String  in Java


String Handling provides a lot of concepts that can be performed on a string such as concatenating string, comparing string, substring etc.


In java, string is basically an immutable object. We will discuss about immutable string later. Let's first understand what is string and how we can create the string object.String Handling provides a lot of concepts that can be performed on a string such as concatenating string, comparing string, substring etc.

String


Generally string is a sequence of characters. But in java, string is an object. String class is used to create string object.



Do You Know ?
  • Why String objects are immutable ?
  • How to create an immutable class ?
  • What is string constant pool ?
  • What code is written by the compiler if you concat any string by + (string concatenation operator) ?
  • What is the difference between StringBuffer and StringBuilder class ?


How to create String object?

There are two ways to create String object:
  1. By string literal
  2. By new keyword

1) String literal

String literal is created by double quote.For Example:

  1. String s="Hello";  

Each time you create a string literal, the JVM checks the string constant pool first. If the string already exists in the pool, a reference to the pooled instance returns. If the string does not exist in the pool, a new String object instantiates, then is placed in the pool.For example:
  1. String s1="Welcome";  
  2. String s2="Welcome";//no new object will be created  

Why java uses concept of string literal?



To make Java more memory efficient (because no new objects are created if it exists already in string constant pool).

2) By new keyword


  1. String s=new String("Welcome");//creates two objects and one reference variable  

In such case, JVM will create a new String object in normal(nonpool) Heap memory and the literal "Welcome" will be placed in the string constant pool.The variable s will refer to the object in Heap(nonpool).

Immutable String in Java


In java, string objects are immutable. Immutable simply means unmodifiable or unchangeable.
Once string object is created its data or state can't be changed but a new string object is created.

Let's try to understand the immutability concept by the example given below:

  1. class Simple{  
  2.  public static void main(String args[]){  
  3.    String s="Sachin";  
  4.    s.concat(" Tendulkar");//concat() method appends the string at the end  
  5.    System.out.println(s);//will print Sachin because strings are immutable objects  
  6.  }  
  7. }  
Output:Sachin

Now it can be understood by the diagram given below. Here Sachin is not changed but a new object is created with sachintendulkar. That is why string is known as immutable.

As you can see in the above figure that two objects are created but s reference variable still refers to "Sachin" not to "Sachin Tendulkar".
But if we explicitely assign it to the reference variable, it will refer to "Sachin Tendulkar" object.For example:
  1. class Simple{  
  2.  public static void main(String args[]){  
  3.    String s="Sachin";  
  4.    s=s.concat(" Tendulkar");  
  5.    System.out.println(s);  
  6.  }  
  7. }  
Output:Sachin Tendulkar
In such case, s points to the "Sachin Tendulkar". Please notice that still sachin object is not modified.

Why string objects are immutable in java?


Because java uses the concept of string literal.Suppose there are 5 reference variables,all referes to one object "sachin".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.

String comparison in Java

We can compare two given strings on the basis of content and reference.
It is used in authentication (by equals() method),sorting (by compareTo() method), reference matching (by == operator) etc.
There are three ways to compare String objects:
  1. By equals() method
  2. By = = operator
  3. By compareTo() method

1) By equals() method

equals() method compares the original content of the string.It compares values of string for equality.String class provides two methods:
  • public boolean equals(Object another){} compares this string to the specified object.
  • public boolean equalsIgnoreCase(String another){} compares this String to another String, ignoring case.

2) By == operator




The = = operator compares references not values.

By compareTo() method:

compareTo() method compares values and returns an int which tells if the values compare less than, equal, or greater than.
Suppose s1 and s2 are two string variables.If:
  • s1 == s2 :0
  • s1 > s2   :positive value
  • s1 < s2   :negative value
  1. //<b><i>Example of compareTo() method:</i></b>  
  2.   
  3. class Simple{  
  4.  public static void main(String args[]){  
  5.    
  6.    String s1="Sachin";  
  7.    String s2="Sachin";  
  8.    String s3="Ratan";  
  9.   
  10.    System.out.println(s1.compareTo(s2));//0  
  11.    System.out.println(s1.compareTo(s3));//1(because s1>s3)  
  12.    System.out.println(s3.compareTo(s1));//-1(because s3 < s1 )  
  13.  }  
  14. }  
Output:0
       1
       -1

