What is the difference between string buffer and StringBuilder?
Why is String immutable
The String class uses the final keyword to modify the character array to save the String. private final char value [], so the String type is immutable.
variability
Both StringBuffer and StringBuffer inherit the AbstractStringBuilder class. In AbstractStringBuilder, the character group is also used to save the string char[] value, but it is not modified with the final keyword, so both objects are variable.
The construction methods of StringBuilder and StringBuffer are implemented by calling the construction method of the parent class.
Thread safety
The object in String is immutable, that is, constant, thread safe. AbstractStringBuilder is a public class of StringBuilder and StringBuffer. It defines some basic String operations, such as expandCapacity, append, insert, indexOf and other public methods. StringBuffer adds a synchronization lock to the method or adds a synchronization lock to the calling method, so the thread is safe. StringBuilder does not add synchronization locks, so threads are unsafe.
performance
Each time the String type is changed, a new String object will be generated, and then the pointer will point to the new String object. StringBuffer will operate on the StringBuffer object itself every time, instead of generating a new object and changing the reference of the object. In the same case, using StringBuilder can only improve the performance by 10% ~ 15% compared with using StringBuffer, but it takes the risk of multithreading not going to that group.
Source code analysis
StringBuffer source code (part):
public final class StringBuffer
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
private transient char[] toStringCache;
static final long serialVersionUID = 3388685877147921107L;
public StringBuffer() {
super(16);
}
public StringBuffer(int capacity) {
super(capacity);
}
public StringBuffer(String str) {
super(str.length() + 16);
append(str);
}
public StringBuffer(CharSequence seq) {
this(seq.length() + 16);
append(seq);
}
@Override
public synchronized int length() {
return count;
}
@Override
public synchronized int capacity() {
return value.length;
}
@Override
public synchronized void ensureCapacity(int minimumCapacity) {
super.ensureCapacity(minimumCapacity);
}
/**
* @since 1.5
*/
@Override
public synchronized void trimToSize() {
super.trimToSize();
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
@Override
public synchronized void setLength(int newLength) {
toStringCache = null;
super.setLength(newLength);
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
@Override
public synchronized char charAt(int index) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
return value[index];
}
/**
* @since 1.5
*/
@Override
public synchronized int codePointAt(int index) {
return super.codePointAt(index);
}
/**
* @since 1.5
*/
@Override
public synchronized int codePointBefore(int index) {
return super.codePointBefore(index);
}
/**
* @since 1.5
*/
@Override
public synchronized int codePointCount(int beginIndex, int endIndex) {
return super.codePointCount(beginIndex, endIndex);
}
/**
* @since 1.5
*/
@Override
public synchronized int offsetByCodePoints(int index, int codePointOffset) {
return super.offsetByCodePoints(index, codePointOffset);
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public synchronized void getChars(int srcBegin, int srcEnd, char[] dst,
int dstBegin)
{
super.getChars(srcBegin, srcEnd, dst, dstBegin);
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see #length()
*/
@Override
public synchronized void setCharAt(int index, char ch) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
toStringCache = null;
value[index] = ch;
}
@Override
public synchronized StringBuffer append(Object obj) {
toStringCache = null;
super.append(String.valueOf(obj));
return this;
}
@Override
public synchronized StringBuffer append(String str) {
toStringCache = null;
super.append(str);
return this;
}
This is only part of the code. From the source code of StringBuffer, we can see that the synchronized synchronization lock is used to modify the method. Therefore, StringBuffer is thread safe. We can see the function of constructor from the source code. Serialization is used to ensure the sequence of buffer area. The area size is specified. It can be used by default or customized.
StringBuilder source code (part):
public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
/** use serialVersionUID for interoperability */
static final long serialVersionUID = 4383685877147921099L;
/**
* Constructs a string builder with no characters in it and an
* initial capacity of 16 characters.
*/
public StringBuilder() {
super(16);
}
/**
* Constructs a string builder with no characters in it and an
* initial capacity specified by the {@code capacity} argument.
