I got a doubt while trying with the string class.
I came to know that string is immutable in java and even if we are performing any string operation to the current string, it will create a new string object. For example, if I apply trim() to the current string object then it will create new object. So my question is then what happens to the old string object. Whether it is removed from the heap replacing with the new object or in the other case it will keep on creating string garbage if I am performing a lot of string functions. Please clarify me?
You seem to have a misconception. When you invoke a string operation, what happens is that you get a new string with the results of that operation. Nothing happens to the old string.
For example
String strWithSpaces = " Dog ";
String strWithoutSpaces = strWithSpaces.trim();
Now you have two separate strings. One that has spaces before and after the word "Dog", and one that doesn't.
You may ask what happens if I do
String strWithSpaces = " Dog ";
strWithSpaces.trim();
Well, besically, that creates a new string without the spaces before and after "Dog", but that new string is not assigned to anything, so it is immediately eligible for garbage collection, and the old string, with the spaces, stays the same as it was.
And if I do
String strWithSpaces = " Dog ";
strWithSpaces = strWithSpaces.trim();
Again, there are two strings. But now, the new string is assigned to the same variable as the old one. So you have a reference to the new, and you can use it, but you don't have a direct reference to the old. If it wasn't created from a literal string, it would be garbage-collected.
Whether it is removed from the heap replacing with the new object or in the other case it will keep on creating string garbage if I am performing a lot of string functions.
No object is ever "replacing" another object on the heap. They all get created in a new location. Anyone who happens to still reference the "old" object can continue to use it.
So, yes, you will be producing "string garbage". But you have the garbage collector to take care of that: It removes unused objects in the background (reclaiming their memory to be re-used by future objects). If you happen to do a lot of string operations in a tight loop, consider using a StringBuilder or a char[] instead. Those exist precisely to avoid the overhead of creating lots of intermediate objects.
String De-duplication:
Strings consume a lot of memory in any application.Whenever the garbage collector visits String objects it takes note of the char arrays. It takes their hash value and stores it alongside with a weak reference to the array. As soon as it finds another String which has the same hash code it compares them char by char.If they match as well, one String will be modified and point to the char array of the second String. The first char array then is no longer referenced anymore and can be garbage collected.
String Pool:
All strings used by the java program are stored here. If two variables are initialized to the same string value. Two strings are not created in the memory, there will be only one copy stored in memory and both will point to the same memory location.
So java already takes care of not creating duplicate strings in the heap by checking if the string exists in the string pool. Then what is the purpose of string de-duplication?
If there is a code as follows
String myString_1 = new String("Hello World");
String myString_2 = new String("Hello World");
two strings are created in memory even though they are same. I cannot think of any scenario other than this where string de-duplication is useful. Obviously I must be missing something. What I am I missing?
Thanks In Advance
The string pool applies only to strings added to it explicitly, or used as constants in the application. It does not apply to strings created dynamically during the lifetime of the application. String deduplication, however, applies to all strings.
Compile time vs run time
String pool refers to string constants that are known at compile time.
String deduplication would help you if you happen to retrieve (or construct) the same string a million times at run time, e.g. reading it from a file, a HTTP request or any other way.
String de-duplication enjoys the extra level of indirection built into String:
With a string pool, you are limited to returning the same object for two identical strings
String de-duplication lets you have multiple distinct String objects sharing the same content.
This translates into removing a limitation of de-duplicating on creation: your application could keep creating new String objects with identical content while using very little extra memory, because the content of the strings would be shared. This process can be done on a completely unrelated schedule - for example, in the background, while your application does not need much of the CPU resources. Since the identity of the String object does not change, de-duplication can be completely hidden form your application.
Just to add to the answers above, on older VM's the string pool is not garbage collected (this has changed now, but don't rely on that). It contains strings which are used as constants in the application, and so will always be needed. If you continually put all your strings in the string pool, you might quickly run out of memory. On top of that, de-duplication is a relatively expensive process, if you know you only need the string for a very short period of time, and you have enough memory.