String Concatenation in Java


Concating strings form a new string i.e. the combination of multiple strings.
There are two ways to concat string objects:

  1. By + (string concatenation) operator
  2. By concat() method

1) By + (string concatenation) operator

String concatenation operator is used to add strings.For Example:

  1. //Example of string concatenation operator  
  2.   
  3. class Simple{  
  4.  public static void main(String args[]){  
  5.    
  6.    String s="Sachin"+" Tendulkar";  
  7.    System.out.println(s);//Sachin Tendulkar  
  8.  }  
  9. }  
Output:Sachin Tendulkar


The compiler transforms this to:

  1. String s=(new StringBuilder()).append("Sachin").append(" Tendulkar).toString();  


String concatenation is implemented through the StringBuilder(or StringBuffer) class and its append method.String concatenation operator produces a new string by appending the second operand onto the end of the first operand.The string concatenation operator can concat not only string but primitive values also.For Example:

  1. class Simple{  
  2.  public static void main(String args[]){  
  3.    
  4.    String s=50+30+"Sachin"+40+40;  
  5.    System.out.println(s);//80Sachin4040  
  6.  }  
  7. }  
Output:80Sachin4040


Note:If either operand is a string, the resulting operation will be string concatenation. If both operands are numbers, the operator will perform an addition.

2) By concat() method

concat() method concatenates the specified string to the end of current string.
Syntax:public String concat(String another){}



  1. //<b><i>Example of concat(String) method</i></b>  
  2.   
  3. class Simple{  
  4.  public static void main(String args[]){  
  5.    
  6.    String s1="Sachin ";  
  7.    String s2="Tendulkar";  
  8.   
  9.    String s3=s1.concat(s2);  
  10.   
  11.    System.out.println(s3);//Sachin Tendulkar  
  12.   }  
  13. }  
Output:Sachin Tendulkar


Substring in Java


A part of string is called substring. In other words, substring is a subset of another string.
In case of substring startIndex starts from 0 and endIndex starts from 1 or startIndex is inclusive and endIndex is exclusive.

You can get substring from the given String object by one of the two methods:
  1. public String substring(int startIndex): This method returns new String object containing the substring of the given string from specified startIndex (inclusive).
  2. public String substring(int startIndex,int endIndex): This method returns new String object containing the substring of the given string from specified startIndex to endIndex.

In case of string:
  • startIndex:starts from index 0(inclusive).
  • endIndex:starts from index 1(exclusive).


Example of java substring

  1. //Example of substring() method  
  2.   
  3. class Simple{  
  4.  public static void main(String args[]){  
  5.    
  6.    String s="Sachin Tendulkar";  
  7.    System.out.println(s.substring(6));//Tendulkar  
  8.    System.out.println(s.substring(0,6));//Sachin  
  9.  }  
  10. }  
Output:Tendulkar
       Sachin

Methods of String class in Java


java.lang.String class provides a lot of methods to work on string. By the help of these methods, we can perform operations on string such as trimming, concatenating, converting strings etc.
Let's see the important methods of String class.



MethodDescription
1)public boolean equals(Object anObject)Compares this string to the specified object.
2)public boolean equalsIgnoreCase(String another)Compares this String to another String, ignoring case.
3)public String concat(String str)Concatenates the specified string to the end of this string.
4)public int compareTo(String str)Compares two strings and returns int
5)public int compareToIgnoreCase(String str)Compares two strings, ignoring case differences.
6)public String substring(int beginIndex)Returns a new string that is a substring of this string.
7)public String substring(int beginIndex,int endIndex)Returns a new string that is a substring of this string.
8)public String toUpperCase()Converts all of the characters in this String to upper case
9)public String toLowerCase()Converts all of the characters in this String to lower case.
10)public String trim()Returns a copy of the string, with leading and trailing whitespace omitted.
11)public boolean startsWith(String prefix)Tests if this string starts with the specified prefix.
12)public boolean endsWith(String suffix)Tests if this string ends with the specified suffix.
13)public char charAt(int index)Returns the char value at the specified index.
14)public int length()Returns the length of this string.
15)public String intern()Returns a canonical representation for the string object.
16)public byte[] getBytes()Converts string into byte array.
17)public char[] toCharArray()Converts string into char array.
18)public static String valueOf(int i)converts the int into String.
19)public static String valueOf(long i)converts the long into String.
20)public static String valueOf(float i)converts the float into String.
21)public static String valueOf(double i)converts the double into String.
22)public static String valueOf(boolean i)converts the boolean into String.
23)public static String valueOf(char i)converts the char into String.
24)public static String valueOf(char[] i)converts the char array into String.
25)public static String valueOf(Object obj)converts the Object into String.
26)public void replaceAll(String firstString,String secondString)Changes the firstString with secondString.


