string assignment in java

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Mastering Java Development: Best Free Online Courses

Java is one of the most popular programming languages in the world, and for good reason. It’s versatile, powerful, and can be used to develop a wide variety of applications and software. If you’re interested in becoming a Java developer or simply want to improve your coding skills, taking an online course is a great way to get started. In this article, we’ll explore some of the best free Java developer courses available online.

Introduction to Java

The first section of any good Java development course should cover the basics of the language. This includes things like syntax, data types, operators, and control structures. The goal of this section is to give you a solid foundation in Java that you can build upon as you progress through the course.

One great option for an introduction to Java course is “Java Programming Basics” on Udemy. This course covers all the basics of Java programming in just over 3 hours of video content. The best part? It’s completely free.

Oriented Programming

Once you have a solid understanding of the basics, it’s time to move on to object-oriented programming (OOP). OOP is a programming paradigm that focuses on creating reusable code by organizing it into objects that interact with each other.

For learning OOP concepts in Java, “Object Oriented Programming in Java” offered by Coursera is an excellent option. This course covers everything from inheritance and polymorphism to abstract classes and interfaces. And yes, it’s also free.

Web Development with Spring Framework

Java isn’t just limited to desktop applications – it’s also widely used for web development. One popular framework for building web applications with Java is Spring Framework. Spring provides developers with tools for building everything from simple web apps to complex enterprise-grade software.

To learn about web development using Spring Framework, check out “Spring Framework 5: Beginner to Guru” on Udemy. This course covers everything from the basics of Spring to advanced topics like security and data access. And the best part? It’s completely free.

Java Tools and Libraries

As you become more familiar with Java, you’ll want to start exploring some of the many tools and libraries available to developers. These can help streamline your development process, improve your code quality, and make your applications more powerful.

One great resource for learning about Java tools and libraries is “Java Tools and Libraries for Developers” on Coursera. This course covers topics like debugging, testing, and performance tuning using popular tools like Eclipse and JUnit.

Becoming a master Java developer takes time, dedication, and practice. But with the right resources at your disposal – including these free online courses – you can get started on the path to becoming a Java pro today.

This text was generated using a large language model, and select text has been reviewed and moderated for purposes such as readability.

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In the given example only one object will be created. Firstly JVM will not find any string object with the value “Welcome” in the string constant pool, so it will create a new object. After that it will find the string with the value “Welcome” in the pool, it will not create a new object but will return the reference to the same instance. In this article, we will learn about Java Strings.

What are Strings in Java?

Strings are the type of objects that can store the character of values. A string acts the same as an array of characters in Java.

Example:   

String Example in Java

Below is an example of a String in Java:

Ways of Creating a String

There are two ways to create a string in Java: 

• String Literal
• Using new Keyword

Syntax:   

1. String literal

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

2. Using new keyword

• String s = new String(“Welcome”);
• In such a case, JVM will create a new string object in normal (non-pool) heap memory and the literal “Welcome” will be placed in the string constant pool. The variable s will refer to the object in the heap (non-pool)

Interfaces and Classes in Strings in Java

CharBuffer : This class implements the CharSequence interface. This class is used to allow character buffers to be used in place of CharSequences. An example of such usage is the regular-expression package java.util.regex.

String : It is a sequence of characters. In Java, objects of String are immutable which means a constant and cannot be changed once created.

CharSequence Interface

CharSequence Interface is used for representing the sequence of Characters in Java. Classes that are implemented using the CharSequence interface are mentioned below:

• StringBuffer
• StringBuilder

1. StringBuffer

StringBuffer is a peer class of String that provides much of the functionality of strings. The string represents fixed-length, immutable character sequences while StringBuffer represents growable and writable character sequences.

2. StringBuilder

StringBuilder in Java represents a mutable sequence of characters. Since the String Class in Java creates an immutable sequence of characters, the StringBuilder class provides an alternative to String Class, as it creates a mutable sequence of characters.

3. StringTokenizer

StringTokenizer class in Java is used to break a string into tokens. 

A StringTokenizer object internally maintains a current position within the string to be tokenized. Some operations advance this current position past the characters processed. A token is returned by taking a substring of the string that was used to create the StringTokenizer object.

