String

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1. Describe String in Java.

Strings are immutable. A string can’t be altered once created. Applying a method on string won’t change string’s value; rather it will create a new string. String is also final, and so can’t be sub classed. When you assign one String variable to another, no copy is made. Even when you take a substring there is no new String created.

Creating Strings
String can be created by assigning string literals:
String stringLiteral = "Java";

The text between double quotes is a string literal. By assigning a string literal, you can avoid using the new keyword. In fact, this special shortcut syntax was designed to improve String performance: Each JVM only keeps one copy of each string literal. The following code creates a single string literal that both String references point to:

String firstString = "I am a String Literal";
String secondString = "I am a String Literal"; // Points to the same object as firstString
String thirdString = new String("I am another String Literal"); // By using constructor, will create a new Objet in JVM

Note: You almost always want to initialize String references with literals to avoid creating unnecessary objects in the JVM? This can really add up if you are creating hundreds of identical Strings.
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2. What are the differences between the == operator and the equals() method?

Equals method compare values of the receiver and sender object. It returns true if the values are same. While == operator is a fundamental operator, it compares reference of the sender and receiver object.

E.g.
String s1 = "Hello";
String s2 = "Hello";
String s3 = new String("Hello");

System.out.println(s1 == s2); // true
System.out.println(s1 == s3); // false
System.out.println(s1.equals(s2)); // true
System.out.println(s1.equals(s3)); // true

From the example, we can see that reference and value for s1 & s2 are same. At the same time s3 is a newly created instance which has same value as of s1 and s2 but reference is different.

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3. What is String.intern()?

String.intern returns a canonical representation for the string object. A pool of strings, initially empty, is maintained privately by the class String. All literal strings and string-valued constant expressions are stored in this pool.
When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, 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.
It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.
By using intern one can get better performance as compared to equals method in String class. equals does the character by character that you probably want.
There are some disadvantages of intern. If we start using intern with every string then pool size will increase and could lead to memory problem. Secondly in case of bigger pool, finding a string could be more costly that having character by character comparison.
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4. Which one of the following is more efficient?

a. String str1 = "Hello " + "Guest";
b. String str2 = (new StringBuffer().append("Hello ")).append("Guest").toString();

str1 is resolved at compile time, while str2 is resolved at runtime time with an extra StringBuffer and String. The version that can be resolved at compile time is more efficient. It avoids the overhead of creating a String and an extra StringBuffer, as well as avoiding the runtime cost of several method calls.
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