StringBuffer Class in the java.lang Package

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StringBuffer Constructors

Constructor Description
StringBuffer() Default constructor. Creates an object that can initially store 16 characters.
StringBuffer(int capacity) Creates an object that can store the number of characters specified by capacity.
StringBuffer(String str) Creates an object that can store the string specified by str plus 16 additional characters.

Main StringBuffer Methods

Method Description
StringBuffer append(boolean b) Adds b to the end of the current string.
StringBuffer append(char c) Adds c to the end of the current string.
StringBuffer append(double d) Adds d to the end of the current string.
StringBuffer append(float f) Adds f to the end of the current string.
StringBuffer append(int i) Adds i to the end of the current string.
StringBuffer append(long l) Adds l to the end of the current string.
StringBuffer append(Object obj) Adds obj to the end of the current string.
StringBuffer append(String str) Adds str to the end of the current string.
int capacity() Returns the current string buffer capacity.
char charAt(int index) Returns the character at index.
StringBuffer delete(int start, int end) Deletes the range from start to end.
StringBuffer deleteCharAt(int index) Deletes the character at index.
StringBuffer insert(int offset, boolean b) Inserts b before offset.
StringBuffer insert(int offset, char c) Inserts c before offset.
StringBuffer insert(int offset, int i) Inserts i before offset.
StringBuffer insert(int offset, long l) Inserts l before offset.
StringBuffer insert(int offset, Object obj) Inserts obj before offset.
StringBuffer insert(int offset, String str) Inserts str before offset.
int length() Returns the number of characters in the character buffer.
StringBuffer replace(int start, int end, String str) Replaces the range from start to end with the string str.
StringBuffer reverse() Returns the string in reverse order.
void setCharAt(int index, char ch) Sets the character at index to ch.
void setLength(int newLength) Sets the buffer size to newLength.
String toString() Returns the current string as a String object.

String Buffer Size

  • int capacity()
  • int length()
  • void setLength(int newLength)

The capacity method is used to get the total capacity allocated to a StringBuffer object.
The length method is used to get the current length of a StringBuffer object.

Example

package com.devkuma.tutorial.stringbuffer;

public class StringBufferCapacity {

    public static void main(String[] args) {

        StringBuffer sb = new StringBuffer("abcdef");

        System.out.println("capacity : " + sb.capacity());
        System.out.println("length : " + sb.length());

        sb.setLength(2);
        System.out.println("setLength : " + sb);
    }
}

Execution result:

capacity : 22
length : 6
setLength : ab

In this result, the length method displays 6, the length of “abcdef”, while the capacity method displays 22 because it automatically adds space for 16 more characters to the 6-character length of “abcdef”. The setLength method changes the buffer size to 2, so “ab” is displayed.

Characters in a String Buffer

Characters in a StringBuffer object can be read and replaced.

  • char charAt(int index)
  • void setCharAt(int index, char ch)
package com.devkuma.tutorial.stringbuffer;

public class StringBufferChar {

    public static void main(String[] args) {

        StringBuffer sb = new StringBuffer("abcde");
        System.out.println("charAt(3) : " + sb.charAt(3));

        sb.setCharAt(4, 'f');
        System.out.println("setCharAt(4, 'f') : " + sb);
    }
}

Execution result:

charAt(3) : d
setCharAt(4, 'f') : abcdf

First, charAt displays the value ’d’ at index 3. Second, setCharAt changes the value at index 4 to ‘f’, so the final result abcdf is displayed.

Appending Data to a String Buffer

To convert data of another type to a string and append it to the end of a StringBuffer object, use the append method.