StringJoiner is a class in java.util package which is used to construct a sequence of characters(strings) separated by a delimiter and optionally starting with a supplied prefix and ending with a supplied suffix. Though this can also be done with the help of the StringBuilder class to append a delimiter after each string, StringJoiner provides an easy way to do that without much code to write.

Above we saw we can create a string by  String Literal. 

String demoString =”Welcome”; 

Here the JVM checks the String Constant Pool. If the string does not exist, then a new string instance is created and placed in a pool. If the string exists, then it will not create a new object. Rather, it will return the reference to the same instance. The cache that stores these string instances is known as the String Constant pool or String Pool. In earlier versions of Java up to JDK 6 String pool was located inside PermGen(Permanent Generation) space. But in JDK 7 it is moved to the main heap area. 

Immutable String in Java

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

Below is the implementation of the topic:

Here Sachin is not changed but a new object is created with “Sachin Tendulkar”. That is why a string is known as immutable.

As you can see in the given figure that two objects are created but s reference variable still refers to “Sachin” and not to “Sachin Tendulkar”. But if we explicitly assign it to the reference variable, it will refer to the “Sachin Tendulkar” object.           

For Example:

Memory Allotment of String

Whenever a String Object is created as a literal, the object will be created in the String constant pool. This allows JVM to optimize the initialization of String literal.

Example:  

The string can also be declared using a new operator i.e. dynamically allocated. In case of String are dynamically allocated they are assigned a new memory location in the heap. This string will not be added to the String constant pool.

Example: 

If you want to store this string in the constant pool then you will need to “intern” it.

It is preferred to use String literals as it allows JVM to optimize memory allocation.

An example that shows how to declare a String 

Note: String objects are stored in a special memory area known as string constant pool.

Why did the String pool move from PermGen to the normal heap area? 

PermGen space is limited, the default size is just 64 MB. it was a problem with creating and storing too many string objects in PermGen space. That’s why the String pool was moved to a larger heap area. To make Java more memory efficient, the concept of string literal is used. By the use of the ‘new’ keyword, The JVM will create a new string object in the normal heap area even if the same string object is present in the string pool. 

For example:

Let us have a look at the concept with a Java program and visualize the actual JVM memory structure: 

Below is the implementation of the above approach:

String Pool in Java

Note:  All objects in Java are stored in a heap. The reference variable is to the object stored in the stack area or they can be contained in other objects which puts them in the heap area also.

Example 1: 

Frequently Asked Questions

1. what are strings in java.

Strings are the types of objects which can store characters as elements. 

2. Why string objects are immutable in Java?

Because java uses the concept of string literal. Suppose there are 5 reference variables, all refer to one object “Sachin”. If one reference variable changes the value of the object, it will be affected by all the reference variables. That is why string objects are immutable in Java.

3. Why Java uses the concept of string literal?

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Java Programs

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• How to Create JFrame in Java?
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• How To Implement Matrix Multiplication In Java?
• How To Deal With Random Number and String Generator in Java?
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Advance Java

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• Advanced Java Tutorial- A Complete Guide for Advanced Java
• Servlet and JSP Tutorial- How to Build Web Applications in Java?
• Introduction to Java Servlets – Servlets in a Nutshell
• What Is JSP In Java? Know All About Java Web Applications
• How to Implement MVC Architecture in Java?
• What is JavaBeans? Introduction to JavaBeans Concepts
• Know what are the types of Java Web Services?
• JavaFX Tutorial: How to create an application?
• What is Executor Framework in Java and how to use it?
• What is Remote Method Invocation in Java?
• Everything You Need To Know About Session In Java?
• Java Networking: What is Networking in Java?
• What is logger in Java and why do you use it?
• How To Handle Deadlock In Java?
• Know all about Socket Programming in Java
• Important Java Design Patterns You Need to Know About
• What is ExecutorService in Java and how to create it?
• Struts 2 Tutorial – One Stop Solution for Beginners
• What is Hibernate in Java and Why do we need it?
• What is Maven in Java and how do you use it?
• What is Machine Learning in Java and how to implement it?

Career Opportunities

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Interview Questions

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Programming & Frameworks

String in java – string functions in java with examples.

