Java
Java Strings
StringBuilder Class In Java

Java StringBuilder Class

Java StringBuilder class is used to create mutable (modifiable) String. The Java StringBuilder class is same as StringBuffer class except that it is non-synchronized. It is available since JDK 1.5.

Important Constructors of StringBuilder class

ConstructorDescription
StringBuilder()It creates an empty String Builder with the initial capacity of 16.
StringBuilder(String str)It creates a String Builder with the specified string.
StringBuilder(int length)It creates an empty String Builder with the specified capacity as length.

Important Methods of StringBuilder Class

MethodDescription
public StringBuilder append(String s)Appends the specified string with this string. The append() method is overloaded like append(char), append(boolean), append(int), append(float), append(double), etc.
public StringBuilder insert(int offset, String s)Inserts the specified string with this string at the specified position. The insert() method is overloaded like insert(int, char), insert(int, boolean), insert(int, int), insert(int, float), insert(int, double), etc.
public StringBuilder replace(int startIndex, int endIndex, String str)Replaces the string from specified startIndex to endIndex.
public StringBuilder delete(int startIndex, int endIndex)Deletes the string from specified startIndex to endIndex.
public StringBuilder reverse()Reverses the string.
public int capacity()Returns the current capacity.
public void ensureCapacity(int minimumCapacity)Ensures the capacity is at least equal to the given minimum.
public char charAt(int index)Returns the character at the specified position.
public int length()Returns the length of the string i.e. total number of characters.
public String substring(int beginIndex)Returns the substring from the specified beginIndex.
public String substring(int beginIndex, int endIndex)Returns the substring from the specified beginIndex to endIndex.

Java StringBuilder Examples

1 StringBuilder append() method

The append() method concatenates the given argument with this String.

class StringBuilderExample {  
    public static void main(String args[]) {  
        StringBuilder sb = new StringBuilder("Hello ");  
        sb.append("Java"); // now original string is changed  
        System.out.println(sb); // prints Hello Java  
    }  
}

Output:

Hello Java

2 StringBuilder insert() method

The insert() method inserts the given string with this string at the given position.

class StringBuilderExample2 {  
    public static void main(String args[]) {  
        StringBuilder sb = new StringBuilder("Hello ");  
        sb.insert(1, "Java"); // now original string is changed  
        System.out.println(sb); // prints HJavaello  
    }  
}

Output:

HJavaello

3 StringBuilder replace() method

The replace() method replaces the given string from the specified beginIndex to endIndex.

class StringBuilderExample3 {  
    public static void main(String args[]) {  
        StringBuilder sb = new StringBuilder("Hello");  
        sb.replace(1, 3, "Java");  
        System.out.println(sb); // prints HJavalo  
    }  
}

Output:

HJavalo

4 StringBuilder delete() method

The delete() method deletes the string from the specified beginIndex to endIndex.

class StringBuilderExample4 {  
    public static void main(String args[]) {  
        StringBuilder sb = new StringBuilder("Hello");  
        sb.delete(1, 3);  
        System.out.println(sb); // prints Hlo  
    }  
}

Output:

Hlo

5 StringBuilder reverse() method

The reverse() method reverses the current string.

class StringBuilderExample5 {  
    public static void main(String args[]) {  
        StringBuilder sb = new StringBuilder("Hello");  
        sb.reverse();  
        System.out.println(sb); // prints olleH  
    }  
}

Output:

olleH

6 StringBuilder capacity() method

The capacity() method returns the current capacity of the Builder. The default capacity is 16. If the number of characters increases from its current capacity, it increases the capacity by (oldCapacity * 2) + 2.

class StringBuilderExample6 {  
    public static void main(String args[]) {  
        StringBuilder sb = new StringBuilder();  
        System.out.println(sb.capacity()); // default 16  
        sb.append("Hello");  
        System.out.println(sb.capacity()); // now 16  
        sb.append("Java is my favourite language");  
        System.out.println(sb.capacity()); // now (16*2)+2=34  
    }  
}

Output:

16
16
34

7 StringBuilder ensureCapacity() method

The ensureCapacity() method ensures the given capacity is the minimum current capacity. If greater, it increases capacity by (oldCapacity * 2) + 2.

class StringBuilderExample7 {  
    public static void main(String args[]) {  
        StringBuilder sb = new StringBuilder();  
        System.out.println(sb.capacity()); // default 16  
        sb.append("Hello");  
        System.out.println(sb.capacity()); // now 16  
        sb.append("Java is my favourite language");  
        System.out.println(sb.capacity()); // now (16*2)+2=34  
        sb.ensureCapacity(10); // now no change  
        System.out.println(sb.capacity()); // now 34  
        sb.ensureCapacity(50); // now (34*2)+2  
        System.out.println(sb.capacity()); // now 70  
    }  
}

Output:

16
16
34
34
70
StringBuffer Class In JavaString vs StringBuffer