# Strings — Core Java

Source: https://www.geekswithgeeks.com/en/core-java/java-strings

> See why Java Strings are immutable, why you compare them with equals() rather than ==, and how StringBuilder avoids O(n²) concatenation in loops.

## Strings are immutable

Every method that seems to "change" a `String` (`toUpperCase`, `replace`, `substring`, `trim`) returns a **new** object; the original is untouched. This makes strings safe to share but wasteful in loops.

## Compare with equals()

`==` tests whether two references point to the same object. To compare **contents**, use `.equals()` (or `.equalsIgnoreCase()`).

```java
String a = "hi";
String b = new String("hi");
System.out.println(a == b);        // false
System.out.println(a.equals(b));   // true
```

## StringBuilder for loops

Concatenating with `+` inside a loop creates a new string each iteration — O(n²). Use a `StringBuilder` and call `toString()` once at the end.

```java
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 5; i++) sb.append(i).append(',');
String csv = sb.toString();   // "0,1,2,3,4,"
```