Doing Minus Operation on String in Java

1. Overview

Java’s String class is a fundamental part of the language, widely used for text manipulation. While Java supports various operations on Strings, such as concatenation with the + operator, there’s no direct support for a “minus” operation. This limitation stems from the design of Java’s operator system.

In this article, we’ll explore why the minus operator isn’t supported for Strings, examine the operations Java does support, and provide practical solutions to achieve a “minus-like” functionality.

2. Java String Operators

Java provides limited operator support for Strings. The most common operation is concatenation, achieved using the + operator or the += shorthand. For example:

String greeting = "Hello, " + "World!";

This concatenates “Hello, “ and “World!” into “Hello, World!”. Internally, the + operator leverages the StringBuilder class for efficiency.

However, Java does not support a minus operator (–) for strings. Attempting to use it results in a compilation error:

String result = "Hello, World!" - "World!"; // Compilation error

The reason lies in Java’s design. String operators like – are not defined for String in the language specification. Subtracting one String from another is ambiguous: should it remove a substring, a specific character, or something else? Without a clear semantic definition, Java leaves such operations to explicit methods.

To achieve a “minus” effect, we can use String manipulation methods like substring, replace, or Stream-based approaches. Let’s explore these solutions.

3. Removing Characters from a String

To simulate a minus operation, we can remove characters from a String either by their position or by specifying the characters to exclude. Below, we’ll look at the implementations of those two common scenarios.

3.1. Removing Trailing Character

One simple way to remove a character at a specific position is by using the substring() method. For example, to remove the last character of a String, we can extract a substring from the start to the second-to-last character:

public static String removeLastCharBySubstring(String sentence) {
    return sentence.substring(0, sentence.length() - 1);
}

However, we must ensure the input String is not empty to avoid an IndexOutOfBoundsException. In a production environment, adding input validation is recommended.

To ensure our method works as expected, let’s write unit tests using JUnit and AssertJ:

@Test
public void givenNotBlankString_whenRemovingLastChar_thenReturnOriginalWithoutLastChar() {
    var original = "Don't give up!";

    var result = StringMinusOperations.removeLastCharBySubstring(original);

    assertThat(result)
      .doesNotContain("!")
      .isEqualTo("Don't give up"); // no '!' at the end
}

In the scenario, we pass a sentence “Don’t give up!” to the removeLastCharBySubstring method and expect the same String without the last character ‘!’.

Of course, it would be possible to design an implementation that removes a trailing String instead of ‘just’ a char.

3.2. Removing Specific Characters

The second approach, to remove specific characters or substrings, Java offers several approaches. The replace() method is the most intuitive, while a stream-based solution provides more control at the cost of complexity.

Let’s examine the implementation using String.replace(). This method can remove a single character or an entire substring by replacing it with an empty String. Here are two variants:

public static String minusByReplace(String sentence, char removeMe) {
    return sentence.replace(String.valueOf(removeMe), "");
}

public static String minusByReplace(String sentence, String removeMe) {
    return sentence.replace(removeMe, "");
}

The first method removes all occurrences of a specific character (for example, removing all ‘l‘ characters from “Hello” results in “Heo”). The second method removes all occurrences of a substring (for example, removing “lo” from “Hello” results in “Hel”).

Both are concise and easy to understand, leveraging Java’s built-in String manipulation.

Let’s test them:

 @Test
 public void givenNotBlankString_whenRemovingSpecificChar_thenReturnOriginalWithoutThatChar() {
     var original = "Don't give up!";
     var toRemove = 't';

     var result = StringMinusOperations.minusByReplace(original, toRemove);

     assertThat(result)
       .doesNotContain(String.valueOf(toRemove))
       .isEqualTo("Don' give up!"); // no 't'
 }

Here, we remove all ‘t’ characters from “Don’t give up!” and confirm the result is “Don’ give up!”.

And the test for the second variant, with String as input:

@Test
public void givenNotBlankString_whenRemovingSpecificString_thenReturnOriginalWithoutThatString() {
    var original = "Don't give up!";
    var toRemove = "Don't";

    var result = StringMinusOperations.minusByReplace(original, toRemove);

    assertThat(result)
      .doesNotContain(toRemove)
      .isEqualTo(" give up!"); // no 'Don't'
}

3.3. Custom Solution Using Streams

Sometimes, we have a more complex problem to solve, with multiple filters and mappings in mind.

For more sophisticated scenarios, such as conditional character filtering, we can use Java streams:

public static String minusByStream(String sentence, char removeMe) {
    return sentence.chars()
      .mapToObj(c -> (char) c)
      .filter(it -> !it.equals(removeMe))
      .map(String::valueOf)
      .collect(Collectors.joining());
}

This method converts the String into a Stream of characters, filters out the specified character, and collects the result back into a String.

While this approach is more verbose, it’s highly flexible. For example, you could modify the filter condition to remove characters based on custom logic (removing all vowels, for example). The trade-off is readability and possibly performance. Streams are less efficient than replace for simple tasks, so use them when you need their flexibility.

Let’s test out this implementation:

@Test
 public void givenNotBlankString_whenRemovingSpecificStringByStream_thenReturnOriginalWithoutThatString() {
     var original = "Don't give up!";
     var toRemove = ' ';

     var result = StringMinusOperations.minusByStream(original, toRemove);

     assertThat(result)
       .doesNotContain(String.valueOf(toRemove))
       .isEqualTo("Don'tgiveup!"); // no blanks
 }

Similar to the other test cases, we remove all spaces/blanks from “Don’t give up!” and confirm the result is “Don’tgiveup!”.

4. Removing Trailing String

Finally, let’s look at a way to remove the whole String from the sentence.

The closest implementation to a minus-like operation – removing a trailing String from another String – could look like:

public static String removeTrailingStringBySubstring(String sentence, String lastSequence) {
    var trailing = sentence.substring(sentence.length() - lastSequence.length());
    if (trailing.equals(lastSequence)) {
        return sentence.substring(0, sentence.length() - lastSequence.length());
    } else {
        return sentence;
    }
}

This method first extracts the trailing portion of the String with the same length as lastSequence. If it matches lastSequence, the method returns the String without that trailing part using substring. Otherwise, it returns the original String unchanged.

Let’s see a unit test for it:

@Test
public void givenNotBlankString_whenRemovingLastString_thenReturnOriginalWithoutLastString() {
    var original = "Don't give up!";
    
    var result = StringMinusOperations.removeTrailingStringBySubstring(original, "up!");

    assertThat(result).isEqualTo("Don't give "); 
} 

@Test 
public void givenNotBlankString_whenRemovingLastStringThatDoesNotMatch_thenReturnOriginalString() { 
    var original = "Don't give up!"; 

    var result = StringMinusOperations.removeTrailingStringBySubstring(original, "foo");
 
    assertThat(result).isEqualTo(original); 
}

Both tests cover the two possible outcomes: return original without trailing String, or give it back unchanged.

5. Conclusion

Java’s String class doesn’t support a minus operator due to the lack of a defined semantics for subtraction. However, we can achieve similar functionality using methods like substring for index-based removals and replace or streams for removing specific characters or substrings.

The substring approach is ideal for simple, position-based removals, such as trimming the last character. The replace method excels at removing specific characters or substrings with minimal code. For advanced use cases requiring custom logic, streams offer unparalleled flexibility, though at the cost of complexity.

By combining these techniques and validating them with tests, we can confidently manipulate Strings to achieve the desired “minus” effect.

As always, the entire code used in this article can be found over on GitHub.