String - sivakrsna/Java-Interview GitHub Wiki

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.

Creating immutable objects

The following rules define a simple strategy for creating immutable objects. Not all classes documented as "immutable" follow these rules. This does not necessarily mean the creators of these classes were sloppy — they may have good reason for believing that instances of their classes never change after construction. However, such strategies require sophisticated analysis and are not for beginners.

• Don't provide "setter" methods — methods that modify fields or objects referred to by fields. Make all fields final and private.
• Don't allow subclasses to override methods. The simplest way to do this is to declare the class as final. A more sophisticated approach is to make the constructor private and construct instances in factory methods. If the instance fields include references to mutable objects, don't allow those objects to be changed:
• Don't provide methods that modify the mutable objects.
• Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.

Why is String immutable in Java?

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

1. Requirement of String Pool

String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference.

The following code will create only one string object in the heap.

String string1 = "abcd"; String string2 = "abcd"; Here is how it looks: java-string-pool

If string is not immutable, changing the string with one reference will lead to the wrong value for the other references.

2. Allow String to Cache its Hashcode

The hashcode of string is frequently used in Java. For example, in a HashMap. Being immutable guarantees that hashcode will always the same, so that it can be cashed without worrying the changes.That means, there is no need to calculate hashcode every time it is used. This is more efficient.

3. Security

String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Were String not immutable, a connection or file would be changed and lead to serious security threat. The method thought it was connecting to one machine, but was not. Mutable strings could cause security problem in Reflection too, as the parameters are strings.

Here is a code example:

boolean connect(string s){
    if (!isSecure(s)) { 
throw new SecurityException(); 
}
    //here will cause problem, if s is changed before this by using other references.    
    causeProblem(s);
}

In summary, the reasons include design, efficiency, and security.

How do I compare strings in Java?

== tests for reference equality (whether they are the same object).

.equals() tests for value equality (whether they are logically "equal").

What is String Interning ?

String Interning is a method of storing only one copy of each distinct String Value, which must be immutable.

In Java, String class has a public method intern() that returns a canonical representation for the string object. Java’s String class privately maintains a pool of strings, where String literals are automatically interned.

When the intern() method is invoked on a String object it looks the string contained by this String object in the pool, if the string is found there then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

Why and When to Intern ?

Though Java automatically interns all Strings by default, remember that we only need to intern strings when they are not constants, and we want to be able to quickly compare them to other interned strings. The intern() method should be used on strings constructed with new String() in order to compare them by == operator.

public class TestString {
    public static void main(String[] args) {
        String s1 = "Test";
        String s2 = "Test";
        String s3 = new String("Test");
        final String s4 = s3.intern();
        System.out.println(s1 == s2);
        System.out.println(s2 == s3);
        System.out.println(s3 == s4);
        System.out.println(s1 == s3);
        System.out.println(s1 == s4);
        System.out.println(s1.equals(s2));
        System.out.println(s2.equals(s3));
        System.out.println(s3.equals(s4));
        System.out.println(s1.equals(s4));
        System.out.println(s1.equals(s3));
    }
}
//Output
true
false
false
false
true
true
true
true
true
true

String.valueOf() vs. Object.toString()

According to the Java documentation, String.valueOf() returns:

if the argument is null, then a string equal to "null"; otherwise, the value of obj.toString() is returned. So there shouldn't really be a difference except for an additional method invocation.

Also, in case of Object#toString, if the instance is null, a NullPointerException will be thrown, so, arguably, it's less safe.

String, StringBuffer, and StringBuilder

Mutability Difference:

String is immutable, if you try to alter their values, another object gets created, whereas StringBuffer and StringBuilder are mutable so they can change their values.

Thread-Safety Difference:

The difference between StringBuffer and StringBuilder is that StringBuffer is thread-safe. So when the application needs to be run only in a single thread then it is better to use StringBuilder. StringBuilder is more efficient than StringBuffer. StringBuilder is faster than StringBuffer because it's not synchronized.

Situations:

• If your string is not going to change use a String class because a String object is immutable.
• If your string can change (example: lots of logic and operations in the construction of the string) and will only be accessed from a single thread, using a StringBuilder is good enough.
• If your string can change, and will be accessed from multiple threads, use a StringBuffer because StringBuffer is synchronous so you have thread-safety.