What is the capacity of a StringBuffer?

问题:

When I run this code:

StringBuffer name = new StringBuffer("stackoverflow.com");
System.out.println("Length: " + name.length() + ", capacity: " + name.capacity());

it gives output:

Length: 17, capacity: 33

Obvious length is related to number of characters in string, but I am not sure what capacity is? Is that number of characters that StringBuffer can hold before reallocating space?

回答1:

See: JavaSE 6 java.lang.StringBuffer capacity()

But your assumption is correct:

The capacity is the amount of storage available for newly inserted characters, beyond which an allocation will occur

回答2:

Yes, you're correct, see the JavaDoc for more information:

As long as the length of the character sequence contained in the string buffer does not exceed the capacity, it is not necessary to allocate a new internal buffer array. If the internal buffer overflows, it is automatically made larger.

回答3:

Internally StringBuffer uses a char array in order to store characters. Capacity is the initial size of that char array.

More INFO can be found from http://download.oracle.com/javase/6/docs/api/java/lang/StringBuffer.html

回答4:

From http://download.oracle.com/javase/6/docs/api/java/lang/StringBuffer.html#capacity%28%29

public int capacity()

Returns the current capacity. The capacity is the amount of storage available for newly inserted characters, beyond which an allocation will occur.

Also from the same document

As of release JDK 5, this class has been supplemented with an equivalent class designed for use by a single thread, StringBuilder. The StringBuilder class should generally be used in preference to this one, as it supports all of the same operations but it is faster, as it performs no synchronization.

回答5:

Yes, it's exactly that. You can think of StringBuffer as being a bit like a Vector<char> in that respect (except obviously you can't use char as a type argument in Java...)

回答6:

Every string buffer has a capacity. As long as the length of the character sequence contained in the string buffer does not exceed the capacity, it is not necessary to allocate a new internal buffer array. If the internal buffer overflows, it is automatically made larger.

From: http://download.oracle.com/javase/1.4.2/docs/api/java/lang/StringBuffer.html

回答7:

StringBuffer has a char[] in which it keeps the strings that you append to it. The amount of memory currently allocated to that buffer is the capacity. The amount currently used is the length.

回答8:

It's the size of internal buffer. As Javadoc says:

Every string buffer has a capacity. As long as the length of the character sequence contained in the string buffer does not exceed the capacity, it is not necessary to allocate a new internal buffer array. If the internal buffer overflows, it is automatically made larger.

回答9:

Taken from the official J2SE documentation

The capacity is the amount of storage available for newly inserted characters, beyond which an allocation will occur.

Its generally length+16, which is the minimum allocation, but once the number of character ie its size exceed the allocated one, StringBuffer also increases its size (by fixed amount), but by how much amount will be assigned,we can't calculate it.

回答10:

"Every string buffer has a capacity. As long as the length of the character sequence contained in the string buffer does not exceed the capacity, it is not necessary to allocate a new internal buffer array. If the internal buffer overflows, it is automatically made larger."

http://download.oracle.com/javase/1.3/docs/api/java/lang/StringBuffer.html -see capacity() and ensurecapacity()

回答11:

Capacity is amount of storage available for newly inserted characters.It is different from length().The length() returns the total number of characters and capacity returns value 16 by default if the number of characters are less than 16.But if the number of characters are more than 16 capacity is number of characters + 16. In this case,no of characters=17 SO,Capacity=17+16=33

回答12:

Ivan, just read the documentation for capacity() - it directly answers your question...

回答13:

The initial capacity of StringBuffer/StringBuilder class is 16. For the first time if the length of your String becomes >16. The capacity of StringBuffer/StringBuilder class increases to 34 i.e [(16*2)+2]

But when the length of your String becomes >34 the capacity of StringBuffer/StringBuilder class becomes exactly equal to the current length of String.