String.equals() / equalsIgnoreCase() / compareTo()

// Checks whether two strings have the same content.
string.equals(Object anObject);

// Compares two strings, ignoring case differences.
string.equalsIgnoreCase(String anotherString);

// Compares strings lexicographically and returns an integer indicating the difference.
string.compareTo(String anotherString);

// Compares strings lexicographically, ignoring case differences.
string.compareToIgnoreCase(String str);

// Use equals() to compare string contents.
String a = "Java";
String b = "Java";
System.out.println(a.equals(b)); // Prints "true".

// == compares references and may produce unexpected results.
String c = new String("Java");
System.out.println(a == c);      // Prints "false".
System.out.println(a.equals(c)); // Prints "true".

// Use equalsIgnoreCase() to compare strings without regard to case.
System.out.println("JAVA".equalsIgnoreCase("java")); // Prints "true".

// Check for null before calling equals() to avoid a NullPointerException.
String input = null;
System.out.println("Java".equals(input)); // Prints "false". (Avoids NullPointerException)

// Use compareTo() to compare strings lexicographically.
System.out.println("apple".compareTo("banana")); // Prints a negative value. ("apple" comes before "banana")
System.out.println("Java".compareTo("Java"));    // Prints "0".

// compareTo() can be used as a sorting criterion.
java.util.List<String> langs = new java.util.ArrayList<>(java.util.Arrays.asList("Python", "Java", "C", "Swift"));
langs.sort(String::compareTo);
System.out.println(langs); // Prints the list sorted in lexicographic order.

When comparing strings in Java, always use equals() instead of ==. == compares object references (memory addresses), so two strings with identical content created with new String() will return false. Using == with string literals may coincidentally return true due to the string pool mechanism, but you should not rely on this behavior intentionally.

If there is any chance the calling object is null, calling null.equals(...) will throw a NullPointerException. To avoid this, place the string literal on the left side (e.g., "Java".equals(input)) or use Objects.equals(a, b).

To convert string case, use toUpperCase() / toLowerCase(). To convert between strings and numbers, use String.valueOf().