Learn Validating Strings in Java

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When working with text input, users rarely give you exactly what you expect — some leave fields empty, some type only spaces, some forget required symbols. Because of that, before using any string in a program, we often need to validate it first — checking whether it follows certain rules.

We already saw basic tools like isEmpty(), length(), and contains(), but now we'll add a few more that are especially useful in real input validation scenarios.

isBlank()

A very common issue is when a string is not technically empty, but still contains only spaces. For example, a user might "type" a name but only hit the spacebar a few times. .isBlank() handles this by treating whitespace-only strings as invalid input. It returns a boolean.

Main.java


              123456789101112
            
package com.example;

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

       String customerName = "   ";

       boolean isBlankName = customerName.isBlank();

       System.out.println(isBlankName);
   }
}

Even though customerName is not completely empty, it contains only spaces — so .isBlank() returns true, signaling that there is no meaningful content.

startsWith()

Sometimes we need to ensure a string follows a specific structure. For example, every order code in our system might need to start with "ORD". .startsWith() checks the beginning of the string and returns a boolean.

Main.java


              123456789101112
            
package com.example;

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

       String orderCode = "ORD-2459";

       boolean validOrderCode = orderCode.startsWith("ORD");

       System.out.println(validOrderCode);
   }
}

.startsWith("ORD") checks the first three characters of orderCode — since they match, it returns true, confirming the code follows the expected format.

endsWith()

We can also validate the end of a string — especially useful for file names or formats. For example, ensuring a receipt file is actually a PDF. It returns a boolean.

Main.java


              123456789101112
            
package com.example;

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

       String receiptFile = "receipt.pdf";

       boolean validReceiptFile = receiptFile.endsWith(".pdf");

       System.out.println(validReceiptFile);
   }
}

.endsWith(".pdf") checks the last four characters of receiptFile — since they match ".pdf", it returns true.

Combined Validation

Real validation is usually not just one check — it's multiple rules working together. For example, validating a username might require that it is not blank, has at least 5 characters, and contains no spaces:

Main.java


              123456789101112131415
            
package com.example;

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

       String username = "coffeeMaster";

       boolean validUsername =
               !username.isBlank() &&
               username.length() >= 5 &&
               !username.contains(" ");

       System.out.println(validUsername);
   }
}

All three conditions are combined with && — the username must pass every single check to be considered valid. "coffeeMaster" is not blank, has more than 5 characters, and contains no spaces, so validUsername is true.

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