For these reasons, strings are not put in the string pool automatically. You have to do it explicitly by calling string.intern().
From documentation :
"Initializes a newly created 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 original is needed, use of this constructor is
unnecessary since Strings are immutable."
So my sense says, this constructor in String class is not needed normally like you have used above. I guess that constructor is provided merely for the sake of completeness or if you do not want to share that copy (kind of unnecessary now, refer here what I am talking about) but still other constructors are useful like getting an String object from char array and so on..
I cannot think of any scenario other than this where string de-duplication is useful.
Well one other (much more) frequent scenario is the use of StringBuilders. In the toString() method of the StringBuilder class, it clearly creates a new instance in memory:
public final class StringBuilder extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
...
#Override
public String toString() {
// Create a copy, don't share the array
return new String(value, 0, count);
}
...
}
Same thing for its thread-safe version StringBuffer:
public final class StringBuffer extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
...
#Override
public synchronized String toString() {
if (toStringCache == null) {
toStringCache = Arrays.copyOfRange(value, 0, count);
}
return new String(toStringCache, true);
}
...
}
In applications that rely heavily on this, string de-duplication may reduce memory usage.
In java String can be created in 2 ways given below
String foo="Test";
String fooobj=new String("Test");
Everywhere it is mentioned about difference between these 2 ways of creating String. I want to know more about What are appropriate scenario's ,
where we should go for
String foo="Test";
And when to go for
String fooobj=new String("Test"); ?
The short answer: If you're in any doubt, you don't want new String("literal here"). If you need it, you'll know you need it, and why.
The long answer:
Essentially the only time you want to use new String("literal here") is if you want to ensure that the resulting string object is not == any other string object. Literals get interned automatically; strings created via new String("literal here") are not.
So why would you want that? The answer is you almost never would, because String instances are immutable, so you don't care if you're sharing a String instance with something else. Just about the only scenario I can imagine is if you had an API that accepted a String and you wanted to have a flag value other than null, and you wanted to check that flag/marker value via ==, like this:
public static final String MARKER = new String("Marker");
public void someFictionalMethod(String arg) {
if (arg == MARKER) {
// Take action on the marker
}
else {
// Take action on the string
}
}
...and even then I'd tend to find it a bit suspect and would explore other ways to do it.
It is never necessary to create a new String object with
String fooobj = new String("Test");
So, you should never do this. Just write String fooobj = "Test"; instead.
Class String is immutable. That means that the content of a String object cannot be changed after it has been constructed. It's never necessary to create an explicit copy of a string literal.
You should never use new String(). Anytime you create string in this way, new object in memory is created. If you write String s = "aaa", then there is a chance such object was already created in a given JVM, it is stored in string pool and thanks to that your variable will be just reference to that existing object - you safe memory in this way.
When you use the second approach, you actually rely on the first approach to initialize the constructor arguments.
So the question is if you could create the String object via first approach, why you spend extra effort to use the second approach ?
I don't see any scenary to use the second constructor style, so always insist in using the first approach.
There is a subtle differences between String object and string literal.
String s = "abc"; // creates one String object and one reference variable
In this simple case, "abc" will go in the pool and s will refer to it.
String s = new String("abc"); // creates two objects,and one reference variable
In this case, because we used the new keyword, Java will create a new String object in normal (non-pool) memory, and s will refer to it. In addition, the literal "abc" will be placed in the pool.
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. For example:
String str = "abc";
is equivalent to:
String str = new String("abc");
You should avoid the second way to declare a string, for reasons explained into other answers
What exactly does immutable mean - that is, what are the consequences of an object being mutable or immutable? In particular, why are Java's Strings immutable?
My understanding is that the StringBuilder type is something like a mutable equivalent to String. When would I use StringBuilder rather than String, and vice-versa?
Immutable means that once the constructor for an object has completed execution that instance can't be altered.
