Using the string concatenation operator repeatedly to concatenate n strings requires time quadratic in n. It's an unfortunate consequence of fact that strings are immutable(Item 15).

// Inappropriate use of string concatenation - Performs horribly!

public String statement() {

String result = "";

for (int i = 0; i < numItems(); i++)

result += lineForItem(i); // String concatenation

return result;

}

To achieve acceptable performance, use a StringBuilder in place of a String

public String statement() {

StringBuilder b = new StringBuilder(numItems() * LINE_WIDTH);

for (int i = 0; i < numItems(); i++)

b.append(lineForItem(i));

return b.toString();

}

Summary

Don't use the string concatenation operator to combine more than a few strings unless performance is irrelevant. Use StringBuilder's append method instead. Alternatively, use a character array, or process the strings one at a time instead of combining them.