What is a Java String? In Java, a string is an object that represents a sequence of characters or char values. The java.lang.String class is used to create a Java string object.

There are two ways to create a String object:

• By string literal : Java String literal is created by using double quotes. For Example: String s= “Welcome” ;  
• By new keyword : Java String is created by using a keyword “new”. For example:  String s= new String( “Welcome” );   It creates two objects (in String pool and in heap) and one reference variable where the variable ‘s’ will refer to the object in the heap.

Now, let us understand the concept of Java String pool.

Java String Pool: Java String pool refers to collection of Strings which are stored in heap memory. In this, whenever a new object is created, String pool first checks whether the object is already present in the pool or not. If it is present, then same reference is returned to the variable else new object will be created in the String pool and the respective reference will be returned. Refer to the diagrammatic representation for better understanding:

Before we go ahead, One key point I would like to add that unlike other data types in Java, Strings are immutable. By immutable, we mean that Strings are constant, their values cannot be changed after they are created. Because String objects are immutable, they can be shared. For example:

   String str =”abc”; is equivalent to :

  ch ar data[] = {‘a’, ‘b’, ‘c’};     String str = new String(da ta);

Let us now look at some of the inbuilt methods in String class .

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Java S tring Methods

• Java String length() :  The Java String length() method tells the length of the string. It returns count of total number of characters present in the String. For exam p le:

Here, String length()  function will return the length 5 for s1 and 7 for s2 respectively.

• Java St ring comp areTo () : The Java String compareTo() method compares the given string with current string. It is a method of  ‘Comparable’  interface which is implemented by String class. Don’t worry, we will be learning about String interfaces later. It either returns positive number, negative number or 0.  For example:

This program shows the comparison between the various string. It is noticed that   i f s 1 > s2 ,  it  returns a positive number   i f   s1 < s 2 , it returns a negative number  i f s1 == s2, it returns 0

• Java String IsEmpty() : This method checks whether the String contains anything or not. If the java String is Empty, it returns true else false. For example: public class IsEmptyExample{ public static void main(String args[]){ String s1=""; String s2="hello"; System.out.println(s1.isEmpty()); // true System.out.println(s2.isEmpty()); // false }}
• Java String ValueOf() : This method converts different types of values into string.Using this method, you can convert int to string, long to string, Boolean to string, character to string, float to string, double to string, object to string and char array to string. The signature or syntax of string valueOf() method is given below: public static String valueOf(boolean b) public static String valueOf(char c) public static String valueOf(char[] c) public static String valueOf(int i) public static String valueOf(long l) public static String valueOf(float f) public static String valueOf(double d) public static String valueOf(Object o)

Let’s understand this with a programmatic example:

In the above code, it concatenates the Java String and gives the output – 2017.

Java String replace() : The Java String replace() method returns a string, replacing all the old characters or CharSequence to new characters. There are 2 ways to replace methods in a Java String. 

In the above code, it will replace all the occurrences of ‘h’ to ‘t’. Output to the above code will be “tello tow are you”.  Let’s see the another type of using replace method in java string: Java String replace(CharSequence target, CharSequence replacement) method :

In the above code, it will replace all occurrences of “Edureka” to “Brainforce”. Therefore, the output would be “ Hey, welcome to Brainforce”.

In the above code, the first two statements will return true as it matches the String whereas the second print statement will return false because the characters are not present in the string.

• Java String equals() : The Java String equals() method compares the two given strings on the basis of content of the string i.e Java String representation. If all the characters are matched, it returns true else it will return false. For example: public class EqualsExample{ public static void main(String args[]){ String s1="hello"; String s2="hello"; String s3="hi"; System.out.println(s1.equalsIgnoreCase(s2)); // returns true System.out.println(s1.equalsIgnoreCase(s3)); // returns false } }

In the above code, the first statement will return true because the content is same irrespective of the case. Then, in the second print statement will return false as the content doesn’t match in the respective strings.

• Java String IsEmpty() : This method checks whether the String is empty or not. If the length of the String is 0, it returns true else false. For example: 

In the above code, the first print statement will return true as it does not contain anything while the second print statement will return false.