This is useful as it means you can pass references to the object around, without worrying that someone else is going to change its contents. Especially when dealing with concurrency, there are no locking issues with objects that never change
e.g.
class Foo
{
private final String myvar;
public Foo(final String initialValue)
{
this.myvar = initialValue;
}
public String getValue()
{
return this.myvar;
}
}
Foo doesn't have to worry that the caller to getValue() might change the text in the string.
If you imagine a similar class to Foo, but with a StringBuilder rather than a String as a member, you can see that a caller to getValue() would be able to alter the StringBuilder attribute of a Foo instance.
Also beware of the different kinds of immutability you might find: Eric Lippert wrote a blog article about this. Basically you can have objects whose interface is immutable but behind the scenes actual mutables private state (and therefore can't be shared safely between threads).
An immutable object is an object where the internal fields (or at least, all the internal fields that affect its external behavior) cannot be changed.
There are a lot of advantages to immutable strings:
Performance: Take the following operation:
String substring = fullstring.substring(x,y);
The underlying C for the substring() method is probably something like this:
// Assume string is stored like this:
struct String { char* characters; unsigned int length; };
// Passing pointers because Java is pass-by-reference
struct String* substring(struct String* in, unsigned int begin, unsigned int end)
{
struct String* out = malloc(sizeof(struct String));
out->characters = in->characters + begin;
out->length = end - begin;
return out;
}
Note that none of the characters have to be copied! If the String object were mutable (the characters could change later) then you would have to copy all the characters, otherwise changes to characters in the substring would be reflected in the other string later.
Concurrency: If the internal structure of an immutable object is valid, it will always be valid. There's no chance that different threads can create an invalid state within that object. Hence, immutable objects are Thread Safe.
Garbage collection: It's much easier for the garbage collector to make logical decisions about immutable objects.
However, there are also downsides to immutability:
Performance: Wait, I thought you said performance was an upside of immutability! Well, it is sometimes, but not always. Take the following code:
foo = foo.substring(0,4) + "a" + foo.substring(5); // foo is a String
bar.replace(4,5,"a"); // bar is a StringBuilder
The two lines both replace the fourth character with the letter "a". Not only is the second piece of code more readable, it's faster. Look at how you would have to do the underlying code for foo. The substrings are easy, but now because there's already a character at space five and something else might be referencing foo, you can't just change it; you have to copy the whole string (of course some of this functionality is abstracted into functions in the real underlying C, but the point here is to show the code that gets executed all in one place).
struct String* concatenate(struct String* first, struct String* second)
{
struct String* new = malloc(sizeof(struct String));
new->length = first->length + second->length;
new->characters = malloc(new->length);
int i;
for(i = 0; i < first->length; i++)
new->characters[i] = first->characters[i];
for(; i - first->length < second->length; i++)
new->characters[i] = second->characters[i - first->length];
return new;
}
// The code that executes
struct String* astring;
char a = 'a';
astring->characters = &a;
astring->length = 1;
foo = concatenate(concatenate(slice(foo,0,4),astring),slice(foo,5,foo->length));
Note that concatenate gets called twice meaning that the entire string has to be looped through! Compare this to the C code for the bar operation:
bar->characters[4] = 'a';
The mutable string operation is obviously much faster.
In Conclusion: In most cases, you want an immutable string. But if you need to do a lot of appending and inserting into a string, you need the mutability for speed. If you want the concurrency safety and garbage collection benefits with it the key is to keep your mutable objects local to a method:
// This will have awful performance if you don't use mutable strings
String join(String[] strings, String separator)
{
StringBuilder mutable;
boolean first = true;
for(int i = 0; i < strings.length; i++)
{
if(first) first = false;
else mutable.append(separator);
mutable.append(strings[i]);
}
return mutable.toString();
}
Since the mutable object is a local reference, you don't have to worry about concurrency safety (only one thread ever touches it). And since it isn't referenced anywhere else, it is only allocated on the stack, so it is deallocated as soon as the function call is finished (you don't have to worry about garbage collection). And you get all the performance benefits of both mutability and immutability.
Actually String is not immutable if you use the wikipedia definition suggested above.
