How do I check if a Java String contains at least one capital letter, lowercase letter, and number?

For a simple string check, a single sweep through the string is enough. Since Regex will not offer any significant benefit, here is a simple for loop to achieve the same :

private static boolean checkString(String str) {
    char ch;
    boolean capitalFlag = false;
    boolean lowerCaseFlag = false;
    boolean numberFlag = false;
    for(int i=0;i < str.length();i++) {
        ch = str.charAt(i);
        if( Character.isDigit(ch)) {
            numberFlag = true;
        }
        else if (Character.isUpperCase(ch)) {
            capitalFlag = true;
        } else if (Character.isLowerCase(ch)) {
            lowerCaseFlag = true;
        }
        if(numberFlag && capitalFlag && lowerCaseFlag)
            return true;
    }
    return false;
}

Test run:

System.out.println(checkString("aBCd1")); // output is true
System.out.println(checkString("abcd")); //output is false

I think this should help OP's particular problem.

Example of @goshDeveloper's answer.

First create a Pattern variable with regular expression you want.

public final Pattern textPattern = Pattern.compile("^(?=.*[a-z])(?=.*[A-Z])(?=.*\\d).+$");

Second you can use it like this:

public boolean isTextValid(String textToCheck) {
return textPattern.matcher(textToCheck).matches();
}

Try regular expression

^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).+$

descriptions are as follow
(?=.*[a-z])  -- check lower case letter
(?=.*[A-Z]) -- check upper case letter
(?=.*\d) -- check one digit exists