String Immutability & StringBuilder

Why Strings create new objects, and how StringBuilder avoids that • Workbook 2a, p.26-27

Code

          String greeting = "Hello";
          greeting = greeting + " World";
          greeting = greeting.toUpperCase();
        

Click Step to execute each line and watch what happens in memory.

Heap Memory

greeting ➜

Objects created: 0

Key insight: Strings in Java are immutable — once created, a String object can never be changed. Every operation that seems to "modify" a String actually creates a brand new object in memory. The old object is abandoned and eventually garbage collected.

Code

          StringBuilder sb = new StringBuilder("Hello");
          sb.append(" World");
          String result = sb.toString();
        

Click Step to see how StringBuilder works differently from String.

Heap Memory

sb ➜

Objects created: 0

Tip: StringBuilder modifies characters in place — no new objects needed. The internal character array grows as needed, but it is still the same object. When you are done building, call .toString() to get a regular String.

Scenario: Building a Grocery List in a Loop

Watch what happens when we build a string with 5 items — one approach creates many objects, the other stays efficient.

String Concatenation

String list = "";
for (int i = 0; i < items.length; i++) {
list = list + items[i] + "\n";
}

Objects created: 0

StringBuilder

StringBuilder sb = new StringBuilder();
for (int i = 0; i < items.length; i++) {
sb.append(items[i]).append("\n");
}

Objects created: 0

Tip: For simple one-line concatenations like String name = first + " " + last;, regular String is fine. StringBuilder shines when you are building strings in loops or across many operations.

Use the tabs above to explore each concept • Click Step to advance through the animation