The java.lang package contains the classes that are most central to the Java language. Object is the ultimate superclass of all Java classes and is therefore at the top of all class hierarchies. Class is a class that describes a Java class. There is one Class object for each class that is loaded into Java.

Boolean, Character, Byte, Short, Integer, Long, Float, and Double are immutable class wrappers around each of the primitive Java data types. These classes are useful when you need to manipulate primitive types as objects. They also contain useful conversion and utility methods. Void is a related class that defines a representation for the void method return type, but that defines no methods. String and StringBuffer are objects that represent strings. String is an immutable type, while StringBuffer can have its string changed in place. In Java 5.0, StringBuilder is like StringBuffer but without synchronized methods, which makes it the preferred choice in most applications. String, StringBuffer and StringBuilder implement the Java 1.4 interface CharSequence which allows instances of these classes to be manipulated through a simple shared API.

String and the various primitive type wrapper classes all implement the Comparable interface which defines an ordering for instances of those classes and enables sorting and searching algorithms (such as those of java.util.Arrays and java.util.Collections, for example). Cloneable is an important marker interface that specifies that the Object.clone( ) method is allowed to make copies of an object.

The Math class (and, in Java 1.3, the StrictMath class) defines static methods for various floating-point mathematical functions.

The THRead class provides support for multiple threads of control running within the same Java interpreter. The Runnable interface is implemented by objects that have a run( ) method that can serve as the body of a thread.

System provides low-level system methods. Runtime provides similar low-level methods, including an exec( ) method that, along with the Process class, defines a platform-dependent API for running external processes. Java 5.0 allows Process objects to be created more easily with the ProcessBuilder class.

Throwable is the root class of the exception and error hierarchy. THRowable objects are used with the Java tHRow and catch statements. java.lang defines quite a few subclasses of Throwable. Exception and Error are the superclasses of all exceptions and errors. RuntimeException defines a special class or "unchecked exceptions" that do not need to be declared in a method's throws clause. The Throwable class was overhauled in Java 1.4, adding the ability to "chain" exceptions, and the ability to obtain the stack trace of an exception as an array of StackTraceElement objects.

Java 5.0 adds three important interfaces to this package. Iterable marks types that have an iterator( ) method and enables iteration with the for/in looping statement introduced in Java 5.0. The Appendable interface is implemented by classes (such as StringBuilder and character output streams) that can have characters appended to them. Implementing this interface enables formatted text output with a java.util.Formatter . The Readable interface is implemented by classes (such as character input streams) that can sequentially copy characters into a buffer. It enables interaction with a java.util.Scanner.

Also new in Java 5.0 is Enum, which serves as the superclass of all enumerated types declared with the new enum keyword. Deprecated , Override, and SuppressWarnings are annotation types that provide metadata for the compiler.

Interfaces

public interface Appendable;
public interface CharSequence;
public interface Cloneable;
public interface Comparable<T>;
public interface Iterable<T>;
public interface Readable;
public interface Runnable;
public interface Thread.UncaughtExceptionHandler;

Enumerated Types

public enum Thread.State;

Annotation Types

public @interface Deprecated;
public @interface Override;
public @interface SuppressWarnings;

Classes

public class Object;
   abstract class AbstractStringBuilder implements Appendable, CharSequence;
      public final class StringBuffer extends AbstractStringBuilder implements 
      CharSequence, Serializable;
      public final class StringBuilder extends AbstractStringBuilder implements 
      CharSequence, Serializable;
   public final class Boolean implements Serializable, Comparable<Boolean>;
   public final class Character implements Serializable, Comparable<Character>;
   public static class Character.Subset;
      public static final class Character.UnicodeBlock extends Character.Subset;
   public final class Class<T> implements Serializable, java.lang.reflect.
      GenericDeclaration, java.lang.reflect.Type, java.lang.reflect.AnnotatedElement;
   public abstract class ClassLoader;
   public final class Compiler;
   public abstract class Enum<E extends Enum<E>> implements Comparable<E>, 
   Serializable;
   public final class Math;
   public abstract class Number implements Serializable;
      public final class Byte extends Number implements Comparable<Byte>;
      public final class Double extends Number implements Comparable<Double>;
      public final class Float extends Number implements Comparable<Float>;
      public final class Integer extends Number implements Comparable<Integer>;
      public final class Long extends Number implements Comparable<Long>;
      public final class Short extends Number implements Comparable<Short>;
   public class Package implements java.lang.reflect.AnnotatedElement;
   public abstract class Process;
   public final class ProcessBuilder;
   public class Runtime;
   public class SecurityManager;
   public final class StackTraceElement implements Serializable;
   public final class StrictMath;
   public final class String implements Serializable, Comparable<String>, CharSequence;
   public final class System;
   public class Thread implements Runnable;
   public class ThreadGroup implements Thread.UncaughtExceptionHandler;
   public class ThreadLocal<T>;
      public class InheritableThreadLocal<T> extends ThreadLocal<T>;
   public class Throwable implements Serializable;
   public final class Void;
public final class RuntimePermission extends java.security.BasicPermission;

Exceptions

public class Exception extends Throwable;
   public class ClassNotFoundException extends Exception;
   public class CloneNotSupportedException extends Exception;
   public class IllegalAccessException extends Exception;
   public class InstantiationException extends Exception;
   public class InterruptedException extends Exception;
   public class NoSuchFieldException extends Exception;
   public class NoSuchMethodException extends Exception;
   public class RuntimeException extends Exception;
      public class ArithmeticException extends RuntimeException;
      public class ArrayStoreException extends RuntimeException;
      public class ClassCastException extends RuntimeException;
      public class EnumConstantNotPresentException extends RuntimeException;
      public class IllegalArgumentException extends RuntimeException;
         public class IllegalThreadStateException extends IllegalArgumentException;
         public class NumberFormatException extends IllegalArgumentException;
      public class IllegalMonitorStateException extends RuntimeException;
      public class IllegalStateException extends RuntimeException;
      public class IndexOutOfBoundsException extends RuntimeException;
         public class ArrayIndexOutOfBoundsException extends IndexOutOfBoundsException;
         public class StringIndexOutOfBoundsException extends IndexOutOfBoundsException;
      public class NegativeArraySizeException extends RuntimeException;
      public class NullPointerException extends RuntimeException;
      public class SecurityException extends RuntimeException;
      public class TypeNotPresentException extends RuntimeException;
      public class UnsupportedOperationException extends RuntimeException;

Errors

public class Error extends Throwable;
   public class AssertionError extends Error;
   public class LinkageError extends Error;
      public class ClassCircularityError extends LinkageError;
      public class ClassFormatError extends LinkageError;
         public class UnsupportedClassVersionError extends ClassFormatError;
      public class ExceptionInInitializerError extends LinkageError;
      public class IncompatibleClassChangeError extends LinkageError;
         public class AbstractMethodError extends IncompatibleClassChangeError;
         public class IllegalAccessError extends IncompatibleClassChangeError;
         public class InstantiationError extends IncompatibleClassChangeError;
         public class NoSuchFieldError extends IncompatibleClassChangeError;
         public class NoSuchMethodError extends IncompatibleClassChangeError;
      public class NoClassDefFoundError extends LinkageError;
      public class UnsatisfiedLinkError extends LinkageError;
      public class VerifyError extends LinkageError;
   public class ThreadDeath extends Error;
   public abstract class VirtualMachineError extends Error;
      public class InternalError extends VirtualMachineError;
      public class OutOfMemoryError extends VirtualMachineError;
      public class StackOverflowError extends VirtualMachineError;
      public class UnknownError extends VirtualMachineError;