intern() method


A pool of strings, initially empty, is maintained privately by the class String.
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.

StringBuffer class:


The StringBuffer class is used to created mutable (modifiable) string. The StringBuffer class is same as String except it is mutable i.e. it can be changed.

Note: StringBuffer class is thread-safe i.e. multiple threads cannot access it simultaneously .So it is safe and will result in an order.


Commonly used Constructors of StringBuffer class:

  1. StringBuffer(): creates an empty string buffer with the initial capacity of 16.
  2. StringBuffer(String str): creates a string buffer with the specified string.
  3. StringBuffer(int capacity): creates an empty string buffer with the specified capacity as length.

Commonly used methods of StringBuffer class:

  1. public synchronized StringBuffer append(String s): is used to append the specified string with this string. The append() method is overloaded like append(char), append(boolean), append(int), append(float), append(double) etc.
  2. public synchronized StringBuffer insert(int offset, String s): is used to insert the specified string with this string at the specified position. The insert() method is overloaded like insert(int, char), insert(int, boolean), insert(int, int), insert(int, float), insert(int, double) etc.
  3. public synchronized StringBuffer replace(int startIndex, int endIndex, String str): is used to replace the string from specified startIndex and endIndex.
  4. public synchronized StringBuffer delete(int startIndex, int endIndex): is used to delete the string from specified startIndex and endIndex.
  5. public synchronized StringBuffer reverse(): is used to reverse the string.
  6. public int capacity(): is used to return the current capacity.
  7. public void ensureCapacity(int minimumCapacity): is used to ensure the capacity at least equal to the given minimum.
  8. public char charAt(int index): is used to return the character at the specified position.
  9. public int length(): is used to return the length of the string i.e. total number of characters.
  10. public String substring(int beginIndex): is used to return the substring from the specified beginIndex.
  11. public String substring(int beginIndex, int endIndex): is used to return the substring from the specified beginIndex and endIndex.

What is mutable string?

A string that can be modified or changed is known as mutable string. StringBuffer and StringBuilder classes are used for creating mutable string.

see more details..


StringBuilder class:


The StringBuilder class is used to create mutable (modifiable) string. The StringBuilder class is same as StringBuffer class except that it is non-synchronized. It is available since JDK1.5.

Commonly used Constructors of StringBuilder class:

  1. StringBuilder(): creates an empty string Builder with the initial capacity of 16.
  2. StringBuilder(String str): creates a string Builder with the specified string.
  3. StringBuilder(int length): creates an empty string Builder with the specified capacity as length.

Commonly used methods of StringBuilder class:

  1. public StringBuilder append(String s): is used to append the specified string with this string. The append() method is overloaded like append(char), append(boolean), append(int), append(float), append(double) etc.
  2. public StringBuilder insert(int offset, String s): is used to insert the specified string with this string at the specified position. The insert() method is overloaded like insert(int, char), insert(int, boolean), insert(int, int), insert(int, float), insert(int, double) etc.
  3. public StringBuilder replace(int startIndex, int endIndex, String str): is used to replace the string from specified startIndex and endIndex.
  4. public StringBuilder delete(int startIndex, int endIndex): is used to delete the string from specified startIndex and endIndex.
  5. public StringBuilder reverse(): is used to reverse the string.
  6. public int capacity(): is used to return the current capacity.
  7. public void ensureCapacity(int minimumCapacity): is used to ensure the capacity at least equal to the given minimum.
  8. public char charAt(int index): is used to return the character at the specified position.
  9. public int length(): is used to return the length of the string i.e. total number of characters.
  10. public String substring(int beginIndex): is used to return the substring from the specified beginIndex.
  11. public String substring(int beginIndex, int endIndex): is used to return the substring from the specified beginIndex and endIndex.


see more details..


