String empty constructor in java [closed]

It creates the empty string, which appears to have some limited use.

If you'll be building up a String by concatenating, and aren't using e.g. StringBuiler, your code can begin as one of the following.

String result = new String();
String result = "";
String result = "first part of string";

// ...
result += "append to the result";

The first two aren't equivalent, and you should prefer to initialize with "" since this can take advantage of string interning.

Small example... String can be garbage collected

System.out.println(1 + new String() + 2);

instead of

System.out.println(1 + "" + 2);

According to the documentation, this constructor creates an empty sequence.

public String()

Initializes a newly created String object so that it represents an empty character sequence. Note that use of this constructor is unnecessary since Strings are immutable.

If you want an empty sequence, it makes sense.

But normally, it wouldn't be necessary to use the empty constructor before you make changes to it, since you are not changing the String. In fact, when you change using the operator += for example, you are creating another immutable String, and not changing one.

Check this question about this subject: How do String objects work (like immutable objects)?

Some background first

Because Strings in Java are immutable, they are also "interned" - that means that all the string literals in the loaded classes are kept in a pool, so there is usually only one instance of each unique string literal in memory at one time. It is an application of the flyweight pattern, similar pools are also kept for Integer and other primitive wrapper objects (but only for a limited number of small values).

Because of this mechanism, identity comparison of string literals (even from different classes) is usually possible (although you should always use equals method when comparing strings for safety and consistency):

System.out.println("hello" == "hello"); // true

Now, if you use the default string constructor, you get an instance of an empty string, but it is a new instance, as stated in JavaDoc:

Initializes a newly created String object so that it represents an empty character sequence. Note that use of this constructor is unnecessary since Strings are immutable.

Such new instance is different from the interned empty string, resulting in:

System.out.println(new String() == ""); // false

But as I said, only string literals are automatically interned - that means strings created manually by StringBuilders, from char arrays etc. are not interned. You can use the String.intern() method to put such a string into the pool manually.

Now for some real scenario

Well all this is nice indeed, but I still haven't answered why this constructor exists. Well, Java strings are just smart wrappers over char arrays and some distinct string objects can share their internal arrays.

If I create a very long string (by reading from a stream for example), then this instance isn't interned (as said above), so it will be garbage collected after the variable that referenced it gets out of scope. But if do this:

String longString = readVeryLongString();
String shortString = longString.subString(0, 10);

... then the new shortString will not copy first 10 characters from the longString and put them into its own new char array. No, it will reference the original array, using only first 10 chars from it.

Now, if the shortString variable has longer life (for example is put into some static context), then the underlying char array will not be garbage collected (even if the original longString variable already got out of scope). This is one of the ways how to create a memory leak in Java.

Now, the default string constructor comes to the rescue! If I change the code above to this:

String longString = readVeryLongString();
String shortString = new String(longString.subString(0, 10));

... then the shortString will be a new string instance that made a new internal char array by copying only the 10 required chars from the original string returned by the subString method.

A nice article illustrating this subject:

http://illya-keeplearning.blogspot.cz/2009/03/java-string-internals.html

to create an empty string,call default constructor as String s new String();

will create an instance of String with no characters in it.