Which of these method of class string is used to remove leading and trailing whitespaces

Overview

In Java, removing whitespace from a given String is a common need when manipulating strings. This article will discuss eliminating all or a portion of the existing whitespace in the string. We'll look at numerous built-in Java methods to accomplish the task of remove whitespaces from a string:

• trim() method
• strip() method
• stripLeading() method
• stripTrailing() method
• replaceAll() method

Scope

The article aims to:

• Discuss various methods to remove whitespaces in strings.

Introduction

Suppose you have to display a list of strings, but each of these strings has some spaces at the end, added mistakenly. This needs to be removed before displaying. Likewise, there can be spaces in between or at the beginning of the strings as well.

We shall be looking at Java programs to remove these whitespaces. Also, Java provides numerous in-built methods to perform this task. To list a few:

• trim(): It removes the leading and trailing spaces present in the string. You can see this in the image below:
  

• strip(): It removes the leading and trailing spaces present in the string. Also, it is Uniset/Unicode character aware. We shall learn more about it in a section ahead.
• stripLeading(): It removes the spaces present at the beginning of the string.
• stripTrailing(): It removes the spaces present at the end of the string.
• replaceAll(): It replaces all the matched substrings with a new string.

Let us discuss each of these methods to remove unnecessary spaces from a String one by one.

trim() method in Java

This is the most common method to remove white spaces present at the beginning and end of the string. For trim() method, space character is a character having ASCII values <= 32. The Unicode value of space character is '\u0020'. It removes all characters between '\u0000' (NUL) and '\u0020' (SPACE) from the start and end of the string. It checks for these Unicode values at the beginning and end of the string. If present, it removes the spaces and returns the modified string.

Signature

trim() method is defined as:

Syntax

trim() method is written as

Parameters

trim() method takes no parameters.

Return Values

It returns the modified version of the original string, with no spaces at the end and beginning.

Exceptions

It throws

• NullPointerException: If the string on which method is called is null.

Usage of trim() method

Program to Remove Leading and Trailing Whitespaces

class Main {
  public static void main(String args[]) {
    // trims the trailing and leading spaces
    //string with single trailing and leading spaces
    String s = " Hey! Let us learn Java! ";
    System.out.println(s.trim());
      
    //string with multiple trailing and leading spaces
    String s1 = "    Learning Java is fun!   ";
    System.out.println(s1.trim());
  }
}

Output:

Hey! Let us learn Java!
Learning Java is fun!

Explanation:

The trim() method is used in the program above to trim the strings s and s1. In contrast to s1, which contains numerous spaces, s only has one space at the beginning and one at the end. These spaces are eliminated once the method trim() is called and the output is printed.

Program where exception is thrown:

Let us look at an example where we call trim() method on a null string.

Code:

class Main {
  public static void main(String args[]) {     
    // trims the trailing and leading spaces
    String s = null;
    System.out.println(s.trim());
  }
}

Output:

Exception in thread "main" java.lang.NullPointerException at Main.main(Main.java:10)

Explanation:

In the above program, the value of the string is null; hence exception is thrown.

strip() method Java 11

The strip() method was introduced in Java 11. It also removes leading and trailing spaces present in the string. But that is exactly what trim() does. What's the difference between the two?

strip() method is Unicode aware, a modification over trim().The reason behind the addition of this method is that there exist numerous space characters as per the Unicode standards having ASCII value > 32(‘U+0020’); the strip() method can remove them as well.

Signature

strip() method is defined as:

Syntax

strip() method is written as

Parameters

strip() method takes no parameters.

Return Values

It returns the modified version of the original string, i.e., with no spaces at the end and beginning.

Exceptions

It throws

• NullPointerException: If the string on which method is called is null.

Usage of strip() method

Program to Remove Leading and Trailing Whitespaces

public class Main {
  public static void main(String[] args) {
    // string with whitespace characters as per unicode values
    String str1 = '\u2003' + "Learn Java ";

    System.out.println("Original String:" + "==" + str1 + "==");

    System.out.println("with trim method: " + "==" + str1.trim() + "==");

    System.out.println("with strip method: " + "==" + str1.strip() + "==");

  }
}

Output:

Original String: == Learn Java ==
with trim method: == Learn Java==
with strip method: ==Learn Java==

Explanation:

In the above program, we have appended a whitespace character with Unicode value: ‘\u2003’. So as you can see, the trim() method does not remove it as the value is not less than '\u0020'. Whereas in the case of strip(), it gets removed. This pretty much explains the advantage of strip() over trim().

stripLeading() method Java 11

stripLeading() method removes all the leading spaces or the spaces present at the beginning of the string. It was introduced in Java 11. Just like strip(), it can remove the space characters as per the Unicode standards having ASCII value greater than 32(‘\u0020’) as well.

Signature

stripLeading() method is defined as:

public String stripLeading()

Syntax

stripLeading() method is written as

Parameters

stripLeading() method takes no parameters.

Return Values

It returns the modified version of the original string, i.e., with no spaces at the beginning. Exceptions: It throws

• NullPointerException: If the string on which method is called is null.

Usage of stripLeading() method

Program to Remove Leading Whitespaces

public class Main {
  public static void main(String[] args) {

    //Original String
    String str1 = " Learn Java ";
    System.out.println("String 1:" + "==" + str1 + "==");

    // string with whitespace character as per Unicode values
    String str2 = '\u2003' + "Learn Java ";

    // trims the  leading spaces
    System.out.println("String 2:" + "==" + str2 + "==");
    System.out.println("modified string 1: " + "==" + str1.stripLeading() + "==");
    System.out.println("modified string 2: " + "==" + str2.stripLeading() + "==");
  }
}

Output:

String 1: == Learn Java ==
String 2: == Learn Java ==
modified string 1: ==Learn Java ==
modified string 2: ==Learn Java ==

Explanation:

In the above program, the stripLeading() method is called on two strings. One of these has a space character with a codepoint value > 32(‘\u0020’). As you can see, in both cases, the spaces present at the beginning get removed; however, the ones at the end are still present.

stripTrailing() method Java 11

This method is exactly the opposite of the stripLeading() method. It removes all the trailing or the spaces present at the end of the string. It was also introduced in Java11. See the image below for reference:

Signature

stripTrailing() method is defined as:

public String stripTrailing()

Syntax

stripTrailing() method is written as

Parameters

stripTrailing method takes no parameters.