String's state does change post construction. Take a look at the hashcode() method. String caches the hashcode value in a local field but does not calculate it until the first call of hashcode(). This lazy evaluation of hashcode places String in an interesting position as an immutable object whose state changes, but it cannot be observed to have changed without using reflection.
So maybe the definition of immutable should be an object that cannot be observed to have changed.
If the state changes in an immutable object after it has been created but no-one can see it (without reflection) is the object still immutable?
Immutable objects are objects that can't be changed programmatically. They're especially good for multi-threaded environments or other environments where more than one process is able to alter (mutate) the values in an object.
Just to clarify, however, StringBuilder is actually a mutable object, not an immutable one. A regular java String is immutable (meaning that once it's been created you cannot change the underlying string without changing the object).
For example, let's say that I have a class called ColoredString that has a String value and a String color:
public class ColoredString {
private String color;
private String string;
public ColoredString(String color, String string) {
this.color = color;
this.string = string;
}
public String getColor() { return this.color; }
public String getString() { return this.string; }
public void setColor(String newColor) {
this.color = newColor;
}
}
In this example, the ColoredString is said to be mutable because you can change (mutate) one of its key properties without creating a new ColoredString class. The reason why this may be bad is, for example, let's say you have a GUI application which has multiple threads and you are using ColoredStrings to print data to the window. If you have an instance of ColoredString which was created as
new ColoredString("Blue", "This is a blue string!");
Then you would expect the string to always be "Blue". If another thread, however, got ahold of this instance and called
blueString.setColor("Red");
You would suddenly, and probably unexpectedly, now have a "Red" string when you wanted a "Blue" one. Because of this, immutable objects are almost always preferred when passing instances of objects around. When you have a case where mutable objects are really necessary, then you would typically guard the objet by only passing copies out from your specific field of control.
To recap, in Java, java.lang.String is an immutable object (it cannot be changed once it's created) and java.lang.StringBuilder is a mutable object because it can be changed without creating a new instance.
In large applications its common for string literals to occupy large bits of memory. So to efficiently handle the memory, the JVM allocates an area called "String constant pool".(Note that in memory even an unreferenced String carries around a char[], an int for its length, and another for its hashCode. For a number, by contrast, a maximum of eight immediate bytes is required)
When complier comes across a String literal it checks the pool to see if there is an identical literal already present. And if one is found, the reference to the new literal is directed to the existing String, and no new 'String literal object' is created(the existing String simply gets an additional reference).
Hence : String mutability saves memory...
But when any of the variables change value, Actually - it's only their reference that's changed, not the value in memory(hence it will not affect the other variables referencing it) as seen below....
String s1 = "Old string";
//s1 variable, refers to string in memory
reference | MEMORY |
variables | |
[s1] --------------->| "Old String" |
String s2 = s1;
//s2 refers to same string as s1
| |
[s1] --------------->| "Old String" |
[s2] ------------------------^
s1 = "New String";
//s1 deletes reference to old string and points to the newly created one
[s1] -----|--------->| "New String" |
| | |
|~~~~~~~~~X| "Old String" |
[s2] ------------------------^
The original string 'in memory' didn't change, but the
reference variable was changed so that it refers to the new string.
And if we didn't have s2, "Old String" would still be in the memory but
we'll not be able to access it...
"immutable" means you cannot change value. If you have an instance of String class, any method you call which seems to modify the value, will actually create another String.
String foo = "Hello";
foo.substring(3);
<-- foo here still has the same value "Hello"
To preserve changes you should do something like this
foo = foo.sustring(3);
Immutable vs mutable can be funny when you work with collections. Think about what will happen if you use mutable object as a key for map and then change the value (tip: think about equals and hashCode).
java.time
It might be a bit late but in order to understand what an immutable object is, consider the following example from the new Java 8 Date and Time API (java.time). As you probably know all date objects from Java 8 are immutable so in the following example
LocalDate date = LocalDate.of(2014, 3, 18);
date.plusYears(2);
System.out.println(date);
Output:
2014-03-18
This prints the same year as the initial date because the plusYears(2) returns a new object so the old date is still unchanged because it's an immutable object. Once created you cannot further modify it and the date variable still points to it.
