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17. The java.util Package

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The package java.util contains a number of useful classes and interfaces. Although the name of the package might imply that these are utility classes, they are really more important than that. In fact, Java depends directly on several of the classes in this package, and many programs will find these classes indispensable. The classes and interfaces in java.util include:

Figure 17.1 shows the class hierarchy for the java.util package.

Figure 17.1: The java.util package

[Graphic: Figure 17-1]

BitSet

Name

BitSet

Synopsis

Class Name:

java.util.BitSet

Superclass:

java.lang.Object

Immediate Subclasses:

None

Interfaces Implemented:

java.lang.Cloneable, java.io.Serializable

Availability:

JDK 1.0 or later

Description

The BitSet class implements a set of bits. The set grows in size as needed. Each element of a BitSet has a boolean value. When a BitSet object is created, all of the bits are set to false by default. The bits in a BitSet are indexed by nonnegative integers, starting at 0. The size of a BitSet is the number of bits that it currently contains. The BitSet class provides methods to set, clear, and retrieve the values of the individual bits in a BitSet. There are also methods to perform logical AND, OR, and XOR operations.

Class Summary

public final class java.util.BitSet extends java.lang.Object
                   implements java.lang.Cloneable, java.io.Serializable {
  // Constructors
  public BitSet();
  public BitSet(int nbits);
  // Instance Methods
  public void and(BitSet set);
  public void clear(int bit);
  public Object clone();
  public boolean equals(Object obj);
  public boolean get(int bit);
  public int hashCode();
  public void or(BitSet set);
  public void set(int bit);
  public int size();
  public String toString();
  public void xor(BitSet set);
}

Constructors

BitSet

public BitSet()

Description

This constructor creates a BitSet with a default size of 64 bits. All of the bits in the BitSet are initially set to false.

public BitSet(int nbits)

Parameters

nbits

The initial number of bits.

Description

This constructor creates a BitSet with a size of nbits. All of the bits in the BitSet are initially set to false.

Instance Methods

and

public void and(BitSet set)

Parameters

set

The BitSet to AND with this BitSet.

Description

This method computes the logical AND of this BitSet and the specified BitSet and stores the result in this BitSet. In other words, for each bit in this BitSet, the value is set to only true if the bit is already true in this BitSet and the corresponding bit in set is true.

If the size of set is greater than the size of this BitSet, the extra bits in set are ignored. If the size of set is less than the size of this BitSet, the extra bits in this BitSet are set to false.

clear

public void clear(int bit)

Parameters

bit

The index of the bit to clear.

Description

This method sets the bit at the given index to false. If bit is greater than or equal to the number of bits in the BitSet, the size of the BitSet is increased so that it contains bit values. All of the additional bits are set to false.

clone

public Object clone()

Returns

A copy of this BitSet.

Overrides

Object.clone()

Description

This method creates a copy of this BitSet and returns it. In other words, the returned BitSet has the same size as this BitSet, and it has the same bits set to true.

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 BitSet and it contains the same bit values as the object this method is associated with. In other words, this method compares each bit of this BitSet with the corresponding bit of obj. If any bits do not match, the method returns false. If the size of this BitSet is different than obj, the extra bits in either this BitSet or in obj must be false for this method to return true.

get

public boolean get(int bit)

Parameters

bit

The index of the bit to retrieve.

Returns

The boolean value of the bit at the given index.

Description

This method returns the value of the given bit. If bit is greater than or equal to the number of bits in the BitSet, the method returns false.

hashCode

public int hashCode()

Returns

The hashcode for this BitSet.

Overrides

Object.hashCode()

Description

This method returns a hashcode for this object.

or

public void or(BitSet set)

Parameters

set

The BitSet to OR with this BitSet.

Description

This method computes the logical OR of this BitSet and the specified BitSet, and stores the result in this BitSet. In other words, for each bit in this BitSet, the value is set to true if the bit is already true in this BitSet or the corresponding bit in set is true.

If the size of set is greater than the size of this BitSet, this BitSet is first increased in size to accommodate the additional bits. All of the additional bits are initially set to false.

set

public void set(int bit)

Parameters

bit

The index of the bit to set.

Description

This method sets the bit at the given index to true. If bit is greater than or equal to the number of bits in the BitSet, the size of the BitSet is increased so that it contains bit values. All of the additional bits except the last one are set to false.

size

public int size()

Returns

The size of this BitSet.

Description

This method returns the size of this BitSet, which is the number of bits currently in the set.

toString

public String toString()

Returns

A string representation of this BitSet.

Overrides

Object.toString()

Description

This method returns a string representation of this BitSet. The string lists the indexes of all the bits in the BitSet that are true.

xor

public void xor(BitSet set)

Parameters

set

The BitSet to XOR with this BitSet.

Description

This method computes the logical XOR (exclusive OR) of this BitSet and the specified BitSet and stores the result in this BitSet. In other words, for each bit in this BitSet, the value is set to true only if the bit is already true in this BitSet, and the corresponding bit in set is false, or if the bit is false in this BitSet and the corresponding bit in set is true.

If the size of set is greater than the size of this BitSet, this BitSet is first increased in size to accommodate the additional bits. All of the additional bits are initially set to false.

Inherited Methods

Method

Inherited From

Method

Inherited From

finalize()

Object

getClass()

Object

notify()

Object

notifyAll()

Object

wait()

Object

wait(long)

Object

wait(long, int)

Object

   

See Also

Cloneable, Serializable


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