classes provide support for the operation of numeric types and strings.
Java.lang.Math and Java.lang.StrictMath provide basic numerical operations such as exponential, logarithmic, square root, and trigonometric functions. In general, math calls Strictmath's method to complete the implementation (it's worth noting that there is also a Java.math package in Java that mainly provides classes for execution (BigInteger) and (BigDecimal). ）
Java.lang.StringBuffer and Java.lang.StringBuilder are variable sequences of characters, unlike StringBuffer, which is thread-safe, and StringBuilder is StringBuffer A simple substitution that can be used when a string buffer is used by a single thread, usually faster than StringBuffer.

Processes and Threads
Java.lang.Process is a process in a program.
The Java.lang.ProcessBuilder class is used to create operating system processes.
Java.lang.Thread is the execution thread in the program.
Java.lang.ThreadGroup (thread group) represents a collection of threads that form a tree structure that can contain other thread groups, except for the thread group of the root node, each with a parent thread group.
Java.lang.threadlocal<t> provides thread-local variables.
The Java.lang.inheritablethreadlocal<t> class is an extended class of threadlocal that provides a value that the thread inherits from the parent thread.
Run-time
Each instance of Java.lang.Runtime is a run-time object associated with a Java application that associates the application with the environment it is running on.
The Runtimepermission class is used for run-time permissions.
Character.
An instance of the Java.lang.Character.Subset class represents a specific subset of the Unicode character set.
The Java.lang.Character.UnicodeBlock class represents a series of character subsets in the Unicode specification for Word converts sequential blocks.
The Java.lang.enum<e> class is the public base class for all Java enumeration types.
The Java.lang.System class contains useful class fields and methods that provide default I/O flows, environment variables, automatic garbage collection, system time, and System properties. This class cannot be instantiated.
Java.lang.Throwable is a superclass of all errors or exceptions in Java.
An instance of the Java.lang.Package class contains version information about the implementation and specification of the Java package.
Java.lang.SecurityManager is a class that allows applications to implement security policies.
Java.lang.StackTraceElement is an element in a stack trace, and each instance of it represents a separate stack frame (representing a method call).
The Java.lang.Void class holds a reference to a class object that represents the Java keyword void and cannot be instantiated.

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Java.lang classes in a software package