So, that code example should capture and use the new object instantiated and returned by that call to plusYears.
LocalDate date = LocalDate.of(2014, 3, 18);
LocalDate dateAfterTwoYears = date.plusYears(2);
date.toString()… 2014-03-18
dateAfterTwoYears.toString()… 2016-03-18
I really like the explaination from SCJP Sun Certified Programmer for Java 5 Study Guide.
To make Java more memory efficient, the JVM sets aside a special area of memory called the "String constant pool." When the compiler encounters a String literal, it checks the pool to see if an identical String already exists. If a match is found, the reference to the new literal is directed to the existing String, and no new String literal object is created.
Objects which are immutable can not have their state changed after they have been created.
There are three main reasons to use immutable objects whenever you can, all of which will help to reduce the number of bugs you introduce in your code:
It is much easier to reason about how your program works when you know that an object's state cannot be changed by another method
Immutable objects are automatically thread safe (assuming they are published safely) so will never be the cause of those hard-to-pin-down multithreading bugs
Immutable objects will always have the same Hash code, so they can be used as the keys in a HashMap (or similar). If the hash code of an element in a hash table was to change, the table entry would then effectively be lost, since attempts to find it in the table would end up looking in the wrong place. This is the main reason that String objects are immutable - they are frequently used as HashMap keys.
There are also some other optimisations you might be able to make in code when you know that the state of an object is immutable - caching the calculated hash, for example - but these are optimisations and therefore not nearly so interesting.
One meaning has to do with how the value is stored in the computer, For a .Net string for example, it means that the string in memory cannot be changed, When you think you're changing it, you are in fact creating a new string in memory and pointing the existing variable (which is just a pointer to the actual collection of characters somewhere else) to the new string.
String s1="Hi";
String s2=s1;
s1="Bye";
System.out.println(s2); //Hi (if String was mutable output would be: Bye)
System.out.println(s1); //Bye
s1="Hi" : an object s1 was created with "Hi" value in it.
s2=s1 : an object s2 is created with reference to s1 object.
s1="Bye" : the previous s1 object's value doesn't change because s1 has String type and String type is an immutable type, instead compiler create a new String object with "Bye" value and s1 referenced to it. here when we print s2 value, the result will be "Hi" not "Bye" because s2 referenced to previous s1 object which had "Hi" value.
Immutable means that once the object is created, non of its members will change. String is immutable since you can not change its content.
For example:
String s1 = " abc ";
String s2 = s1.trim();
In the code above, the string s1 did not change, another object (s2) was created using s1.
Immutable simply mean unchangeable or unmodifiable. Once string object is created its data or state can't be changed
Consider bellow example,
class Testimmutablestring{
public static void main(String args[]){
String s="Future";
s.concat(" World");//concat() method appends the string at the end
System.out.println(s);//will print Future because strings are immutable objects
}
}
Let's get idea considering bellow diagram,
In this diagram, you can see new object created as "Future World". But not change "Future".Because String is immutable. s, still refer to "Future". If you need to call "Future World",
String s="Future";
s=s.concat(" World");
System.out.println(s);//print Future World
Why are string objects immutable in java?
Because Java uses the concept of string literal. Suppose there are 5 reference variables, all refers to one object "Future".If one reference variable changes the value of the object, it will be affected to all the reference variables. That is why string objects are immutable in java.
Once instanciated, cannot be altered. Consider a class that an instance of might be used as the key for a hashtable or similar. Check out Java best practices.
As the accepted answer doesn't answer all the questions. I'm forced to give an answer after 11 years and 6 months.
Can somebody clarify what is meant by immutable?
Hope you meant immutable object (because we could think about immutable reference).
An object is immutable: iff once created, they always represent the same value (doesn't have any method that change the value).
Why is a String immutable?