Return Values

It returns the modified string with no spaces at the end.

Exceptions

It throws

• NullPointerException: If the string on which method is called is null.

Usage of stripTrailing() method

Program to Remove Trailing Whitespaces

public class Main {
  public static void main(String[] args) {
    //Original String
    String str1 = " Learn Java ";

    System.out.println("String 1:" + "==" + str1 + "==");

    // string with whitespace character as per Unicode values
    String str2 = " Learn Java" + '\u2003';
    System.out.println("String 2:" + "==" + str2 + "==");

    // trims the  trailing spaces
    System.out.println("modified string 1: " + "==" + str1.stripTrailing() + "==");
    System.out.println("modified string 2: " + "==" + str2.stripTrailing() + "==");
  }
}

Output:

String 1: == Learn Java ==
String 2: == Learn Java ==
modified string 1: == Learn Java==
modified string 2: == Learn Java==

Explanation:

In the above program stripLeading() method is called on two strings. One of these has a space character with a codepoint value > 32(‘\u0020’). As you can see, in both cases, the spaces present at the end get removed. However, the ones at the beginning are still present.

replaceAll(String regex, String replacement)

replaceAll() is a very commonly used method for string manipulation in Java. This method replaces all the substrings of the string matching the specified regex expression with the specified replacement text.

A regular expression or regex is a string pattern that we use for some searching or matching kind of operations.

We can use this method to remove spaces from the string. All we need to do is specify the regex expression for the kind of spaces that need to be removed, i.e., trailing or leading, or both. The following table shows the regex expressions for the same:

Signature

replaceAll() method is defined as:

  public String replaceAll(String regex, String replacement)

Syntax

replaceAll() method is written as

string.replaceAll(regex, replacement)

Parameters

replaceAll method takes two parameters:

• String regex: The regular expression that is used for matching the substring.
• String replacement: The string that replaces the matched substring.

Return Values

It returns the modified string, where all the occurrences of matched substrings are replaced by the string specified as a parameter.

Exceptions

It throws :

• PatternSyntaxException : If the syntax of the regex is invalid.

Usage of replaceAll() method

Program to Remove All Whitespaces and Leading, Trailing Whitespaces

public class Main {

  public static void main(String[] args) {
    String str1 = "  Learn Java  ";
    //original string
    System.out.println("String 1:" + "==" + str1 + "==");

    System.out.println(
      "Replace all the spaces " + "==" + str1.replaceAll(" ", "") + "=="
    );

    //regex to remove all the spaces
    System.out.println(
      "Replace all using regex  " + "==" + str1.replaceAll("\\s+", "") + "=="
    );

    //regex to remove all the leading spaces
    System.out.println(
      "Replace all the leading spaces: " +
      "==" +
      str1.replaceAll("^\\s+", "") +
      "=="
    );

    //regex to remove all the trailing spaces
    System.out.println(
      "Replace all the trailing spaces: " +
      "==" +
      str1.replaceAll("\\s+$", "") +
      "=="
    );
  }
}

Output

String 1:==  Learn Java  ==
Replace all the spaces ==LearnJava==
Replace all using regex  ==LearnJava==
Replace all the leading spaces: ==Learn Java  ==
Replace all the trailing spaces: ==  Learn Java==

Explanation

In the above program, replaceAll() is called with different regular expressions as parameters to remove leading, trailing, and all the spaces.

Program where Exception is thrown

Code:

public class Main {

  public static void main(String[] args) {
    String str1 = "  Learn Java  ";
    //original string
    System.out.println("String 1:" + "==" + str1 + "==");

    System.out.println(
      "Replace all using regex  " + "==" + str1.replaceAll("[s", "") + "=="
    );
  }
}

Output:

String 1:==  Learn Java  ==
Exception in thread "main" java.util.regex.PatternSyntaxException: Unclosed character class near index 1
[s
 ^
at java.base/java.util.regex.Pattern.error(Pattern.java:2027)
	at java.base/java.util.regex.Pattern.clazz(Pattern.java:2695)
at java.base/java.util.regex.Pattern.sequence(Pattern.java:2138)
	at java.base/java.util.regex.Pattern.expr(Pattern.java:2068)
	at java.base/java.util.regex.Pattern.compile(Pattern.java:1782)
at java.base/java.util.regex.Pattern.<init>(Pattern.java:1428)
	at java.base/java.util.regex.Pattern.compile(Pattern.java:1068)
at java.base/java.lang.String.replaceAll(String.java:2126)
	at Main.main(Main.java:9)

Explanation:

So in the above program, the syntax defined for regex is incorrect; hence exception is thrown.

Note: We can not directly use "" in Java, escape character is required. Thus, "\" is used instead.

Conclusion

• Java has inbuilt methods to remove the whitespaces from the string, whether leading, trailing, both, or all.
• trim() method removes the leading and trailing spaces present in the string.
• strip() method removes the leading and trailing spaces present in the string. Also, it is Uniset/Unicode character aware.
• stripleading() method removes the spaces present at the beginning of the string.
• striptrailing() method removes the spaces present at the end of the string.
• replaceall(String regex, String replacement) method replaces all the matched substring with a new string.