The java.lang Package

Contents:
Byte
Character
Class
ClassLoader
Cloneable
Compiler
Double
Float
Integer
Long
Math
Number
Object
Process
Runnable
Runtime
SecurityManager
Short
String
StringBuffer
System
Thread
ThreadGroup
Throwable
Void

The package java.lang contains classes and interfaces that are essential to the Java language. These include:

• Object, the ultimate superclass of all classes in Java
• Thread, the class that controls each thread in a multithreaded program
• Throwable, the superclass of all error and exception classes in Java
• Classes that encapsulate the primitive data types in Java
• Classes for accessing system resources and other low-level entities
• Math, a class that provides standard mathematical methods
• String, the class that is used to represent strings

Because the classes in the java.lang package are so essential, the java.lang package is implicitly imported by every Java source file. In other words, you can refer to all of the classes and interfaces in java.lang using their simple names.

Figure 10.1 shows the class hierarchy for the java.lang package.

Figure 10.1: The java.lang package

Boolean

Name

Synopsis

• Class Name:
• java.lang.Boolean
• Superclass:
• java.lang.Object
• Immediate Subclasses:
• None
• Interfaces Implemented:
• java.io.Serializable
• Availability:
• JDK 1.0 or later

Description

The Boolean class provides an object wrapper for a boolean value. This is useful when you need to treat a boolean value as an object. For example, there are a number of utility methods that take a reference to an Object as one of their arguments. You cannot specify a boolean value for one of these arguments, but you can provide a reference to a Boolean object that encapsulates the boolean value. Furthermore, as of JDK 1.1, the Boolean class is necessary to support the Reflection API and class literals.

Class Summary

public final class java.lang.Boolean {
 // Constants public final static Boolean FALSE;
public final static Boolean TRUE;
public final static Class TYPE; // New in 1.1 // Constructors public Boolean(boolean value);
public Boolean(String s); // Class Methods public static boolean getBoolean(String name);
public static Boolean valueOf(String s); // Instance Methods public boolean booleanValue();
public boolean equals(Object obj);
public int hashCode();
public String toString();
}

Constants

TRUE

public static final Boolean TRUE

• Description
• A constant Boolean object that has the value true.

FALSE

public static final Boolean FALSE

• Description
• A constant Boolean object that has the value false.

TYPE

public static final Class TYPE

• Availability
• New as of JDK 1.1
• Description
• The Class object that represents the type boolean. It is always true that Boolean.TYPE == boolean.class.

Constructors

Boolean

public Boolean(boolean value)

• Parameters
• • value
  • The boolean value to be made into a Boolean object.
• Description
• Constructs a Boolean object with the given value.

public Boolean(String s)

• Parameters
• • s
  • The string to be made into a Boolean object.
• Description
• Constructs a Boolean object with the value specified by the given string. If the string equals 'true' (ignoring case), the value of the object is true; otherwise it is false.

Class Methods

getBoolean

public static boolean getBoolean(String name)

• Parameters
• • name
  • The name of a system property.
• Returns
• The boolean value of the system property.
• Description
• This methods retrieves the boolean value of a named system property.

valueOf

public static Boolean valueOf(String s)

• Parameters
• • s
  • The string to be made into a Boolean object.
• Returns
• A Boolean object with the value specified by the given string.
• Description
• This method returns a Boolean object with the value true if the string equals "true" (ignoring case); otherwise the value of the object is false.

Instance Methods

booleanValue

public boolean booleanValue()

• Returns
• The boolean value contained by the object.

equals

public boolean equals(Object obj)

• Parameters
• • obj
  • The object to be compared with this object.
• Returns
• true if the objects are equal; false if they are not.
• Overrides
• Object.equals()
• Description
• This method returns true if obj is an instance of Boolean, and it contains the same value as the object this method is associated with.

hashCode

public int hashCode()

• Returns
• A hashcode based on the boolean value of the object.
• Overrides
• Object.hashCode()

toString

public String toString()

• Returns
• "true" if the value of the object is true; "false" otherwise.
• Overrides
• Object.toString()
• Description
• This method returns a string representation of the Boolean object.

Inherited Methods

See Also

Boolean Type; Boolean literals; Class; Object; System