16 Java Coding Best Practices for Performance and Clean Code

1. Avoid "where 1=1" in MyBatis

Using where 1=1 can prevent the database from using indexes, leading to full‑table scans and potential SQL‑injection risks.

Incorrect example:

<select id="queryBookInfo" parameterType="com.tjt.platform.entity.BookInfo" resultType="java.lang.Integer">
  select count(*) from t_rule_BookInfo t where 1=1
  <if test="title !=null and title !='' ">
    AND title = #{title}
  </if>
  <if test="author !=null and author !='' ">
    AND author = #{author}
  </if>
</select>

Correct example (use <where> tag):

<select id="queryBookInfo" parameterType="com.tjt.platform.entity.BookInfo" resultType="java.lang.Integer">
  select count(*) from t_rule_BookInfo t
  <where>
    <if test="title !=null and title !='' ">
      AND title = #{title}
    </if>
    <if test="author !=null and author !='' ">
      AND author = #{author}
    </if>
  </where>
</select>

2. Iterate Maps with entrySet() when both key and value are needed

Iterating over keySet() and then calling get() incurs extra lookups; entrySet() is more efficient.

Incorrect example:

// Map get value – anti‑pattern
HashMap<String, String> map = new HashMap<>();
for (String key : map.keySet()) {
    String value = map.get(key);
}

Correct example:

// Map get key & value – proper usage
HashMap<String, String> map = new HashMap<>();
for (Map.Entry<String, String> entry : map.entrySet()) {
    String key = entry.getKey();
    String value = entry.getValue();
}

3. Use Collection.isEmpty() instead of size() == 0

isEmpty()

is O(1) and makes intent clearer.

Incorrect example:

LinkedList<Object> collection = new LinkedList<>();
if (collection.size() == 0) {
    System.out.println("collection is empty.");
}

Correct example:

LinkedList<Object> collection = new LinkedList<>();
if (collection.isEmpty()) {
    System.out.println("collection is empty.");
}
// Null‑safe check using Spring's CollectionUtils
if (CollectionUtils.isEmpty(collection)) {
    System.out.println("collection is null.");
}

4. Pre‑size collections when possible

Specifying an initial capacity reduces costly resizing operations.

Incorrect example:

int[] arr = new int[]{1,2,3,4};
List<Integer> list = new ArrayList<>();
for (int i : arr) {
    list.add(i);
}

Correct example:

int[] arr = new int[]{1,2,3,4};
List<Integer> list = new ArrayList<>(arr.length);
for (int i : arr) {
    list.add(i);
}

5. Use StringBuilder for string concatenation inside loops

Plain += concatenation cannot be optimized inside loops.

Incorrect example:

String str = "";
for (int i = 0; i < 10; i++) {
    // Java does not optimize this inside a loop
    str += i;
}

Correct example:

String str1 = "Love";
String str2 = "Courage";
String strConcat = str1 + str2; // compile‑time optimization
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 10; i++) {
    sb.append(i);
}

6. Replace frequent List.contains() checks with a Set

Contain checks on a List are O(n); a HashSet provides O(1).

Incorrect example:

List<Object> list = new ArrayList<>();
for (int i = 0; i <= Integer.MAX_VALUE; i++) {
    // O(n) lookup
    if (list.contains(i)) {
        System.out.println("list contains " + i);
    }
}

Correct example:

List<Object> list = new ArrayList<>();
Set<Object> set = new HashSet<>();
for (int i = 0; i <= Integer.MAX_VALUE; i++) {
    // O(1) lookup
    if (set.contains(i)) {
        System.out.println("list contains " + i);
    }
}

7. Initialise static collections in a static block, not via anonymous inner classes

Using an anonymous subclass for initialisation adds hidden references and can cause memory leaks.

Incorrect example:

private static Map<String, Integer> map = new HashMap<String, Integer>(){
    {
        map.put("Leo",1);
        map.put("Family-loving",2);
        map.put("Cold on the out side passionate on the inside",3);
    }
};
private static List<String> list = new ArrayList<>(){
    {
        list.add("Sagittarius");
        list.add("Charming");
        list.add("Perfectionist");
    }
};

Correct example:

private static Map<String, Integer> map = new HashMap<String, Integer>();
static {
    map.put("Leo",1);
    map.put("Family-loving",2);
    map.put("Cold on the out side passionate on the inside",3);
}
private static List<String> list = new ArrayList<>();
static {
    list.add("Sagittarius");
    list.add("Charming");
    list.add("Perfectionist");
}

8. Hide utility‑class constructors

Utility classes should not be instantiated; declare a private constructor to suppress the default public one.

