String Handling
String is probably the most commonly used class in java library. String class is encapsulated under
java.lang package. In java, every string that you create is actually an object of type String. One important thing to notice about string object is that string objects are immutable that means once a string object is created it cannot be altered.What is an Immutable object?
An object whose state cannot be changed after it is created is known as an Immutable object. String, Integer, Byte, Short, Float, Double and all other wrapper classes objects are immutable.
Creating an Immutable class
public final class MyString
{
final String str;
MyString(String s)
{
this.str = s;
}
public String get()
{
return str;
}
}Creating a String object
String can be created in number of ways, here are a few ways of creating string object.
1) Using a String literal
String literal is a simple string enclosed in double quotes
" ". A string literal is treated as a String object.String str1 = "Hello";2) Using another String object
String str2 = new String(str1);3) Using new Keyword
String str3 = new String("Java");4) Using + operator (Concatenation)
String str4 = str1 + str2;
or,
String str5 = "hello"+"Java";
Each time you create a String literal, the JVM checks the string pool first. If the string literal already exists in the pool, a reference to the pool instance is returned. If string does not exist in the pool, a new string object is created, and is placed in the pool. String objects are stored in a special memory area known as string constant pool inside the heap memory.
String object and How they are stored
When we create a new string object using string literal, that string literal is added to the string pool, if it is not present there already.
String str= "Hello";
And, when we create another object with same string, then a reference of the string literal already present in string pool is returned.
String str2=str;
But if we change the new string, its reference gets modified.
str2=str2.concat("world");
Concatenating String
There are 2 methods to concatenate two or more string.
- Using concat() method
- Using
+operator
1) Using concat() method
string s = "Hello";
string str = "Java";
string str2 = s.concat(str);
String str1 = "Hello".concat("Java"); //works with string literals too.2) Using + operator
string str = "Rahul";
string str1 = "Dravid";
string str2 = str + str1;
string st = "Rahul"+"Dravid";String Comparison
String comparison can be done in 3 ways.
- Using equals() method
- Using
==operator - By CompareTo() method
Using equals() method
equals() method compares two strings for equality. Its general syntax is,
boolean equals (Object str)
It compares the content of the strings. It will return true if string matches, else returns false.
String s = "Hell";
String s1 = "Hello";
String s2 = "Hello";
s1.equals(s2); //true
s.equals(s1) ; //falseUsing == operator
== operator compares two object references to check whether they refer to same instance. This also, will return true on successful match.String s1 = "Java";
String s2 = "Java";
String s3 = new string ("Java");
test(s1 == s2) //true
test(s1 == s3) //false
Reason:
Its because we are creating a new object using new operator, and thus it gets created in a non-pool memory area of the heap. s1 is pointing to the String in string pool while s3 is pointing to the String in heap and hence, when we compare s1 and s3, the answer is false.
By compareTo() method
compareTo() method compares values and returns an int which tells if the string compared is less than, equal to or greater than the other string. It compares the String based on natural ordering i.e alphabetically. Its general syntax is,
int compareTo(String str)String s1 = "Abhi";
String s2 = "Viraaj";
String s3 = "Abhi";
s1.compareTo(S2); //return -1 because s1 < s2
s1.compareTo(S3); //return 0 because s1 == s3
s2.compareTo(s1); //return 1 because s2 > s1
