[Chapter 10] StringBuffer

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| Chapter 10 The java.lang Package |

# StringBuffer

## Name

StringBuffer

## Synopsis

Class Name:

  `java.lang.StringBuffer`

Superclass:

  `java.lang.Object`

Immediate Subclasses:

  None

Interfaces Implemented:

  `java.io.Serializable`

Availability:

  JDK 1.0 or later

## Description

The `StringBuffer` class represents a variable-length sequence of characters. `StringBuffer` objects are used in computations that involve creating new `String` objects. The `StringBuffer` class provides a number of utility methods for working with `StringBuffer` objects, including `append()` and `insert()` methods that add characters to a `StringBuffer` and methods that fetch the contents of `StringBuffer` objects.

When a `StringBuffer` object is created, the constructor determines the initial contents and capacity of the `StringBuffer`. The capacity of a `StringBuffer` is the number of characters that its internal data structure can hold. This is distinct from the length of the contents of a `StringBuffer`, which is the number of characters that are actually stored in the `StringBuffer` object. The capacity of a `StringBuffer` can vary. When a `StringBuffer` object is asked to hold more characters than its current capacity allows, the `StringBuffer` enlarges its internal data structure. However, it is more costly in terms of execution time and memory when a `StringBuffer` has to repeatedly increase its capacity than when a `StringBuffer` object is created with sufficient capacity.

Because the intended use of `StringBuffer` objects involves modifying their contents, all methods of the `StringBuffer` class that modify `StringBuffer` objects are `synchronized`. This means that is it safe for multiple threads to try to modify a `StringBuffer` object at the same time.

`StringBuffer` objects are used implicitly by the string concatenation operator. Consider the following code:

    String s, s1, s2;
    s = s1 + s2;

To compute the string concatenation, the Java compiler generates code like:

    s = new StringBuffer().append(s1).append(s2).toString();

## Class Summary

    public class java.lang.StringBuffer extends java.lang.Object {
        // Constructors
        public StringBuffer();
        public StringBuffer(int length);
        public StringBuffer(String str);
        // Instance Methods
        public StringBuffer append(boolean b);
        public synchronized StringBuffer append(char c);
        public synchronized StringBuffer append(char[] str);
        public synchronized StringBuffer append(char[] str, int offset, int len);
        public StringBuffer append(double d);
        public StringBuffer append(float f);
        public StringBuffer append(int i);
        public StringBuffer append(long l);
        public synchronized StringBuffer append(Object obj);
        public synchronized StringBuffer append(String str);
        public int capacity();
        public synchronized char charAt(int index);
        public synchronized void ensureCapacity(int minimumCapacity);
        public synchronized void getChars(int srcBegin, int srcEnd,
                                 char[] dst, int dstBegin);
        public StringBuffer insert(int offset, boolean b);
        public synchronized StringBuffer insert(int offset, char c);
        public synchronized StringBuffer insert(int offset, char[] str);
        public StringBuffer insert(int offset, double d);
        public StringBuffer insert(int offset, float f);
        public StringBuffer insert(int offset, int i);
        public StringBuffer insert(int offset, long l);
        public synchronized StringBuffer insert(int offset, Object obj);
        public synchronized StringBuffer insert(int offset, String str);
        public int length();
        public synchronized StringBuffer reverse();
        public synchronized void setCharAt(int index, char ch);
        public synchronized void setLength(int newLength);
        public String toString();
    }

## Constructors

### StringBuffer

#### `public StringBuffer()`

Description

  Creates a `StringBuffer` object that does not contain any characters and has a capacity of 16 characters.

#### `public StringBuffer(int capacity)`

Parameters

  `capacity`

    The initial capacity of this `StringBufffer` object.

Throws

  `NegativeArraySizeException`

    If `capacity` is negative.

Description

  Creates a `StringBuffer` object that does not contain any characters and has the specified capacity.

#### `public StringBuffer(String str)`

Parameters

  `str`

    A `String` object.

Description

  Creates a `StringBuffer` object that contains the same sequence of characters as the given `String` object and has a capacity 16 greater than the length of the `String`.