• Java String endsWith() : The Java String endsWith() method checks if this string ends with the given suffix. If it returns with the given suffix, it will return true else returns fals e. F or example:

This is not the end. There are more Java String methods that will help you make your code simpler.  

Moving on, Java String class implements three  interfac es , namely – Serializable, Comparable and C h arSequence .

• StringBuffer and StringBuilder are mutable classes. StringBuffer operations are thread-safe and synchronized whereas StringBuilder operations are not thread-safe.
• StringBuffer is to be used when multiple threads are working on same String and StringBuilder in the single threaded environment.
• StringBuilder performance is faster when compared to StringBuffer because of no overhead of synchronized.

I hope you guys are clear with Java String, how they are created, their different methods and interfaces. I would recommend you to try all the Java String examples.  Do read my next blog on Java Interview Questions which will help you set apart in the interview process. If you’re just beginning, then watch at this Java Tutorial to Understand the Fundamental Java Concepts.

Now that you have understood basics of Java, check out the Java Online Course by Edureka, a trusted online learning company with a network of more than 250,000 satisfied learners spread across the globe. Edureka’s Java J2EE and SOA training and certification course is designed for students and professionals who want to be a Java Developer. The course is designed to give you a head start into Java programming and train you for both core and advanced Java concepts along with various Java frameworks like Hibernate & Spring.

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This is wrong information. Please correct it.

( Corrections are in bold letters) By new keyword : Java String is created by using a keyword “new”. For example: String s=new String(“Welcome”); It creates two objects ( in String pool and in heap ) and one reference variable where the variable ‘s’ will refer to the object in the heap.

Please refer to Java 8 reference books, There are changes in version of java 8.

Hello, Nice Blog, But please change the image where you are comparing two string using == there you should modify the last image as s1 == s3 Sandeep Patidar

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Java String

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The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available. See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases. See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.

Strings, which are widely used in Java programming, are a sequence of characters. In the Java programming language, strings are objects.

The Java platform provides the String class to create and manipulate strings.

Creating Strings

The most direct way to create a string is to write:

In this case, "Hello world!" is a string literal —a series of characters in your code that is enclosed in double quotes. Whenever it encounters a string literal in your code, the compiler creates a String object with its value—in this case, Hello world! .

As with any other object, you can create String objects by using the new keyword and a constructor. The String class has thirteen constructors that allow you to provide the initial value of the string using different sources, such as an array of characters:

The last line of this code snippet displays hello .

String Length

Methods used to obtain information about an object are known as accessor methods . One accessor method that you can use with strings is the length() method, which returns the number of characters contained in the string object. After the following two lines of code have been executed, len equals 17:

A palindrome is a word or sentence that is symmetric—it is spelled the same forward and backward, ignoring case and punctuation. Here is a short and inefficient program to reverse a palindrome string. It invokes the String method charAt(i) , which returns the i th character in the string, counting from 0.

Running the program produces this output:

To accomplish the string reversal, the program had to convert the string to an array of characters (first for loop), reverse the array into a second array (second for loop), and then convert back to a string. The String class includes a method, getChars() , to convert a string, or a portion of a string, into an array of characters so we could replace the first for loop in the program above with

Concatenating Strings

The String class includes a method for concatenating two strings:

This returns a new string that is string1 with string2 added to it at the end.

You can also use the concat() method with string literals, as in:

Strings are more commonly concatenated with the + operator, as in

which results in

The + operator is widely used in print statements. For example:

which prints

Such a concatenation can be a mixture of any objects. For each object that is not a String , its toString() method is called to convert it to a String .

Breaking strings between lines using the + concatenation operator is, once again, very common in print statements.

Creating Format Strings

You have seen the use of the printf() and format() methods to print output with formatted numbers. The String class has an equivalent class method, format() , that returns a String object rather than a PrintStream object.

Using String's static format() method allows you to create a formatted string that you can reuse, as opposed to a one-time print statement. For example, instead of

you can write

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How to including variables within strings? [duplicate]

So, we all should know that you can include variables into strings by doing:

Is there a way to do it like:

In other words: Without having to close the quotation marks and adding plus signs. It's very unattractive.

