String.indexOf() / lastIndexOf() / contains()

// Returns the index of the first match (or -1 if not found).
string.indexOf(String str);
string.indexOf(String str, int fromIndex);

// Returns the index of the last match (or -1 if not found).
string.lastIndexOf(String str);
string.lastIndexOf(String str, int fromIndex);

// Returns whether the string contains the specified sequence.
string.contains(CharSequence s);

// Use indexOf() to get the position of the first occurrence.
String str = "banana";
System.out.println(str.indexOf("an")); // Prints "1".

// Returns -1 if the substring is not found.
System.out.println(str.indexOf("xyz")); // Prints "-1".

// Search starting from a specified position.
System.out.println(str.indexOf("an", 2)); // Prints "3".

// Use lastIndexOf() to get the position of the last occurrence.
String text = "abcabc";
System.out.println(text.lastIndexOf("b")); // Prints "4".

// Use contains() to check whether a substring is present.
String sentence = "Java is an object-oriented language.";
System.out.println(sentence.contains("Java")); // Prints "true".
System.out.println(sentence.contains("Python")); // Prints "false".

// You can also use indexOf() to check for the existence of a substring.
if (str.indexOf("nan") != -1) {
    System.out.println("'nan' was found.");
}

indexOf() returns the index of the first occurrence of the specified substring within the string. Indexes are zero-based, and -1 is returned if the substring is not found. You can pass a starting position as the second argument to begin the search partway through the string.

contains() internally calls indexOf() and is convenient when you want a boolean result rather than a position. Both methods are case-sensitive. If you need a case-insensitive search, convert the string with toLowerCase() or similar before comparing.

To check for matches at the beginning or end of a string, use startsWith() / endsWith(). To extract a substring, use substring().