Creating Strings

String a = "hello";              // string literal (most common)
String b = new String("hello");  // constructor (less common)

Strings Are Immutable

Once created, a String object’s character sequence cannot change. Every String method that “modifies” a String actually returns a new String object; the original is untouched.

String s = "hello";
s.toUpperCase();           // returns new String "HELLO" -- s unchanged
System.out.println(s);     // prints: hello
s = s.toUpperCase();       // now s points to the new "HELLO" object
System.out.println(s);     // prints: HELLO

The + and += Operators

Two Strings joined with + produce a new String. A primitive concatenated with a String is automatically converted to its String representation.

String name = "Java";
int version = 17;
String result = name + " " + version;  // "Java 17"
result += "!";                          // "Java 17!"

length()

int length() — returns the number of characters.

"computer".length()   // 8
"".length()           // 0

substring(from, to) and substring(from)

String substring(int from, int to) — returns characters at indices from through to - 1. The to index is exclusive. Length of result = to - from.

String substring(int from) — returns characters from from to the end. Equivalent to substring(from, length()).

"computer".substring(0, 4)  // "comp"  (indices 0,1,2,3)
"computer".substring(4)     // "uter"  (indices 4,5,6,7)
"computer".substring(3, 4)  // "p"     (single char at index 3)

indexOf(str)

int indexOf(String str) — returns the index of the first occurrence of str in the String. Returns -1 if not found.

"computer".indexOf("put")   // 3
"computer".indexOf("xyz")   // -1
"abcabc".indexOf("bc")      // 1  (first occurrence)

equals(other)

boolean equals(Object other) — returns true if the two Strings have the same character sequence, case-sensitive.

"hello".equals("hello")   // true
"hello".equals("Hello")   // false (case matters)
"hello".equals(null)      // false (does not throw NPE)

compareTo(other)

int compareTo(String other) — compares lexicographically (dictionary order).

Returns a value < 0 if this comes before other.
Returns 0 if the strings are equal.
Returns a value > 0 if this comes after other.

"apple".compareTo("banana")   // negative (a before b)
"cat".compareTo("cat")        // 0
"zebra".compareTo("ant")      // positive (z after a)

Uppercase letters come before lowercase in Unicode: "A".compareTo("a") is negative.