## Instance Methods

### append

#### `public StringBuffer append(boolean b)`

Parameters

  `b`

    A `boolean` value.

Returns

  This `StringBuffer` object.

Description

  This method appends either `"true"` or `"false"` to the end of the sequence of characters stored in ths `StringBuffer` object, depending on the value of `b`.

#### `public synchronized StringBuffer append(char c)`

Parameters

  `c`

    A `char` value.

Returns

  This `StringBuffer` object.

Description

  This method appends the given character to the end of the sequence of characters stored in this `StringBuffer` object.

#### `public synchronized StringBuffer append(char str[])`

Parameters

  `str`

    An array of `char` values.

Returns

  This `StringBuffer` object.

Description

  This method appends the characters in the given array to the end of the sequence of characters stored in this `StringBuffer` object.

####

     public synchronized StringBuffer append(char str[], int offset, int len)

Parameters

  `str`

    An array of `char` values.

  `offset`

    An offset into the array.

  `len`

    The number of characters from the array to be appended.

Returns

  This `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `offset` or `len` are out of range.

Description

  This method appends the specified portion of the given array to the end of the character sequence stored in this `StringBuffer` object. The portion of the array that is appended starts `offset` elements from the beginning of the array and is `len` elements long.

#### `public StringBuffer append(double d)`

Parameters

  `d`

    A `double` value.

Returns

  This `StringBuffer` object.

Description

  This method converts the given `double` value to a string using `Double.toString(d)` and appends the resulting string to the end of the sequence of characters stored in this `StringBuffer` object.

#### `public StringBuffer append(float f)`

Parameters

  `f`

    A `float` value.

Returns

  This `StringBuffer` object.

Description

  This method converts the given `float` value to a string using `Float.toString(f)` and appends the resulting string to the end of the sequence of characters stored in this `StringBuffer` object.

#### `public StringBuffer append(int i)`

Parameters

  `i`

    An `int` value.

Returns

  This `StringBuffer` object.

Description

  This method converts the given `int` value to a string using `Integer.toString(i)` and appends the resulting string to the end of the sequence of characters stored in this `StringBuffer` object.

#### `public StringBuffer append(long l)`

Parameters

  `l`

    A `long` value.

Returns

  This `StringBuffer` object.

Description

  This method converts the given `long` value to a string using `Long.toString(l)` and appends the resulting string to the end of the sequence of characters stored in this `StringBuffer` object.

#### `public synchronized StringBuffer append(Object obj)`

Parameters

  `obj`

    A reference to an object.

Returns

  This `StringBuffer` object.

Description

  This method gets the string representation of the given object by calling `String.valueOf(obj)` and appends the resulting string to the end of the character sequence stored in this `StringBuffer` object.

#### `public synchronized StringBuffer append(String str)`

Parameters

  `str`

    A `String` object.

Returns

  This `StringBuffer` object.

Description

  This method appends the sequence of characters represented by the given `String` to the characters in this `StringBuffer` object. If `str` is `null`, the string `"null"` is appended.

### capacity

#### `public int capacity()`

Returns

  The capacity of this `StringBuffer` object.

Description

  This method returns the current capacity of this object. The capacity of a `StringBuffer` object is the number of characters that its internal data structure can hold. A `StringBuffer` object automatically increases its capacity when it is asked to hold more characters than its current capacity allows.

### charAt

#### `public synchronized char charAt(int index)`

Parameters

  `index`

    An index into the `StringBuffer`.

Returns

  The character stored at the specified position in this `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `index` is less than zero or greater than or equal to the length of the `StringBuffer` object.

Description

  This method returns the character at the specified position in the `StringBuffer` object. The first character in the `StringBuffer` is at index `0`.

### ensureCapacity

#### `public synchronized void ensureCapacity(int minimumCapacity)`

Parameters

  `minimumCapacity`

    The minimum desired capacity.

Description

  This method ensures that the capacity of this `StringBuffer` object is at least the specified number of characters. If necessary, the capacity of this object is increased to the greater of `minimumCapacity` or double its current capacity plus two.

