Introduction To Strings
The String is an object in java.
The string represents a sequence of characters, But unlike many other languages that implement String as character arrays, java implements string as objects of type string.
Here is a string “Hello World” OR more accurately it’s a string literal, which means a string value
A String is Instantiated as
The most appropriate approach to make a string is:
String firstEx = “Hello World!”;
The String is a class but can be used as a data type
String firstEx(refrence) = “Hello World!”(object);
Creating String
Using Literal Using new keyword
String name = “Great”; String str= new String();
Methods Of String
The String class in Java provides a number of useful methods which helps to modifying of string:
Methods Effects
• concate() concatenates two strings
• length() To find the length of String
• replace(“a”, “b”) replaces Characters of ‘a’ with ‘b’
• toUpperCase() converts the string to upper case
• toLowerCase() converts the string to lower case
• trim() removes the whitespaces at the beginning and at the end
• indexOf(“a”) returns the position of first occurrence of a in the string.
• startsWith(“x”) returns a boolean true or false
• endsWith(“x”) returns a boolean true or false
• equals(“x”) returns a boolean true or false
Implementation With Examples
1. How to Concatenate Strings
We can concatenate or joining a string with another one using the plus(+) operator For Example
The string is a class it has members that we can access using the dot(.) operator so we can type Example.method().
We can also use the concate() method to join a string for Example:
2. endsWith()
We have the method endsWith() and with this, we can check to see if our string ends with a character or sequence of characters For example
Output: true
When we run the program we get true. So, this method that we have returns a boolean value which can be true or false
Now take a look one more time what happens
Output: false
3. startsWith()
We also have another method startsWith() let’s take a look
in this case we get false because our Example doesn’t start with “thon”. One more Example
Output: true
4. length()
Another useful method is length so we can call that to get the length of a string which is the number of characters
Output: 4
So in this string, we have 4 characters and this is useful in situations where you want to check the length of the input by the user for example
You might have a Sign-up form with a username field you can check the length of someone’s username and give them an error if the username is longer than 22 characters
5. indexOf()
We also have another method that is indexOf() and this returns the index of the first occurrence of the character or the string for example If you write “J” the index of J is 0
Output: 0
If you write “a” we get 1
Output: 1
because the index of the first “a” in this Example is 1
Now what if you write a character or string that doesn’t exist in this Example
let’s write “python” let’s take a look
Output: -1
we get a negative 1. So with this method, we can check if a string contains certain characters or words or sentences and so on.
6. replace()
We have a method replace() and with this, we can replace one or more characters with something else for example
We can replace “P” with “J” So this replace method has two parameters, one is target the other is replacement and here we’re writing two values for these parameters
here’s the first value “P”, the second value “J” and we have separated these values using a comma(,)
Now in programming terms, we refer to these values as arguments a lot of programmers don’t know the difference between parameters and arguments parameters are the holes that we define in our methods,
arguments are the actual values that we write to these methods. So in this case target and replacement are parameters but “P” and “J” are arguments
Now take a look at what happens
Output: Jython
Our P is replaced with J, now what is important here is that this method does not modify our original string it returns a new string.
So, If we print our original string after
Output: Jython
Output: Python //Original string
You can see the original string is not changed because in Java strings are Immutable.
We cannot mutate them we cannot change them so any methods that modify a string will always return a new string object.
7. toLowerCase()
We also have another useful method toLowerCase() For example
Output: java
Output: JAVA //Original string
toLowerCase() converts all characters to lowercase and once again you can see that the original string is not affected because this method returns a new string.
8. toUpperCase()
We also have toUpperCase() Method
Output: PYTHON
Output: python //Original string
this method converts all characters to Uppercase.
9. trim()
Another useful Method is trim() and with this, we can get rid of extra white spaces that can be at the beginning or the end of a string. Sometimes our users type unnecessary spaces in form fields so using the trim() method we can get rid of these white spaces for example
Output: JAVA & python
Output: JAVA & python //Original string
So we can see this if we add a couple of spaces before and after our Example then when we trim it, these white spaces are gonna get removed.
so the original string you can see two white spaces at the beginning.
10. equals()
The equals() method checks a string is equals with another one for example
Output: true
This example gives a boolean value true. Because the string is the same as after modification
here’s another example
Output: false this example gives Boolean value false.
These are some useful methods in the String class.
Other Concepts
Escape Sequence
We have the string
“Hi, My name is Ankit & I am the Student of Maths”
If we want to surround Maths with double-quote
Now here’s the problem if we add a double quote before Maths, the Java compiler thinks this is the termination of our string so it doesn’t understand what we have after the double quote that’s why the Java compiler gives a compilation error.
to fix this problem we need to prefix this double code with a backslash (\”)
so using this backslash we have escaped the double quote now take a look at what we get
So we get
Output: Hi, My name is Ankit & I am the Student of “Maths”
In this case we learn about a Special Character
Java gives us some more Special Characters :
Escape Sequence Effect
\n Create a new line
\t Insert a TAB
\b Create a backspace character
\’ Create a quote
\” Create a Double quote
\\ Create a Backslash
If we want to write a directory on Windows so that will look like this
c:\user\admin\documents…
Now if you want to write this in a string we need to escape each backslash
so for this, you have to add two backslashes (\\) for example
The Other special character is backslash N(\n) and we use that to add a new line to our strings so let’s write the backslash N now take look at what happens
now our string is broken down onto multiple lines by the first line we have Hello
then we have Students so wherever we had a backslash N Java will insert a new line
We can also add a tab in our strings
To add, a tab write backslash t (\t) in string
let’s take a look
here we have Hello a tab and then Students
The String is immutable:
A string is immutable i.e. we cannot change the string after creation. However, a variable declared as a String reference can be changed to point to some other string object at any time.
We can still perform all types of string operations. Each time we need an altered version of an existing string, a new string object is created that contains the modifications.
The original string is left unchanged. This approach is used because fixed, immutable strings can be implemented more efficiently than changeable ones.
For those cases in which a modified string is desired, there is a companion class called StringBuffer, whose objects contain strings that can be modified after they are created.
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