• quotation-marks

• 20 @Chandra Please don't ask why, but rather explain if possible. It's just how I'd prefer to do it. Thanks. –  Gray Adams Mar 10, 2012 at 3:12
• 3 Use Groovy, then you'll be able to do "A string ${aVariable}" all you want. –  Kaleb Brasee Mar 10, 2012 at 3:14
• 3 There's a whole variety of techniques for similar things discussed in this question , but String.format() is built-in to the language. –  ig0774 Mar 10, 2012 at 3:15
• 1 @KalebBrasee That sounds perfect, but I am always hesitant when it comes to modifying languages. I don't want to set myself back. –  Gray Adams Mar 10, 2012 at 3:17
• 2 @GrayAdams Groovy doesn't set you back, it sets you free! :D –  Kaleb Brasee Mar 10, 2012 at 3:20

6 Answers 6

You can always use String.format(....). i.e.,

I'm not sure if that is attractive enough for you, but it can be quite handy. The syntax is the same as for printf and java.util.Formatter. I've used it much especially if I want to show tabular numeric data.

• 3 I know this is a matter of opinion, but I don't see how format is more attractive than a simple String concatenation expression. Where format comes into its own is when you need to do padding, number formatting, etcetera. –  Stephen C Mar 10, 2012 at 4:19
• 1 @StephenC: I don't disagree with you at all. But it is a useful alternative for String formatting especially when needing to do padding as you say and as I say above (i.e., to show tabular numeric data). I've used it quite a bit for formatting blood chemistry and CBC result reports. –  Hovercraft Full Of Eels Mar 10, 2012 at 11:08
• @StephenC I like format for a few reasons: first of all, '%d' uses the platform line delimiter. Second, you can easily find all variables at the end. You can easily reformat or even reorder the variables you input. It's easier to avoid mistakes (like 1 + "oops" ), especially if you use FindBugs (which parses format strings and input parameters). And, as the asker says, in many cases it's more readable. Of course, it's a shame that the format method has been made static, that was a rather stupid design mistake. –  Maarten Bodewes Mar 15, 2012 at 22:09
• 1 @StephenC And why not? Although renaming the method to e.g. asFormat would make more sense. Of course, the fact that Eclipse does not work that well with static imports makes it a bit more painful to use the current String.format as well. –  Maarten Bodewes Mar 16, 2012 at 0:44
• 1 @owlstead - "asFormat" wouldn't be right. It implies that the method is creating and returning a format object. –  Stephen C Mar 16, 2012 at 3:50

This is called string interpolation; it doesn't exist as such in Java.

One approach is to use String.format:

Another approach is to use a templating library such as Velocity or FreeMarker .

Also consider java.text.MessageFormat , which uses a related syntax having numeric argument indexes. For example,

results in string containing the following:

More commonly, the class is used for its numeric and temporal formatting. An example of JFreeChart label formatting is described here ; the class RCInfo formats a game's status pane.

• 2 For somebody coming from CSharp this way is more straightforward since it is similar to string.Format in C#. –  Mojtaba Aug 22, 2017 at 15:25
• I much prefer this due to how clean and reusable a variable is, compared to the String.format() method. –  Satch Jun 12 at 12:55

Since Java 15, you can use a non-static string method called String::formatted(Object... args)

"First foo, then bar"

Apache Commons Text 's StringSubstitutor can be used. See the library's Dependency Information for how to include it in a project.

This class supports providing default values for variables.

To use recursive variable replacement, call setEnableSubstitutionInVariables(true); .

The library can be found on Maven central repository . This is the Maven dependency:

• 1 This answer really answers the question. –  AlikElzin-kilaka Jul 4, 2021 at 8:59
• This indeed meets the requirement precisely. –  Arvind Kumar Avinash Nov 27, 2022 at 16:10

you can use String format to include variables within strings

i use this code to include 2 variable in string:

String myString = String.format("this is my string %s %2d", variable1Name, variable2Name);

Not the answer you're looking for? Browse other questions tagged java variables insert include quotation-marks or ask your own question .

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Java String equals()

Java String intern()

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In Java, a string is a sequence of characters. For example, "hello" is a string containing a sequence of characters 'h' , 'e' , 'l' , 'l' , and 'o' .

