String is an immutable object, not a primitive. Literals are interned in the string pool, so == sometimes “works” — and that is exactly the trap.

Creating strings

Form	Where it lives	Notes
`String a = "hi";`	String pool (interned)	Identical literals in the whole JVM share one object
`String b = new String("hi");`	A new heap object	Always a distinct object — almost never what you want
`"h" + "i"`	Pool	Constant expression — folded to `"hi"` by the compiler
`s1 + s2` (non-final variables)	New heap object	Built at runtime (via `StringConcatFactory` since Java 9)
`b.intern()`	Pool	Returns the pooled copy (adds it if missing)

`==` vs `equals()`

Java

public class Main {
    public static void main(String[] args) {
        String a = "java";
        String b = "java";
        String c = new String("java");
        String d = "ja" + "va";          // compile-time constant → pooled
        String part = "ja";
        String e = part + "va";          // runtime concatenation → new object

        System.out.println(a == b);          // same pooled object
        System.out.println(a == c);          // different objects
        System.out.println(a.equals(c));     // same characters
        System.out.println(a == d);
        System.out.println(a == e);
        System.out.println(a == e.intern());
    }
}

Output

true
false
true
true
false
true

Immutability in action

Java

public class Main {
    public static void main(String[] args) {
        String s = "hello";
        s.toUpperCase();                 // result discarded — s is unchanged
        System.out.println(s);
        s = s.toUpperCase();             // rebind the variable to the new object
        System.out.println(s);

        String t = s;
        s = s + "!";                     // creates a new String; t still points at the old one
        System.out.println(t + " " + s);
    }
}

Output

hello
HELLO
HELLO HELLO!

Why is String immutable?

Concatenation in loops

Each += on a String allocates a new object and copies all characters so far — O(n²) for a loop of n appends. Use StringBuilder (next topics) or String.join / Collectors.joining.

Java

public class Main {
    public static void main(String[] args) {
        String slow = "";
        for (int i = 0; i < 5; i++) slow += i;      // 5 new String objects

        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 5; i++) sb.append(i);   // one growing buffer
        System.out.println(slow + " " + sb);
        System.out.println(String.join("-", "a", "b", "c"));
    }
}

Output

01234 01234
a-b-c

How many objects does new String("abc") create?

Where is the string pool located?

Why is char[] preferred over String for passwords?

Creating strings

== vs equals()

Immutability in action

Why is String immutable?

Concatenation in loops

`==` vs `equals()`