Respect the above definition which could be checked by looking into the Sting.java source code.
What are the advantages/disadvantages of the immutable objects?
immutable types are :
safer from bugs.
easier to understand.
and more ready for change.
Why should a mutable object such as StringBuilder be preferred over String and vice-verse?
Narrowing the question Why do we need the mutable StringBuilder in programming?
A common use for it is to concatenate a large number of strings together, like this:
String s = "";
for (int i = 0; i < n; ++i) {
s = s + n;
}
Using immutable strings, this makes a lot of temporary copies — the first number of the string ("0") is actually copied n times in the course of building up the final string, the second number is copied n-1 times, and so on. It actually costs O(n2) time just to do all that copying, even though we only concatenated n elements.
StringBuilder is designed to minimize this copying. It uses a simple but clever internal data structure to avoid doing any copying at all until the very end, when you ask for the final String with a toString() call:
StringBuilder sb = new StringBuilder();
for (int i = 0; i < n; ++i) {
sb.append(String.valueOf(n));
}
String s = sb.toString();
Getting good performance is one reason why we use mutable objects. Another is convenient sharing: two parts of your program can communicate more conveniently by sharing a common mutable data structure.
More could be found here : https://web.mit.edu/6.005/www/fa15/classes/09-immutability/#useful_immutable_types
Immutable Objects
An object is considered immutable if its state cannot change after it is constructed. Maximum reliance on immutable objects is widely accepted as a sound strategy for creating simple, reliable code.
Immutable objects are particularly useful in concurrent applications. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state.
Programmers are often reluctant to employ immutable objects, because they worry about the cost of creating a new object as opposed to updating an object in place. The impact of object creation is often overestimated, and can be offset by some of the efficiencies associated with immutable objects. These include decreased overhead due to garbage collection, and the elimination of code needed to protect mutable objects from corruption.
The following subsections take a class whose instances are mutable and derives a class with immutable instances from it. In so doing, they give general rules for this kind of conversion and demonstrate some of the advantages of immutable objects.
Source
An immutable object is the one you cannot modify after you create it. A typical example are string literals.
A D programming language, which becomes increasingly popular, has a notion of "immutability" through "invariant" keyword. Check this Dr.Dobb's article about it - http://dobbscodetalk.com/index.php?option=com_myblog&show=Invariant-Strings.html&Itemid=29 . It explains the problem perfectly.
Given: Class has no fields, every variable is local. littleString was created by refactoring bigString in Eclipse:
public String bigString()
{
StringBuffer bob = new StringBuffer();
this.littleString(bob);
return bob.toString();
}
private void littleString(final StringBuffer bob)
{
bob.append("Hello, I'm Bob");
}
The method littleString should not be passing the StringBuffer back, but yet is is. What kind of Black Magic goes on here? This is breaking all rules of encapsulation that I know. I'm in shock, words fail me.
littleString isn't passing the object back -- it's just using the same object. Both the local variable bob in bigString() and the parameter bob in littleString() refer to the same object, so if you change one of those objects, the changes will appear instantaneously in the other because they're both references to the same object.
The issue is that StringBuffers are mutable and have internal state associated with them. Some types of objects (such as Strings) are immutable, so you can safely pass them around as method parameters, and you know they won't ever get modified. Note that the addition of the final keyword doesn't help here -- it just makes sure that bob never gets assigned to refer to a different StringBuffer object.
It's not passing anything back. It's modifying the StringBuffer you passed a reference to. Objects in Java are not passed by value.
If you meant why does the string buffer get modified, it's because you were passing a reference to the string buffer, which allows you to call the public method append which modifies the string buffer object.
the answers above pretty much got it, except one little thing hasnt been mentioned: java lacks "const-ness", meaning you want an object to be unmodifiable.
"final" is close, but it still doesnt do the job properly. What the code snippet showed is the kind of error that can happen if you have mutable objects passed in as parameters to other methods. This can be fixed either by having immutable objects, or if there is some kind of new keyword added for deep const-ness.