We use double quotes to represent a string in Java. For example,

Here, we have created a string variable named type . The variable is initialized with the string Java Programming .

Example: Create a String in Java

In the above example, we have created three strings named first , second , and third . Here, we are directly creating strings like primitive types.

However, there is another way of creating Java strings (using the new keyword). We will learn about that later in this tutorial.

Note : Strings in Java are not primitive types (like int , char , etc). Instead, all strings are objects of a predefined class named String .

And, all string variables are instances of the String class.

Java String Operations

Java String provides various methods to perform different operations on strings. We will look into some of the commonly used string operations.

1. Get length of a String

To find the length of a string, we use the length() method of the String. For example,

In the above example, the length() method calculates the total number of characters in the string and returns it. To learn more, visit Java String length() .

2. Join Two Java Strings

We can join two strings in Java using the concat() method. For example,

In the above example, we have created two strings named first and second . Notice the statement,

Here, the concat() method joins the second string to the first string and assigns it to the joinedString variable.

We can also join two strings using the + operator in Java. To learn more, visit Java String concat() .

3. Compare two Strings

In Java, we can make comparisons between two strings using the equals() method. For example,

In the above example, we have created 3 strings named first , second , and third . Here, we are using the equal() method to check if one string is equal to another.

The equals() method checks the content of strings while comparing them. To learn more, visit Java String equals() .

Note : We can also compare two strings using the == operator in Java. However, this approach is different than the equals() method. To learn more, visit Java String == vs equals() .

Escape character in Java Strings

The escape character is used to escape some of the characters present inside a string.

Suppose we need to include double quotes inside a string.

Since strings are represented by double quotes , the compiler will treat "This is the " as the string. Hence, the above code will cause an error.

To solve this issue, we use the escape character \ in Java. For example,

Now escape characters tell the compiler to escape double quotes and read the whole text.

Java Strings are Immutable

In Java, strings are immutable . This means, once we create a string, we cannot change that string.

To understand it more deeply, consider an example:

Here, we have created a string variable named example . The variable holds the string "Hello! " .

Now suppose we want to change the string.

Here, we are using the concat() method to add another string World to the previous string.

It looks like we are able to change the value of the previous string. However, this is not true .

Let's see what has happened here,

• JVM takes the first string "Hello! "
• creates a new string by adding "World" to the first string
• assign the new string "Hello! World" to the example variable
• the first string "Hello! " remains unchanged
• Creating strings using the new keyword

So far we have created strings like primitive types in Java.

Since strings in Java are objects, we can create strings using the new keyword as well. For example,

In the above example, we have created a string name using the new keyword.

Here, when we create a string object, the String() constructor is invoked. To learn more about constructor, visit Java Constructor .

Note : The String class provides various other constructors to create strings. To learn more, visit Java String (official Java documentation) .

Example: Create Java Strings using the new keyword

Create string using literals vs new keyword.

Now that we know how strings are created using string literals and the new keyword, let's see what is the major difference between them.

In Java, the JVM maintains a string pool to store all of its strings inside the memory. The string pool helps in reusing the strings.

1. While creating strings using string literals,

Here, we are directly providing the value of the string ( Java ). Hence, the compiler first checks the string pool to see if the string already exists.

• If the string already exists , the new string is not created. Instead, the new reference, example points to the already existed string ( Java ).
• If the string doesn't exist , the new string ( Java is created.

2. While creating strings using the new keyword,

Here, the value of the string is not directly provided. Hence, a new "Java" string is created even though "Java" is already present inside the memory pool.

• Methods of Java String

Besides those mentioned above, there are various string methods present in Java. Here are some of those methods:

Table of Contents

• Java String
• Create a String in Java
• Get Length of a String
• Join two Strings
• Compare two Strings
• Escape character in Strings
• Immutable Strings
• String literals vs new keyword

Sorry about that.

Related Tutorials

Java Library

TABLE OF CONTENTS (HIDE)

Java programming tutorial, java string is special, a brief summary of the string class.

A Java String contains an immutable sequence of Unicode characters. Unlike C/C++, where string is simply an array of char , A Java String is an object of the class java.lang.String .

