In Java, when should I prefer String over StringBuilder vice versa?

Question

I know StringBuilder should be preferred over String because String will be kept in Constant String Pool and assigning a new value to it does not override the previous values. However, StringBuilder is an object that overrides its previous value.

• In what cases should I use String over StringBuilder vice versa.

• Also If I have variables of type String in backing classes of Hibernate should I use StringBuilder? How?

  ...
  Result result = (Result) criteria.list().get(0);
  
  
  class Result{
     String name;
     String fname;
     ...
  }
  

Answer 1

You should use String, because String objects are cached in an object pool and might deliver better performance when you don't change them.

A StringBuilder is only useful when you keep on concatenating String tokens, which shouldn't be the case in a well normalized database table.

The JVM does all sorts of optimizations and chances are that even if you use concatenation the JVM might rewrite that routine to a StringBuilder version.

Answer 2

I'd use String for text values that are fixed.

Use StringBuilder when you are creating a larger text String like:

final StringBuilder sb = new StringBuilder();
for(;;){
    sb.append("more text\n");//add text, StringBuilder's size will increase incrementally
    if(this.stop()){
         break;//stop loop
    }
}
final String result = sb.toString();// Create the final result to work with
System.out.println(result);//print result

Use StringBuffer instead of StringBuilder for synchronized values, see https://stackoverflow.com/a/355092/928952 for difference between StringBuilder and StringBuffer.

JavaDoc: StringBuffer (http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuffer.html):

A thread-safe, mutable sequence of characters. A string buffer is like a String, but can be modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls

JavaDoc: StringBuilder (http://docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html):

A mutable sequence of characters. This class provides an API compatible with StringBuffer, but with no guarantee of synchronization. This class is designed for use as a drop-in replacement for StringBuffer in places where the string buffer was being used by a single thread (as is generally the case). Where possible, it is recommended that this class be used in preference to StringBuffer as it will be faster under most implementations.

JavaDoc: String (http://docs.oracle.com/javase/7/docs/api/java/lang/String.html):

The String class represents character strings. All string literals in Java programs, such as "abc", are implemented as instances of this class. Strings are constant; their values cannot be changed after they are created. String buffers support mutable strings. Because String objects are immutable they can be shared

Basically you'll use String for constants (immutable) of text.

Answer 3

String is immutable object,once created can not be changed. The object created as a String is stored in the Constant String Pool. Every immutable object in Java is thread safe ,that implies String is also thread safe. String can not be used by two threads simultaneously.String once assigned can not be changed.

String  demo = "Test1" ;

The above object is stored in constant string pool and its value can not be modified.

demo="Test2" ;

new "Test2" string is created in constant pool and referenced by the demo variable

StringBuilder objects are mutable, we can make changes to the value stored in the object. What this effectively means is that string operations such as append would be more efficient if performed using StringBuilder objects than String objects.

StringBuilder demo2= new StringBuilder("Test1");

The above object too is stored in the heap and its value can be modified

demo2=new StringBuilder("Test1"); 

Above statement is right as it modifies the value which is allowed in the StringBuilder.

So you should be using stringBuilder where you required to update/modify the string frequently.

Answer 4

The java.lang.StringBuilder classes should be used when you have to make a lot of modifications to strings of characters.As we know String objects are immutable, so if you choose to do a lot of manipulations with String objects, you will end up with a lot of abandoned String objects in the String pool. (Even in these days of gigabytes of RAM, it's not a good idea to waste precious memory on discarded String pool objects.) On the other hand, objects of type StringBuilder can be modified over and over again without leaving behind a great effluence of discarded String objects.

String x = "abc";
x.concat("def");
System.out.println("x = " + x); // output is "x = abc"


StringBuffer sb = new StringBuffer("abc");
sb.append("def");
System.out.println("sb = " + sb); // output is "sb = abcdef"

Answer 5

A simple rule of thumb (String is a type that represents character strings. StringBuilder a stream of mutable characters)

Use String to represent text values. By definition Java provides pooling of string values and thus providing you some space optimization. Think of this in a scenario where your application is dealing with millions of text values during a file batch processing. So as an example.

  String str1 = "Test";
  String str2 = "Test";

Here, str1 == str2 ( same reference)

Also, + operator is overloaded in String to construct String from different types. This can be used when constructing small Strings ( internally it gets done using StringBuilder so no worries) - but not while looping.

Use StringBuilder(or old counterpart StringBuffer) only when you are constructing a target String using small pieces of different types - and especially inside a loop - this will help you to avoid placing unnecessary string fragments in the string pool.

   StringBuilder s1 = new StringBuilder("test");
   StringBuilder s2 = new StringBuilder("test");

Here, s1 != s2

Also, I do not think there is someway you can manipulate the encoding of StringBuilder/Buffer - Meanwhile String allows this.

Edit: Representing Hibernate entities : Always use String to represent a text type in your class. For reasons stated above.This should come to you like muscle memory. For example, int, float, char etc for primitive types and String for text type. Use the builder only to build strings and not to represent a type unless that is some strange requirement.