Java String Tricks: replace vs replaceAll, Integer Caching, BigDecimal Precision

1. replace vs replaceAll – String.replace replaces all literal occurrences, while String.replaceAll treats the first argument as a regular expression and also replaces all matches. replaceFirst replaces only the first occurrence. Example:

<code>String str = "ACDAB$%^&A*Y";
System.out.println(str.replace('A', 'X'));
System.out.println(str.replace("A", "X"));
System.out.println(str.replaceAll("A", "X"));
System.out.println(str.replaceAll("\\*", "XO"));
System.out.println(str.replaceFirst("A", "-"));
</code>

2. Integer caching – Integer.valueOf caches values in the range [-128, 127]; therefore a == b is true for values inside this range but may be false outside it. The cache size can be changed with the JVM option -Djava.lang.Integer.IntegerCache.high=128 . Example output shows the difference for 1, 128, -128, and -129.

<code>Integer a = 1; Integer b = 1; System.out.println(a == b); // true
Integer a = 128; Integer b = 128; System.out.println(a == b); // false (unless cache enlarged)
</code>

3. BigDecimal precision – Creating a BigDecimal directly from a double introduces binary floating‑point errors (e.g., new BigDecimal(0.1) is not exactly 0.1). Use the String constructor or BigDecimal.valueOf for exact values.

<code>BigDecimal a = new BigDecimal(0.02);
BigDecimal b = new BigDecimal(0.03);
System.out.println(b.subtract(a)); // 0.009999… (unexpected)

BigDecimal a2 = new BigDecimal("0.02");
BigDecimal b2 = new BigDecimal("0.03");
System.out.println(b2.subtract(a2)); // 0.01

BigDecimal a3 = BigDecimal.valueOf(0.02);
BigDecimal b3 = BigDecimal.valueOf(0.03);
System.out.println(b3.subtract(a3)); // 0.01
</code>

4. String concatenation – The + operator is compiled to StringBuilder calls since JDK 5, but explicit use of StringBuilder (or StringBuffer when thread‑safety is required) avoids creating many intermediate objects in large concatenations.

<code>String a = "123";
String b = "456";
String c = a + b; // compiled to StringBuilder

StringBuilder sb = new StringBuilder();
sb.append(a).append(b);
System.out.println(sb);
</code>

5. isEmpty vs isBlank – StringUtils.isEmpty returns false for a string containing only spaces, while StringUtils.isBlank returns true. This distinction matters when validating user input.

<code>StringUtils.isEmpty(" ");   // false
StringUtils.isBlank(" ");   // true
</code>

6. Mapper list null check – MyBatis’s DefaultResultSetHandler returns an empty list, not null . Therefore, using CollectionUtils.isNotEmpty(list) is sufficient without an explicit null check.

<code>List<User> list = userMapper.query(search);
if (CollectionUtils.isNotEmpty(list)) {
    List<Long> ids = list.stream().map(User::getId).collect(Collectors.toList());
}
</code>

7. Correct use of indexOf – indexOf returns 0 for a match at the first character; checking > 0 misses such cases. Use >= 0 or contains instead.

<code>String source = "#ABBXXXOOO*pack";
if (source.indexOf("#") >= 0) {
    System.out.println("contains success");
}
</code>