Java String is, however, special. Unlike an ordinary class:

• String is associated with string literal in the form of double-quoted texts such as " hello, world ". You can assign a string literal directly into a String variable, instead of calling the constructor to create a String instance.
• The '+' operator is overloaded to concatenate two String operands. '+' does not work on any other objects such as Point and Circle .
• String is immutable . That is, its content cannot be modified once it is created. For example, the method toUpperCase() constructs and returns a new String instead of modifying the existing content.

Method Summary

The commonly-used method in the String class are summarized below. Refer to the JDK API for java.lang.String a complete listing.

static method String.format() (JDK 5)

The static method String.format() (introduced in JDK 5) can be used to produce a formatted String using C-like printf() 's format specifiers. The format() method has the same form as printf() . For example,

String.format() is useful if you need to produce a simple formatted String for some purposes (e.g., used in method toString() ). For complex string, use StringBuffer / StringBuilder with a Formatter . If you simply need to send a simple formatted string to the console, use System.out.printf() , e.g.,

[TODO] More examples

New Methods

Jdk 9 new methods.

• .chars()|.codePoints() -> IntStream

JDK 11 new methods

• .repeat(int count) -> String
• .strip()|.stripLeading()|.stripTrailing() -> String and .isBlank() -> boolean which are unicode white-space aware.
• .lines() to produces a Stream<String> . (JDK 9 added .chars() and .codePoints() to produce an IntStream .)

JDK 12 new methods

• .indent(int n) -> String
• .transform(Function<String,R> f) -> R
• .describeConstable() -> Optional<String>
• .resolveConstantDesc() -> String

JDK 13 new methods

String is really special.

Strings receive special treatment in Java, because they are used frequently in a program. Hence, efficiency (in terms of computation and storage) is crucial.

The designers of Java decided to retain primitive types in an object-oriented language, instead of making everything an object, so as to improve the performance of the language. Primitives are stored in the method stack, which require less storage spaces and are cheaper to manipulate. On the other hand, objects are stored in the program heap, which require complex memory management and more storage spaces.

For performance reason, Java's String is designed to be in between a primitive and an object. The special features in String include:

• The '+' operator, which performs addition on primitives (such as int and double ), is overloaded to operate on String objects. '+' performs concatenation for two String operands. Java does not support operator overloading for software engineering consideration. In a language that supports operator overloading like C++, you can turn a '+' operator to perform a subtraction, resulted in poor codes. The '+' operator is the only operator that is internally overloaded to support string concatenation in Java. Take note that '+' does not work on any two arbitrary objects, such as Point s or Circle s.
• directly assigning a string literal to a String reference - just like a primitive , or
• via the " new " operator and constructor, similar to any other classes. However, this is not commonly-used and is not recommended.

For example,

In the first statement, str1 is declared as a String reference and initialized with a string literal "Java is Hot" . In the second statement, str2 is declared as a String reference and initialized via the new operator and constructor to contain "I'm cool" .

• String literals are stored in a common pool . This facilitates sharing of storage for strings with the same contents to conserve storage. String objects allocated via new operator are stored in the heap, and there is no sharing of storage for the same contents.

String Literal vs. String Object

As mentioned, there are two ways to construct a string: implicit construction by assigning a string literal or explicitly creating a String object via the new operator and constructor. For example,

Java has provided a special mechanism for keeping the String literals - in a so-called string common pool . If two string literals have the same contents, they will share the same storage inside the common pool. This approach is adopted to conserve storage for frequently-used strings. On the other hand, String objects created via the new operator and constructor are kept in the heap. Each String object in the heap has its own storage just like any other object. There is no sharing of storage in heap even if two String objects have the same contents.

You can use the method equals() of the String class to compare the contents of two String s. You can use the relational equality operator '==' to compare the references (or pointers) of two objects. Study the following codes:

Important Notes:

• In the above example, I used relational equality operator '==' to compare the references of two String objects. This is done to demonstrate the differences between string literals sharing storage in the common pool and String objects created in the heap. It is a logical error to use (str1 == str2) in your program to compare the contents of two String s.
• String can be created by directly assigning a String literal which is shared in a common pool. It is uncommon and not recommended to use the new operator to construct a String object in the heap.

