String Programs in Java: In Java, a String is an immutable object that represents a sequence of characters. For example, “Tutorial” is a string that consists of 8 characters: ‘T’, ‘u’, ‘t’, ‘o’, ‘r’, ‘i’, ‘a’, ‘l’. Immutable object means once a string is created, it cannot be changed, and if we try to change the string, we end up creating a new string. In this section, we will create different String programs.

The Java String class is used for the creating of a string object. However, we do not need to import the class String. This is because the Java String class is part of the java.lang package, and java.lang package is imported by default in every Java program. In Java, a string is represented by characters enclosed in double quotes (“”). For characters, single quotes are used (‘’);

Creating a String in Java

There are three ways of creating a string/ string object in Java:

  • Using string literal
  • Using the new keyword
  • Using a character array

Using string literal

FileName: StringExample.java

Output:

Explanation: In the above program, we have created three strings: country, country1 and country2 and assigned the strings: “India”, “France”, and “Mexico”. Note that we have not create any object of the class String. However, we know that a string is an object in Java. Hence, under the hood, JVM creates three objects for the three strings and assigns the reference to the string variables. Memory allocation for the three strings is done in the string pool ( a place in the heap memory used by JVM for avoiding string redundancy), whereas the string variables get placed in stack memory.

Using new Keyword

FileName: StringExample1.java

Output:

Explanation: In the above program, we have used the new keyword for creating the string object. The string objects are created in heap memory but not in the string pool. Memory for the string variables is allocated in the stack memory.

Using Character Array

FileName: StringExample2.java

Output:

Explanation: Using the given character array, we are creating string str. This time also, the new keyword comes in handy. Eventually, we are displaying the result.

The length() method

To calculate the size of a string, we use the length() method of String class. The signature of the method length() is:

Since the keyword static is not used, the length() method cannot be invoked using the class name, i.e., String.length(). We have to use objects to call the length() method. It does not accept any parameter. It returns the length of the string. The following example illustrates the same.

FileName: StringExample3.java

Output:

Explanation: The string “Apple” internally gets converted to a character array. Then, the length field of the array is used to find its size, which is 5 in our case. The value of the length field is returned by the method length().

Manipulating Strings

Suppose we have a string s = “Apple”. The task is to change it to “Rpple”, i.e., instead of the letter ‘A’; we have to use the letter ‘R’. In c++, we can easily achieve the task. At the index 0, put the letter R, i.e., S[0] = ‘R’. But it is not possible in Java. Because strings are immutable. Therefore, we need to find a workaround to do the manipulation. One way is to use a character array. Observe the following Java program.

Using Character Array

 FileName: StringExample4.java

Output:

Explanation: We know that string is immutable in Java. However, the same is not true for arrays. This is why we need to generate a character array from the given input string. The above program does the same. First, the program allocates the size of the character array and then fills the array by scanning each and every character of the given string. In order to retrieve characters from the input array, the method charAt() is used. The method is defined in the class String. It accepts an int value as in its argument and returns the character present at that particular index. The signature of the method is:

If the value of the index is negative or greater than the size of the array, the method throws StringIndexOutOfBoundsException.

After assigning all the values to the character array, we update the value present at the index 0. Then, using the character array, we have created a string and assigned its reference to the same variable that was holding the input string. It means that the input string has no reference. Thus, the input string is ready for garbage collection. Eventually, we are displaying the result. Note that, at no place, we have manipulated the input string. We have to create a completely new string to get our result.

Using the replace() Method

Another approach is to use the predefined method replace(). The method is present in the class String. Observe the following Java program.

FileName: StringExample5.java

Output:

Explanation: Instead of creating a character array, we have used the method replace() to accomplish our task. The replace() method, used in our code, takes two arguments: one is the character we want to replace (oldChar), and another is that character that replaces the oldChar (newChar). The signature of the replace() method is:

public String replace(char oldChar, char newChar)

From the signature, it is obvious that the replace() method returns a string. The replace() method creates a character array on the basis of the string upon which the method is called. In our code, str is calling the method. Thus, a character array is created of size equal to the length of string referred by str, which is 5. Then, all the occurrence of letter ‘A’ is replaced by ‘R’. From the character array a new sting a generated (similar to the previous example). The newly generated string is then returned, whose reference is being stored in the variable strFinal. Eventually, we are displaying the result. Thus, we see that even though we are using an in-built method, the immutability of the string remains intact.

