String isBlank() Java with Examples

String isBlank() Java with Examples | In the String class of Java, the isBlank() method is given to check whether the given String is blank or not. A string is treated as blank if it doesn’t contain any character or contains only blank spaces.

The internal implementation of String isBlank() Java Method,

public boolean isBlank() {
   return indexOfNonWhitespace() == length();
}

Method declaration,

public boolean isBlank()

Return values of isBlank() method,

true:- If the string is empty or contains only whitespace (space, tab).
false:- If the string is not empty and contains valid characters except whitespace.

This method was given in the Java 11 version. This method returns true if String doesn’t have characters or it has only spaces/tabs. Otherwise, it returns false.

Program to demonstrate String isBlank() Java method

public class Test {
   public static void main(String[] args) {
      String s1 = "";
      System.out.println(s1.isBlank());

      String s2 = " ";
      System.out.println(s2.isBlank());

      String s3 = "Program";
      System.out.println(s3.isBlank());

      String s4 = new String();
      System.out.println(s4.isBlank());

      String s5 = new String("");
      System.out.println(s5.isBlank());

      String s6 = new String(" ");
      System.out.println(s6.isBlank());

      String s7 = new String("Program");
      System.out.println(s7.isBlank());
   }
}

Output:-

true
true
false
true
true
true
false

This first string literal "" doesn’t contain any valid characters, and contains only ‘\0’ therefore the length of this string is zero (0). Hence s1.isBlank() method returns true. 

The string literal " " contains one space, and the length of the string is 1. But the isBlank() method returns true when the string contains only whitespace. Therefore the s2.isBlank() method returns true.

In the next line, the string literal “Program” contains 7 valid characters. Since the length of the string is greater than 0 and doesn’t contain only white space, therefore, the s3.isBlank() method returns false.

From the above example,

ValueisBlank()
""true
" "true
“Program”false
new String();true
new String("");true
new String(" ");true
new String(“Program”);false

Also see:- isBlank() vs isEmpty() in Java StringDifferent ways to check if String is Empty in Java

Let us see another program to demonstrate String isBlank() Java method by taking String input values from the end-user through the Scanner class,

import java.util.Scanner;

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

      Scanner scan = new Scanner(System.in);
      
      System.out.println("Enter three Strings in three lines,");
      String s1 = scan.nextLine(); // directly press enter key
      String s2 = scan.nextLine(); // enter blank spaces or tabs
      String s3 = scan.nextLine(); // enter normal string
      
      System.out.println(s1.isBlank() + " " + s1.length());
      System.out.println(s2.isBlank() + " " + s2.length());
      System.out.println(s3.isBlank() + " " + s3.length());
   }
}

Output:-

Enter three Strings in three lines,
->(Here directly enter key was pressed)
->(Here one space was entered)
KnowProgram
true 0
true 1
false 11

In this program, The string s1 is an empty string and it is similar to the previous program String s5 = new String(""); Notice that it is different from String s4 = new String();

MCQ on String isBlank() Java Method

Based on the above-discussed points let us solve some quick questions on the String isBlank() Java method. Find the outputs of the below all Java programs.

Q1) Find the output of the below Java program?

package com.kp.test;
public class Q1 {
   public static void main(String[] args) {
      String s1 = "     "; // five spaces
      System.out.println(s1.isBlank());
   }
}

a) true
b) false
c) Compile time error
d) NullPointerException

View Answer Answer:- a) true

Explanation:- The string literal is a valid string of length 5, but it contains only whitespaces, therefore, the isBlank() method returns true.

Q2) Find the output of the below Java program?

package com.kp.test;
public class Q2 {
   public static void main(String[] args) {
      String s2 = "Know Program"; 
      System.out.println(s2.isBlank());
   }
}

a) true
b) false
c) Compile time error
d) NullPointerException

View Answer Answer:- b) false

Explanation:- The string literal “Know Program” is a valid string of length 12, and doesn’t contain only white spaces therefore isBlank() method returns false. Note that if any string literal contains spaces with other characters then the isBlank() method will not return true, but if string literal contains only white spaces then it will return true.

Q3) Find the output of the below Java program?

package com.kp.test;
public class Q3 {
   public static String s3;
   public static void main(String[] args) { 
      System.out.println(s3.isBlank());
   }
}

a) true
b) false
c) Compile time error
d) NullPointerException

View Answer Answer:- d) NullPointerException

Explanation:- If the string is not initialized then we can’t perform any operation on it because there is not any String object available here. Due to this reason, we will get an Exception in thread “main” java.lang.NullPointerException: Cannot invoke “String.isBlank()” because “com.kp.test.Test.s3” is null at com.kp.test.Test.main(Test.java:6)

Q4) Find the output of the below Java program?

package com.kp.test;
public class Q4 {
   public static void main(String[] args) {
      String s4;
      System.out.println(s4.isBlank());
   }
}

a) true
b) false
c) Compile time error
d) NullPointerException

View Answer Answer:- c) Compile time error

Explanation:- In this program, before running compile time itself gets an error because the local variable is not initialized. All the local variables must be initialized before using it. Hence we get a compile-time error: variable s4 might not have been initialized.

Q5) Find the output of the below Java program?

package com.kp.test;
public class Q5 {
   public static void main(String[] args) {
      String s5 = null;
      System.out.println(s5.isBlank());
   }
}

a) true
b) false
c) Compile time error
d) NullPointerException

View Answer Answer:- d) NullPointerException

Explanation:- The string is not initialized, and we are trying to call the isBlank() method with null reference. Due to this reason, we will get a runtime error:- java.lang.NullPointerException

Q6) Find the output of the below Java program?

package com.kp.test;
public class Q6 {
   public static void main(String[] args) {
      String s6 = "null";
      System.out.println(s6.isBlank());
   }
}

a) true
b) false
c) Compile time error
d) NullPointerException

View Answer Answer:- b) false

Explanation:- In this program, “null” is a String literal with characters ‘n’, ‘u’, ‘l’, ‘l’. It is a simple string like “java”, “Know Program”. Hence s6.isBlank() gives the result as false.

The null and “null” are completely different things. The string s5 has null, it is a reference that can be used for initializing any reference variable. The compiler will not give any error if the string is not null but JVM will give an exception when we try to excess string data. String reference is null so JVM gives an exception. 

But “null” is a String literal with 4 characters ‘n’, ‘u’, ‘l’, ‘l’. It is a simple string literal like “Java”, “program”, “learn”, and e.t.c. Hence s6.isBlank() gives the result as false in Q6.

Also see:-

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