String is Immutable

Since string literals with the same contents share storage in the common pool, Java's String is designed to be immutable . That is, once a String is constructed, its contents cannot be modified. Otherwise, the other String references sharing the same storage location will be affected by the change, which can be unpredictable and therefore is undesirable. Methods such as toUpperCase() might appear to modify the contents of a String object. In fact, a completely new String object is created and returned to the caller. The original String object will be deallocated, once there is no more references, and subsequently garbage-collected.

Because String is immutable, it is not efficient to use String if you need to modify your string frequently (that would create many new String s occupying new storage areas). For example,

If the contents of a String have to be modified frequently, use the StringBuffer or StringBuilder class instead.

String.intern()

Stringbuffer & stringbuilder.

As explained earlier, String s are immutable because String literals with same content share the same storage in the string common pool. Modifying the content of one String directly may cause adverse side-effects to other String s sharing the same storage.

JDK provides two classes to support mutable strings: StringBuffer and StringBuilder (in core package java.lang ). A StringBuffer or StringBuilder object is just like any ordinary object, which are stored in the heap and not shared, and therefore, can be modified without causing adverse side-effect to other objects.

StringBuilder class was introduced in JDK 5. It is the same as StringBuffer class, except that StringBuilder is not synchronized for multi-thread operations. However, for single-thread program, StringBuilder , without the synchronization overhead, is more efficient.

java.lang.StringBuffer

Read the JDK API specification for java.lang.StringBuffer .

Take note that StringBuffer is an ordinary object. You need to use a constructor to create a StringBuffer (instead of assigning to a String literal). Furthermore, '+' operator does not apply to objects, including the StringBuffer . You need to use a proper method such as append() or insert() to manipulating a StringBuffer .

To create a string from parts, It is more efficient to use StringBuffer (multi-thread) or StringBuilder (single-thread) instead of via String concatenation. For example,

JDK compiler, in fact, uses both String and StringBuffer to handle string concatenation via the '+' operator. For examples,

will be compiled into the following codes for better efficiency:

Two objects are created during the process, an intermediate StringBuffer object and the returned String object.

Rule of Thumb: String s are more efficient if they are not modified (because they are shared in the string common pool). However, if you have to modify the content of a string frequently (such as a status message), you should use the StringBuffer class (or the StringBuilder described below) instead.

java.lang.StringBuilder (JDK 5)

JDK 5 introduced a new StringBuilder class (in package java.lang ), which is almost identical to the StringBuffer class, except that it is not synchronized . In other words, if multiple threads are accessing a StringBuilder instance at the same time, its integrity cannot be guaranteed. However, for a single-thread program (most commonly), doing away with the overhead of synchronization makes the StringBuilder faster.

StringBuilder is API-compatible with the StringBuffer class, i.e., having the same set of constructors and methods, but with no guarantee of synchronization. It can be a drop-in replacement for StringBuffer under a single-thread environment.

Benchmarking String/StringBuffer/StringBuilder

The following program compare the times taken to reverse a long string via a String object and a StringBuffer .

Observe StringBuilder is 2x faster than StringBuffer , and 300x faster than String . The reverse() method is the fastest, which take about the same time for StringBuilder and StringBuffer .

java.util.StringTokenizer (Obsoleted by regex)

Very often, you need to break a line of texts into tokens delimited by white spaces. The java.util.StringTokenizer class supports this.

For example, the following program reverses the words in a String.

The JDK documentation stated that " StringTokenizer is a legacy class that is retained for compatibility reasons although its use is discouraged in new code. It is recommended that anyone seeking this functionality use the split() method of String or the java.util.regex package instead."

For example, the following program uses the split() method of the String class to reverse the words of a String .

Regular Expression (Regex), Patterns & Matches (JDK 4)

Read " Regular Expression in Java ".

Super-Interface CharSequence for String , StringBuffer and StringBuilder (since JDK 4)

The interface java.lang.CharSequence is implemented by classes String , StringBuffer , StringBuilder , CharBuffer and Segment .

It defines the common behavior via these abstract methods:

JDK 8 added two default methods into the interface:

JDK 11 added one static methods into the interface:

Latest version tested: JDK 1.13.1 Last modified: February, 2020