How to create Immutable class?


There are many immutable classes like String, Boolean, Byte, Short, Integer, Long, Float, Double etc. In short, all the wrapper classes and String class is immutable. We can also create immutable class by creating final class that have final data members as the example given below:

Example to create Immutable class

In this example, we have created a final class named Employee. It have one final datamember, a parameterized constructor and getter method.
  1. public final class Employee{  
  2. final String pancardNumber;  
  3.   
  4. public Employee(String pancardNumber){  
  5. this.pancardNumber=pancardNumber;  
  6. }  
  7.   
  8. public String getPancardNumber(){  
  9. return pancardNumber;  
  10. }  
  11.   
  12. }  

The above class is immutable because:
  • The instance variable of the class is final i.e. we cannot change the value of it after creating an object.
  • The class is final so we cannot create the subclass.
  • There is no setter methods i.e. we have no option to change the value of the instance variable.
These points makes this class as immutable.


Understanding toString() method


If you want to represent any object as a string, toString() method comes into existence.
The toString() method returns the string representation of the object.
If you print any object, java compiler internally invokes the toString() method on the object. So overriding the toString() method, returns the desired output, it can be the state of an object etc. depends on your implementation.


Advantage of the toString() method

By overriding the toString() method of the Object class, we can return values of the object, so we don't need to write much code.

StringTokenizer in Java

The java.util.StringTokenizer class allows you to break a string into tokens. It is simple way to break string.

It doesn't provide the facility to differentiate numbers, quoted strings, identifiers etc. like StreamTokenizer class. We will discuss about the StreamTokenizer class in I/O chapter.The java.util.StringTokenizer class allows you to break a string into tokens. It is simple way to break string.

Constructors of StringTokenizer class

There are 3 constructors defined in the StringTokenizer class.

ConstructorDescription
StringTokenizer(String str)creates StringTokenizer with specified string.
StringTokenizer(String str, String delim)creates StringTokenizer with specified string and delimeter.
StringTokenizer(String str, String delim, boolean returnValue)creates StringTokenizer with specified string, delimeter and returnValue. If return value is true, delimiter characters are considered to be tokens. If it is false, delimiter characters serve to separate tokens.

Methods of StringTokenizer class


The 6 useful methods of StringTokenizer class are as follows:

Public methodDescription
boolean hasMoreTokens()checks if there is more tokens available.
String nextToken()returns the next token from the StringTokenizer object.
String nextToken(String delim)returns the next token based on the delimeter.
boolean hasMoreElements()same as hasMoreTokens() method.
Object nextElement()same as nextToken() but its return type is Object.
int countTokens()returns the total number of tokens.

Simple example of StringTokenizer class

Let's see the simple example of StringTokenizer class that tokenizes a string "my name is khan" on the basis of whitespace.

  1. import java.util.StringTokenizer;  
  2. public class Simple{  
  3.  public static void main(String args[]){  
  4.    StringTokenizer st = new StringTokenizer("my name is khan"," ");  
  5.      while (st.hasMoreTokens()) {  
  6.          System.out.println(st.nextToken());  
  7.      }  
  8.    }  
  9. }  
Output:my
       name
       is
       khan

Example of nextToken(String delim) method of StringTokenizer class

  1. import java.util.*;  
  2.   
  3. public class Test {  
  4.    public static void main(String[] args) {  
  5.        StringTokenizer st = new StringTokenizer("my,name,is,khan");  
  6.         
  7.       // printing next token  
  8.       System.out.println("Next token is : " + st.nextToken(","));  
  9.    }      
  10. }  
Output:Next token is : my

StringTokenizer class is deprecated now. It is recommended to use split() method of String class or regex (Regular Expression).

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