  • StringBuffer append(boolean b)
  • StringBuffer append(char c)
  • StringBuffer append(double d)
  • StringBuffer append(float f)
  • StringBuffer append(int i)
  • StringBuffer append(long l)
  • StringBuffer append(Object obj)
  • StringBuffer append(String str)

The following example uses append.

package com.devkuma.tutorial.stringbuffer;

public class StringBufferAppend {

    public static void main(String[] args) {

        boolean b = true;
        char c = 'a';
        double d = 1.1;
        float f = 3.14f;
        int i = 1;
        long l = 2;
        String str = "A";

        StringBuffer sb = new StringBuffer();
        sb.append(b).append(", ");
        sb.append(c).append(", ");
        sb.append(d).append(", ");
        sb.append(f).append(", ");
        sb.append(i).append(", ");
        sb.append(l).append(", ");
        sb.append(str);

        System.out.println(sb);
    }
}

Execution result:

true, a, 1.1, 3.14, 1, 2, A

The append method has a similar purpose to concat on a String object. Unlike String, however, the changed string is stored in the called object instead of being assigned to a new object. When many operations append another string to a given string, using a StringBuffer object greatly helps performance.

The following example creates a String object and a StringBuffer object, then appends a string 10000 times to each. Let’s measure the speed of each object. The measurement uses System.currentTimeMillis(), a method that gets the current time.

package com.devkuma.tutorial.stringbuffer;

public class StringBufferAppend2 {

    public static void main(String[] args) {

        String str = "A";
        long currentStr = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            str += "A";
        }
        System.out.println("String :" + (System.currentTimeMillis() - currentStr) + "ms");

        StringBuffer sb = new StringBuffer("A");
        long currentSb = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            sb.append("A");
        }
        System.out.println("StringBuffer :" + (System.currentTimeMillis() - currentSb) + "ms");

    }
}

Execution result:

String :68ms
StringBuffer :1ms

From the result, you can see that the StringBuffer object is much faster than the String object.

Inserting Data into a String Buffer

To insert data of another type as a string at a specified position in a StringBuffer object, use the insert method.

  • StringBuffer insert(int offset, boolean b)
  • StringBuffer insert(int offset, char c)
  • StringBuffer insert(int offset, int i)
  • StringBuffer insert(int offset, long l)
  • StringBuffer insert(int offset, Object obj)
  • StringBuffer insert(int offset, String str)
package com.devkuma.tutorial.stringbuffer;

public class StringBufferInsert {

    public static void main(String[] args) {

        boolean b = true;
        char c = 'a';
        double d = 1.1;
        float f = 3.14f;
        int i = 1;
        long l = 2;
        String str = "A";

        StringBuffer sb = new StringBuffer("b=, c=, d=, f=, i=, l=, str=");
        sb.insert(2, b);
        sb.insert(10, c);
        sb.insert(15, d);
        sb.insert(22, f);
        sb.insert(30, i);
        sb.insert(35, l);
        sb.insert(42, str);

        System.out.println(sb);
    }
}

Execution result:

b=true, c=a, d=1.1, f=3.14, i=1, l=2, str=A

Reversing a String Buffer

Returns the string in a StringBuffer object in reverse order.

  • StringBuffer reverse()
package com.devkuma.tutorial.stringbuffer;

public class StringBufferReverse {

    public static void main(String[] args) {
        StringBuffer sb = new StringBuffer("abcdef");
        System.out.println(sb.reverse());
    }
}

Execution result:

fedcba

Deleting from a String Buffer

The delete method of a StringBuffer object deletes the specified range.
The deleteCharAt method of a StringBuffer object deletes the character at the entered index.

  • StringBuffer delete(int start, int end)
  • StringBuffer deleteCharAt(int index)
package com.devkuma.tutorial.stringbuffer;

public class StringBufferDelete {

    public static void main(String[] args) {

        StringBuffer sb = new StringBuffer("abcxxxdef");
        sb.delete(4, 6);
        System.out.println(sb);
        sb.deleteCharAt(3);
        System.out.println(sb);
    }
}

The execution result is as follows.

abcxdef
abcdef

Replacing in a String Buffer

In the string of a StringBuffer object, replaces the specified range with the input string.

  • StringBuffer replace(int start, int end, String str)
package com.devkuma.tutorial.stringbuffer;

public class StringReplace {

    public static void main(String[] args) {

        StringBuffer sb = new StringBuffer("abcxxghi");
        sb.replace(3, 5, "def");
        System.out.println(sb);
    }
}

The execution result is as follows.

abcdefghi