Incorrect example:

public class PasswordUtils {
    private static final Logger LOG = LoggerFactory.getLogger(PasswordUtils.class);
    public static final String DEFAULT_CRYPT_ALGO = "PBEWithMD5AndDES";
    public static String encryptPassword(String aPassword) throws IOException {
        return new PasswordUtils(aPassword).encrypt();
    }
}

Correct example:

public class PasswordUtils {
    private static final Logger LOG = LoggerFactory.getLogger(PasswordUtils.class);
    // Private constructor prevents instantiation
    private PasswordUtils() {}
    public static final String DEFAULT_CRYPT_ALGO = "PBEWithMD5AndDES";
    public static String encryptPassword(String aPassword) throws IOException {
        return new PasswordUtils(aPassword).encrypt();
    }
}

9. Remove redundant catch blocks that only re‑throw

If the catch block does nothing but re‑throw, it can be omitted.

Incorrect example:

private static String fileReader(String fileName) throws IOException {
    try (BufferedReader reader = new BufferedReader(new FileReader(fileName))) {
        String line;
        StringBuilder builder = new StringBuilder();
        while ((line = reader.readLine()) != null) {
            builder.append(line);
        }
        return builder.toString();
    } catch (Exception e) {
        // merely re‑throwing
        throw e;
    }
}

Correct example:

private static String fileReader(String fileName) throws IOException {
    try (BufferedReader reader = new BufferedReader(new FileReader(fileName))) {
        String line;
        StringBuilder builder = new StringBuilder();
        while ((line = reader.readLine()) != null) {
            builder.append(line);
        }
        return builder.toString();
        // optional: handle other exceptions here
        /*catch (Exception e) {
            return "fileReader exception";
        }*/
    }
}

10. Prefer String.valueOf() over concatenation with an empty string

String.valueOf()

avoids creating intermediate String objects.

Incorrect example:

int num = 520;
String strLove = "" + num;

Correct example:

int num = 520;
String strLove = String.valueOf(num);

11. Avoid new BigDecimal(double) due to precision loss

Construct BigDecimal from String or valueOf instead.

Incorrect example: BigDecimal bigDecimal = new BigDecimal(0.11D); Correct example:

BigDecimal bigDecimal1 = BigDecimal.valueOf(0.11D);

12. Return empty collections/arrays instead of null

Returning empty objects prevents NullPointerException and simplifies caller code.

Incorrect example:

public static Result[] getResults() { return null; }
public static List<Result> getResultList() { return null; }
public static Map<String, Result> getResultMap() { return null; }

Correct example:

public static Result[] getResults() { return new Result[0]; }
public static List<Result> getResultList() { return Collections.emptyList(); }
public static Map<String, Result> getResultMap() { return Collections.emptyMap(); }

13. Call equals() on a constant or use Objects.equals()

Calling equals() on a potentially null variable can throw an exception.

Incorrect example:

private static boolean fileReader(String fileName) throws IOException {
    return fileName.equals("Charming"); // may NPE
}

Correct example:

private static boolean fileReader(String fileName) throws IOException {
    return "Charming".equals(fileName);
    // or
    return Objects.equals("Charming", fileName);
}

14. Make enum fields private and final without setters

Enums should be immutable; expose values via getters only.

Incorrect example:

public enum SwitchStatus {
    DISABLED(0, "禁用"),
    ENABLED(1, "启用");
    public int value;
    private String description;
    private SwitchStatus(int value, String description) {
        this.value = value;
        this.description = description;
    }
    public String getDescription() { return description; }
    public void setDescription(String description) { this.description = description; }
}

Correct example:

public enum SwitchStatus {
    DISABLED(0, "禁用"),
    ENABLED(1, "启用");
    private final int value;
    private final String description;
    private SwitchStatus(int value, String description) {
        this.value = value;
        this.description = description;
    }
    public int getValue() { return value; }
    public String getDescription() { return description; }
}

15. Escape regex metacharacters when using String.split()

Characters like . and | are regex operators and must be escaped.

Incorrect example:

String[] split = "a.ab.abc".split(".");
System.out.println(Arrays.toString(split)); // []
String[] split1 = "a|ab|abc".split("|");
System.out.println(Arrays.toString(split1)); // ["a","|","a","b","|","a","b","c"]

Correct example:

String[] split2 = "a.ab.abc".split("\\.");
System.out.println(Arrays.toString(split2)); // ["a","ab","abc"]
String[] split3 = "a|ab|abc".split("\\|");
System.out.println(Arrays.toString(split3)); // ["a","ab","abc"]

By following these sixteen guidelines, Java developers can write code that is faster, safer, and easier to maintain.