...
| Code Block | ||
|---|---|---|
| ||
public class ExampleComparisonStringComparison { public static void main(String[] args) { String str1 = new String("one"); String str2 = new String("one"); System.out.println(isEqual( str1, str2)); } public static boolean isEqual(String str1, String str2) { boolean result; // test for null is redundant in this case, but required for full generality if (str1 == null) { result = str2 == null; } else { result = str1 == str2; } return System.out.println(result); // prints false! } } |
Compliant Solution (Object.equals())
...
| Code Block | ||
|---|---|---|
| ||
public class GoodComparisonStringComparison { public static voidboolean main(String[] args) { isEqual(String str1, = new String("one"); String str2 = new String("one");) { boolean result; // test for null is redundant in this case, but required for full generality if (str1 == null) { result = (str2 == null); } else { result = str1.equals(str2); } return System.out.println(result); // prints true } } |
Compliant Solution (String.intern())
...
| Code Block | ||
|---|---|---|
| ||
public class GoodComparisonStringComparison { public static void main(String[] args) { String str1 = new String("one"); String str2 = new String("one"); boolean result; // test for null is redundant in this case, but required for full generality if (str1 != null) { str1 = str1.intern(); } // test for null is redundant in this case, but required for full generality if (str2 != null) { str2 = str2.intern(); } result = System.out.println(str1 == str2; System.out.println(result); // prints true } } |
Use of String.intern() should be reserved for cases where tokenization of strings either yields an important performance enhancement or dramatically simplifies code. The performance and readability is often improved by the use of code that applies the Object.equals() approach and lacks any dependence on reference equality.
The performance issue arises because:
- the The cost of
String.intern()grows as the number of intern strings grows. Performance should be no worse than n log n, but the JLS makes no guarantee. - strings Strings that have been interned become immortal; they cannot be garbage collected. This can be problematic when large numbers of strings are interned.
...