Java's StringBuilder class
The java.lang.StringBuilder class is used to represent character strings that can be dynamically modified. This is required as String objects are immutable - they cannot have their value altered once created.
Constructing new StringBuilder objects
The StringBuilder class has the following constructors:
StringBuilder()- constructs a new string builder with no characters in it.StringBuilder(String str)- constructs a new string builder which has the contents of the specifiedString.StringBuilder(CharSequence seq)- constructs a new string builder which has the contents of the specifiedCharSequence.CharSequenceis an interface implemented by classes that provide a readable sequence ofcharvalues. Implementations ofCharSequenceincludeString,StringBuilderandStringBuffer.StringBuilder(int capacity)- constructs a new string builder with no characters in it, and an initial capacity specified by thecapacityargument. The inital capacity is the starting size of internal data structure used by theStringBuilderto represent a sequence of characters. If the required size of sequence exceeds the initial capacity the capacity is automatically increased.
StringBuilder methods
Below is a description of some of the methods provided by StringBuilder:
StringBuilder append(String str)- adds the characters in the specifiedStringto the end of the current sequence. There are overloaded versions of theappendmethod that take primitive types,ObjectandStringBuilderas their single arguments.Code snippet:Output from running code snippet:StringBuilder sb = new StringBuilder("qwertyuiop");
sb.append("qwertyuiop");
sb.append(' ');
sb.append(1);
sb.append(false);
sb.append(2.35);
System.out.println(sb);qwertyuiopqwertyuiop 1false2.35StringBuilder insert(int offset, String str)- inserts the characters in the specifiedStringat the specified index.Code snippet:Output from running code snippet:StringBuilder sb = new StringBuilder("qwertyuiop");
sb.insert(3,"AB");
System.out.println(sb);qweABrtyuiopStringBuilder replace(int start, int end, String str)- replaces the characters between the specified start and end points with the characters in the specifiedString.Code snippet:Output from running code snippet:StringBuilder sb = new StringBuilder("qwertyuiop");
sb.replace(3,6,"AB");
System.out.println(sb);qweABuiopStringBuilder delete(int start, int end)- removes the characters between the specified start and end points.Code snippet:Output from running code snippet:StringBuilder sb = new StringBuilder("qwertyuiop");
sb.delete(3,6);
System.out.println(sb);qweuiopStringBuilder reverse()- reverses the sequence of characters.Code snippet:Output from running code snippet:StringBuilder sb = new StringBuilder("qwertyuiop");
sb.reverse();
System.out.println(sb);poiuytrewq
Most of the methods of the StringBuilder class return a reference to themselves - allowing multiple operations on a StringBuilder instance to be chained together in a single statement. This technique is known as method chaining.
StringBuilder.Code snippet:
StringBuilder sb = new StringBuilder("qwerty");
sb.reverse().append("abcdef").insert(8, "Z");
System.out.println(sb);ytrewqabZcdefAdvantages of StringBuilder
When performing concatenation of lots of String objects (e.g. in a loop), a StringBuilder can provide better performance then using the + operator. A StringBuilder can be more efficient as it creates less objects.
Code snippet:
long start = System.nanoTime();
String s = "";
for (int i=0; i<5000; i++) {
s += i;
}
long duration = System.nanoTime()-start;
System.out.println(duration + " nanoseconds for String version");
start = System.nanoTime();
StringBuilder sb = new StringBuilder();
for (int i=0; i<5000; i++) {
sb.append(i);
}
duration = System.nanoTime()-start;
System.out.println(duration + " nanoseconds for StringBuilder version");
start = System.nanoTime();
sb = new StringBuilder(18890);
for (int i=0; i<5000; i++) {
sb.append(i);
}
duration = System.nanoTime()-start;
System.out.println(duration + " nanoseconds for StringBuilder with large initial capacity version");308243046 nanoseconds for String version
972206 nanoseconds for StringBuilder version
581890 nanoseconds for StringBuilder with large initial capacity versionThe java.lang.StringBuffer class
Before StringBuilder was introduced in Java 5 the java.lang.StringBuffer class was the standard way of working with modifiable sequences of characters. Both StringBuffer and StringBuilder extend java.lang.AbstractStringBuilder. The significant difference between the two classes is that StringBuffer is synchronised while StringBuilder is not. The lack of syncronisation makes StringBuilder faster and therefore the recommended choice for single-threaded use. If the object is to be used by multiple threades then StringBuffer is the recommended option.
Code snippet:
StringBuffer sb = new StringBuffer("qwertyuiop");
sb.append("qwertyuiop");
sb.append(' ');
sb.append(1);
sb.append(false);
sb.append(2.35);
System.out.println(sb);qwertyuiopqwertyuiop 1false2.35See Also
- StringBuilder Javadoc - http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html
- StringBuffer Javadoc - http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuffer.html