Pattern Searching Program

In pattern searching, a word/ pattern and a text are given. The task is to find whether the given pattern is present in the text or not. For example, text = “llalalaland” and pattern = “land”. The pattern “land” is present in the text “llalalaland” at index 7. Observe the following code.

FileName: StringExample7.java

Output:

Explanation: In the above program, we have used nested Java for-loop for finding the pattern. Inside the for loop, we have used the two pointers approach. One pointer is the outer loop variable i, for the given text, and another pointer is the inner loop variable j, for the given pattern. In each iteration of the outer loop, we assign the value of i to the variable temp. Because i marks the index, where we have to start the search of pattern in the text. Till this point, temp and i both point to the same index. Then, we enter the inner loop, where j is pointing to the first index of the pattern (j = 0).

Now, the comparison of characters of the text and the pattern starts. If the character pointed by temp matches with the character pointed by the j if-block of the inner loop gets executed. The variables j and temp are incremented by 1, and the flag isPatternFound is true. In the case of mismatch, the else block is executed, where the flag isPatternFound is assigned false value, and the inner loop is terminated.

Beneath the inner loop, there is an if block. The if block only gets executed if isPatternFound is true, which is only possible when the else block of the inner for loop is not executed at all. It means that the inner loop only leaves true value to isPatternFound when there is no mismatch. Inside the if-block, the index stores the value of the outer loop variable i, as i is the beginning index of the pattern matching in the text.

Let’s understand each iteration of the outer for-loop with the help of following steps.

For the 1st iteration of the outer loop,

i = 0, temp = 0, j = 0

llalalaland

land

The first character of the text matches with the first character of the pattern. Hence, the inner loop iterates once more.

i = 0, temp = 1, j = 1

llalalaland

land

The second character of the text does not match with the second character of the pattern. Hence, the else block of the inner block is executed, and the inner loop is terminated.

For the 2nd iteration of the outer loop

i = 1, temp = 1, j = 0

llalalaland

 land

The second character of the text matches with the first character of the pattern. Hence, the iteration continues of the inner loop.

i = 1, temp = 2, j = 1

llalalaland

 land

The second character of the pattern also matches. Therefore, temp and j again get incremented by 1.

i = 1, temp = 3, j = 2

llalalaland

 land

Now, we encounter a mismatch. Hence, the inner loop is terminated. Still, we did not find the pattern in the text. Therefore, the search continues.

For the 3rd iteration of the outer loop,

i = 2, temp = 2, j = 0

llalalaland

  land

In the first iteration of the inner loop, we get a mismatch. Hence, the inner loop terminates.

For the 4th iteration of the outer loop,

i = 3, temp = 3, j = 0

llalalaland

    land

The fourth character of the text matches with the first character of the pattern. Therefore, iteration of the inner loop continues.

i = 3, temp = 4, j = 1

llalalaland

    land

Again, the characters match. The inner loop iterates one more time.

i = 3, temp = 5, j = 2

llalalaland

    land

l ? n. Therefore, because of the mismatch, the inner loop terminates.

For the 5th iteration of the outer loop,

i = 4, temp = 4, j = 0

llalalaland

     land

a ? l. Hence, the inner loop does not iterate further.

For the 6th iteration of the outer loop,

i = 5, temp = 5, j = 0

llalalaland

       land

l = l. Therefore, temp and j are incremented by 1.

i = 5, temp = 6, j = 1

llalalaland

       land

Again, there is a match. Thus, we move to the next iteration of the inner for-loop.

i = 5, temp = 7, j = 2

llalalaland

       land

l ? n.  Because of the mismatch, the inner loop terminates.

For the 7th iteration of the outer loop,

i = 6, temp = 6, j = 0

llalalaland

         land

There is a mismatch. The control moves to the outer loop, again.

For the 8th iteration of the outer loop,

i = 7, temp = 7, j = 0

llalalaland

          land

The characters match. The inner loop iterates.

i = 7, temp = 8, j = 1

llalalaland

          land

Again, there is a match. The inner loop comes into the picture again.

i = 7, temp = 9, j = 2

llalalaland

          land

n = n. Still, there is a match. The inner loop iterates one more time

i = 7, temp = 10, j = 3

llalalaland

          land

The last character of the pattern matches and the inner and outer loop terminates. We see at i = 7; the successful search has occurred. Therefore, “index 7” is printed on the console. Therefore, we have successfully searched our pattern in the given text.

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