  It is more efficient to ensure that the capacity of a `StringBuffer` object is sufficient to hold all of the additions that will be made to its contents, rather than let the `StringBuffer` increase its capacity in multiple increments.

### getChars

####

     public synchronized void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin)

Parameters

  `srcBegin`

    The index of the first character to be copied.

  `srcEnd`

    The index after the last character to be copied.

  `dst`

    The destination `char` array.

  `dstBegin`

    An offset into the destination array.

Throws

  `StringIndexOutOfBoundsException`

    If `srcBegin`, `srcEnd`, or `dstBegin` is out of range.

Description

  This method copies each character in the specified range of this `StringBuffer` object to the given array of `char` values. More specifically, the first character to be copied is at index `srcBegin`; the last character to be copied is at index `srcEnd-1`.

  These characters are copied into `dst`, starting at index `dstBegin` and ending at index:

      dstBegin + (srcEnd-srcBegin) - 1

### insert

#### public StringBuffer insert(int offset, boolean b)

Parameters

  `offset`

    An offset into the `StringBuffer`.

  `b`

    A `boolean` value.

Returns

  This `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `offset` is out of range.

Description

  This method inserts either `"true"` or `"false"` into the sequence of characters stored in this `StringBuffer` object, depending on the value of `b`. The string is inserted at a position `offset` characters from the beginning of the sequence. If `offset` is `0`, the string is inserted before the first character in the `StringBuffer`.

#### `public synchronized StringBuffer insert(int offset, char c)`

Parameters

  `offset`

    An offset into the `StringBuffer`.

  `c`

    A `char` value.

Returns

  This `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `offset` is less than zero or greater than or equal to the length of the `StringBuffer` object.

Description

  This method inserts the given character into the sequence of characters stored in this `StringBuffer` object. The character is inserted at a position `offset` characters from the beginning of the sequence. If `offset` is `0`, the character is inserted before the first character in the `StringBuffer`.

#### public synchronized StringBuffer insert(int offset, char str[])

Parameters

  `offset`

    An offset into the `StringBuffer`.

  `str`

    An array of `char` values.

Returns

  This `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `offset` is less than zero or greater than or equal to the length of the `StringBuffer` object.

Description

  This method inserts the characters in the given array into the sequence of characters stored in this `StringBuffer` object. The characters are inserted at a position `offset` characters from the beginning of the sequence. If `offset` is `0`, the characters are inserted before the first character in the `StringBuffer`.

#### `public StringBuffer insert(int offset, double d)`

Parameters

  `offset`

    An offset into the `StringBuffer`.

  `d`

    A `double` value.

Returns

  This `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `offset` is less than zero or greater than or equal to the length of the `StringBuffer` object.

Description

  This method converts the given `double` value to a string using `Double.toString(d)` and inserts the resulting string into the sequence of characters stored in this `StringBuffer` object. The string is inserted at a position `offset` characters from the beginning of the sequence. If `offset` is `0`, the string is inserted before the first character in the `StringBuffer`.

#### `public StringBuffer insert(int offset, float f)`

Parameters

  `offset`

    An offset into the `StringBuffer`.

  `f`

    A `float` value.

Returns

  This `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `offset` is less than zero or greater than or equal to the length of the `StringBuffer` object.

Description

  This method converts the given `float` value to a string using `Float.toString(f)` and inserts the resulting string into the sequence of characters stored in this `StringBuffer` object. The string is inserted at a position `offset` characters from the beginning of the sequence. If `offset` is `0`, the string is inserted before the first character in the `StringBuffer`.

#### `public StringBuffer insert(int offset, int i)`

Parameters

  `offset`

    An offset into the `StringBuffer`.

  `i`

    An `int` value.

Returns

  This `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `offset` is less than zero or greater than or equal to the length of the `StringBuffer` object.

Description

  This method converts the given `int` value to a string using `Integer.toString(i)` and inserts the resulting string into the sequence of characters stored in this `StringBuffer` object. The string is inserted at a position `offset` characters from the beginning of the sequence. If `offset` is `0`, the string is inserted before the first character in the `StringBuffer`.

