Controlling String Builder Capacity

The following methods are exclusive to the StringBuilder class and can be used to control various capacity-related aspects of a string builder:

int capacity()

Returns the current capacity of the string builder, meaning the number of characters the current builder can accommodate without allocating a new, larger array to hold characters.

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void ensureCapacity(int minCapacity)

Ensures that there is room for at least a minCapacity number of characters. It expands the string builder, depending on the current capacity of the builder.

void trimToSize()

Attempts to reduce the storage used for the character sequence. It may affect the capacity of the string builder.

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void setLength(int newLength)

Ensures that the actual number of characters—that is, the length of the string builder—is exactly equal to the value of the newLength argument, which must be greater than or equal to 0. This operation can result in the string being truncated or padded with null characters (‘\u0000’). This method affects the capacity of the string builder only if the value of the parameter newLength is greater than the current capacity.

Example 8.5 illustrates the various capacity-related methods of the StringBuilder class. It is instructive to go through the output to see how these methods affect the length and the capacity of a string builder.

Example 8.5 Controlling String Builder Capacity

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public class StringBuilderCapacity {
public static void main(String[] args) {
    StringBuilder builder = new StringBuilder(“No strings attached!”);
    System.out.println(“Builder contents: ” + builder);
    System.out.println(“Builder length:   ” + builder.length());
    System.out.println(“Builder capacity: ” + builder.capacity());
    System.out.println(“Ensure capacity of 40”);
    builder.ensureCapacity(40);
    System.out.println(“Builder capacity: ” + builder.capacity());
    System.out.println(“Trim to size”);
    builder.trimToSize();
    System.out.println(“Builder length:   ” + builder.length());
    System.out.println(“Builder capacity: ” + builder.capacity());
    System.out.println(“Set length to 10”);
    builder.setLength(10);
    System.out.println(“Builder length:   ” + builder.length());
    System.out.println(“Builder contents: ” + builder);
  System.out.println(“Set length to 0”);
    builder.setLength(0);
    System.out.println(“Builder is empty: ” + (builder.length() == 0));
}
}

Probable output from the program:

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Builder contents: No strings attached!
Builder length:   20
Builder capacity: 36
Ensure capacity of 40
Builder capacity: 74
Trim to size
Builder length:   20
Builder capacity: 20
Set length to 10
Builder length:   10
Builder contents: No strings
Set length to 0
Builder is empty: true