*
* @param capacity the initial capacity.
* @throws NegativeArraySizeException if the {@code capacity}
* argument is less than {@code 0}.
*/
public StringBuilder(int capacity) {
super(capacity);
}
/**
* Constructs a string builder initialized to the contents of the
* specified string. The initial capacity of the string builder is
* {@code 16} plus the length of the string argument.
*
* @param str the initial contents of the buffer.
*/
public StringBuilder(String str) {
super(str.length() + 16);
append(str);
}
/**
* Constructs a string builder that contains the same characters
* as the specified {@code CharSequence}. The initial capacity of
* the string builder is {@code 16} plus the length of the
* {@code CharSequence} argument.
*
* @param seq the sequence to copy.
*/
public StringBuilder(CharSequence seq) {
this(seq.length() + 16);
append(seq);
}
@Override
public StringBuilder append(Object obj) {
return append(String.valueOf(obj));
}
@Override
public StringBuilder append(String str) {
super.append(str);
return this;
}
/**
* Appends the specified {@code StringBuffer} to this sequence.
* <p>
* The characters of the {@code StringBuffer} argument are appended,
* in order, to this sequence, increasing the
* length of this sequence by the length of the argument.
* If {@code sb} is {@code null}, then the four characters
* {@code "null"} are appended to this sequence.
* <p>
* Let <i>n</i> be the length of this character sequence just prior to
* execution of the {@code append} method. Then the character at index
* <i>k</i> in the new character sequence is equal to the character at
* index <i>k</i> in the old character sequence, if <i>k</i> is less than
* <i>n</i>; otherwise, it is equal to the character at index <i>k-n</i>
* in the argument {@code sb}.
*
* @param sb the {@code StringBuffer} to append.
* @return a reference to this object.
*/
public StringBuilder append(StringBuffer sb) {
super.append(sb);
return this;
}
@Override
public StringBuilder append(CharSequence s) {
super.append(s);
return this;
}
/**
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
@Override
public StringBuilder append(CharSequence s, int start, int end) {
super.append(s, start, end);
return this;
}
@Override
public StringBuilder append(char[] str) {
super.append(str);
return this;
}
From the code, we can see that StringBuilder is almost the same as StringBuffer, but it is not decorated with the synchronized keyword, so it is determined that it is not thread safe.
String source code:
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
/** The value is used for character storage. */
private final char value[];
/** Cache the hash code for the string */
private int hash; // Default to 0
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -6849794470754667710L;
/**
* Class String is special cased within the Serialization Stream Protocol.
*
* A String instance is written into an ObjectOutputStream according to
* <a href="{@docRoot}/../platform/serialization/spec/output.html">
* Object Serialization Specification, Section 6.2, "Stream Elements"</a>
*/
private static final ObjectStreamField[] serialPersistentFields =
new ObjectStreamField[0];
/**
* Initializes a newly created {@code String} object so that it represents
* an empty character sequence. Note that use of this constructor is
* unnecessary since Strings are immutable.
*/
public String() {
this.value = "".value;
}
/**
* Initializes a newly created {@code String} object so that it represents
* the same sequence of characters as the argument; in other words, the
* newly created string is a copy of the argument string. Unless an
* explicit copy of {@code original} is needed, use of this constructor is
* unnecessary since Strings are immutable.
*
* @param original
* A {@code String}
*/
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
/**
* Allocates a new {@code String} so that it represents the sequence of
* characters currently contained in the character array argument. The
* contents of the character array are copied; subsequent modification of
* the character array does not affect the newly created string.
*
* @param value
* The initial value of the string
*/
public String(char value[]) {
this.value = Arrays.copyOf(value, value.length);
}
We can see that the array in String is decorated with the final key, which determines its immutability.
summary
- Operate a small amount of data: applicable to String
- Single thread operation operates a large amount of data under the string buffer: applicable to StringBuilder
- Multithreading operates a large amount of data in the string buffer: use StringBuffer