#### `public StringBuffer insert(int offset, long l)`

Parameters

  `offset`

    An offset into the `StringBuffer`.

  `l`

    A `long` value.

Returns

  This `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `offset` is less than zero or greater than or equal to the length of the `StringBuffer` object.

Description

  This method converts the given `long` value to a string using `Long.toString(l)` and inserts the resulting string into the sequence of characters stored in this `StringBuffer` object. The string is inserted at a position `offset` characters from the beginning of the sequence. If `offset` is `0`, the string is inserted before the first character in the `StringBuffer`.

#### public synchronized StringBuffer insert(int offset, Object obj)

Parameters

  `offset`

    An offset into the `StringBuffer`.

  `obj`

    A reference to an object.

Returns

  This `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `offset` is less than zero or greater than or equal to the length of the `StringBuffer` object.

Description

  This method gets the string representation of the given object by calling `String.valueOf(obj)` and inserts the resulting string into the sequence of characters stored in this `StringBuffer` object. The string is inserted at a position `offset` characters from the beginning of the sequence. If `offset` is `0`, the string is inserted before the first character in the `StringBuffer`.

#### public synchronized StringBuffer insert(int offset, String str)

Parameters

  `offset`

    An offset into the `StringBuffer`.

  `str`

    A `String` object.

Returns

  This `StringBuffer` object.

Throws

  `StringIndexOutOfBoundsException`

    If `offset` is less than zero or greater than or equal to the length of the `StringBuffer` object.

Description

  This method inserts the sequence of characters represented by the given `String` into the sequence of characters stored in this `StringBuffer` object. If `str` is `null`, the string `"null"` is inserted. The string is inserted at a position `offset` characters from the beginning of the sequence. If `offset` is `0`, the string is inserted before the first character in the `StringBuffer`.

### length

#### `public int length()`

Returns

  The number of characters stored in this `StringBuffer` object.

Description

  This method returns the number of characters stored in this `StringBuffer` object. The length is distinct from the capacity of a `StringBuffer`, which is the number of characters that its internal data structure can hold.

### reverse

#### `public synchronized StringBuffer reverse()`

Returns

  This `StringBuffer` object.

Description

  This method reverses the sequence of characters stored in this `StringBuffer` object.

### setCharAt

#### `public synchronized void setCharAt(int index, char ch)`

Parameters

  `index`

    The index of the character to be set.

  `ch`

    A `char` value.

Throws

  `StringIndexOutOfBoundsException`

    If `index` is less than zero or greater than or equal to the length of the `StringBuffer` object.

Description

  This method modifies the character located `index` characters from the beginning of the sequence of characters stored in this `StringBuffer` object. The current character at this position is replaced by the character `ch`.

### setLength

#### `public synchronized void setLength(int newLength)`

Parameters

  `newLength`

    The new length for this `StringBuffer`.

Throws

  `StringIndexOutOfBoundsException`

    If `index` is less than zero.

Description

  This method sets the length of the sequence of characters stored in this `StringBuffer` object. If the length is set to be less than the current length, characters are lost from the end of the character sequence. If the length is set to be more than the current length, NUL characters (`\u0000`) are added to the end of the character sequence.

### toString

#### `public String toString()`

Returns

  A new `String` object that represents the same sequence of characters as the sequence of characters stored in this `StringBuffer` object.

Overrides

  `Object.toString()`

Description

  This method returns a new `String` object that represents the same sequence of characters as the sequence of characters stored in this `StringBuffer` object. Note that any subsequent changes to the contents of this `StringBuffer` object do not affect the contents of the `String` object created by this method.

## Inherited Methods

Method | Inherited From | Method | Inherited From
--- | --- | --- | ---
`clone()` | `Object` | `equals(Object)` | `Object`
`finalize()` | `Object` | `getClass()` | `Object`
`hashCode()` | `Object` | `notify()` | `Object`
`notifyAll()` | `Object` | `wait()` | `Object`
`wait(long)` | `Object` | `wait(long, int)` | `Object`

## See Also

Character; Double; Exceptions; Float; Integer; Long; Object; String; String Concatenation Operator +

String |  | System