filter(), distinct() and count()

public class PangramChecker {
    private final static int LETTERS_IN_ALPHABET = 26;

    public boolean isPangram(String input) {
        return input.toLowerCase().chars()
            .filter(Character::isLetter)
            .distinct()
            .count() == LETTERS_IN_ALPHABET;
    }
}

This approach starts be defining a private final static value for all 26 letters in the English alphabet. It is private because it does not need to be directly accessed from outside the class, and it is final because its value does not need to be changed once it is set. It is static because it doesn't need to differ between object instantiations, so it can belong to the class itself.

The input String is lowercased and chained into the chars() method. Each character is passed as a primitive int representing its Unicode codepoint to the filter() method. The filter() passes each codepoint to the IsLetter() method to filter in only letter characters.

The the count() of the distinct() surviving codepoints are compared with the number of letters expected, which for the English alphabet is 26.

This works as long as the only letters in the input are in the English alphabet. If the input was missing a but had α, then the distinct letters would be 26 but it would not be a Pangram for English.