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作者：王超，花名麟超，
阿里巴巴高级地图技术工程师，一直从事Java研发相关工作。
导读
私欲日生，如地上尘，一日不扫，便又有一层。着实用功，便见道无终穷，愈探愈深，必使精白无一毫不彻方可。
Map<String, String> map = ...;
for (String key : map.keySet()) {
String value = map.get(key);
...
}
正例：
Map<String, String> map = ...;
for (Map.Entry<String, String> entry : map.entrySet()) {
String key = entry.getKey();
String value = entry.getValue();
...
}
if (collection.size() == 0) {
...
}
if (collection.isEmpty()) {
...
}
List<String> list = new ArrayList<>();
list.add("Hello");
list.add("World");
if (list.containsAll(list)) { // 无意义,总是返回true
...
}
list.removeAll(list); // 性能差, 直接使用clear()
int[] arr = new int[]{1, 2, 3};
List<Integer> list = new ArrayList<>();
for (int i : arr) {
list.add(i);
}
int[] arr = new int[]{1, 2, 3};
List<Integer> list = new ArrayList<>(arr.length);
for (int i : arr) {
list.add(i);}
String s = "";
for (int i = 0; i < 10; i++) {
s += i;
}
String a = "a";
String b = "b";
String c = "c";
String s = a + b + c; // 没问题，java编译器会进行优化
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 10; i++) {
sb.append(i); // 循环中，java编译器无法进行优化，所以要手动使用StringBuilder
}List 的随机访问
// 调用别人的服务获取到list
List<Integer> list = otherService.getList();
if (list instanceof RandomAccess) {
// 内部数组实现，可以随机访问
System.out.println(list.get(list.size() - 1));
} else {
// 内部可能是链表实现，随机访问效率低
}频繁调用 Collection.contains 方法请使用 Set
ArrayList<Integer> list = otherService.getList();
for (int i = 0; i <= Integer.MAX_VALUE; i++) {
// 时间复杂度O(n)
list.contains(i);
}
ArrayList<Integer> list = otherService.getList();
Set<Integer> set = new HashSet(list);
for (int i = 0; i <= Integer.MAX_VALUE; i++) {
// 时间复杂度O(1)
set.contains(i);
}
让代码更优雅
长整型常量后添加大写 L
long value = 1l;
long max = Math.max(1L, 5);
long value = 1L;
long max = Math.max(1L, 5L);不要使用魔法值
for (int i = 0; i < 100; i++){
...
}
if (a == 100) {
...
}
private static final int MAX_COUNT = 100;
for (int i = 0; i < MAX_COUNT; i++){
...
}
if (count == MAX_COUNT) {
...
}
private static Map<String, Integer> map = new HashMap<String, Integer>() {
{
put("a", 1);
put("b", 2);
}
};
private static List<String> list = new ArrayList<String>() {
{
add("a");
add("b");
}
};
private static Map<String, Integer> map = new HashMap<>();
static {
map.put("a", 1);
map.put("b", 2);
};
private static List<String> list = new ArrayList<>();
static {
list.add("a");
list.add("b");
};建议使用 try-with-resources 语句
private void handle(String fileName) {
BufferedReader reader = null;
try {
String line;
reader = new BufferedReader(new FileReader(fileName));
while ((line = reader.readLine()) != null) {
...
}
} catch (Exception e) {
...
} finally {
if (reader != null) {
try {
reader.close();
} catch (IOException e) {
...
}
}
}
}
private void handle(String fileName) {
try (BufferedReader reader = new BufferedReader(new FileReader(fileName))) {
String line;
while ((line = reader.readLine()) != null) {
...
}
} catch (Exception e) {
...
}
}
删除未使用的私有方法和字段
public class DoubleDemo1 {
private int unusedField = 100;
private void unusedMethod() {
...
}
public int sum(int a, int b) {
return a + b;
}
}
public class DoubleDemo1 {
public int sum(int a, int b) {
return a + b;
}
}
删除未使用的局部变量
public int sum(int a, int b) {
int c = 100;
return a + b;
}
public int sum(int a, int b) {
return a + b;
}
删除未使用的方法参数
public int sum(int a, int b, int c) {
return a + b;
}
public int sum(int a, int b) {
return a + b;
}
删除表达式的多余括号
return (x);
return (x + 2);
int x = (y * 3) + 1;
int m = (n * 4 + 2);
return x;
return x + 2;
int x = y * 3 + 1;
int m = n * 4 + 2;
工具类应该屏蔽构造函数
public class MathUtils {
public static final double PI = 3.1415926D;
public static int sum(int a, int b) {
return a + b;
}
}
public class MathUtils {
public static final double PI = 3.1415926D;
private MathUtils() {}
public static int sum(int a, int b) {
return a + b;
}
}删除多余的异常捕获并抛出
private static String readFile(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) {
throw e;
}
}
private static String readFile(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();
}
}
公有静态常量应该通过类访问
public class User {
public static final String CONST_NAME = "name";
...
}
User user = new User();
String nameKey = user.CONST_NAME;
public class User {
public static final String CONST_NAME = "name";
...
}
String nameKey = User.CONST_NAME;
不要用NullPointerException判断空
public String getUserName(User user) {
try {
return user.getName();
} catch (NullPointerException e) {
return null;
}
}
正例：
public String getUserName(User user) {
if (Objects.isNull(user)) {
return null;
}
return user.getName();
}
使用String.valueOf(value)代替""+value
int i = 1;
String s = "" + i;
int i = 1;
String s = String.valueOf(i);
过时代码添加 @Deprecated 注解
/**
* 保存
*
* @deprecated 此方法效率较低，请使用{@link newSave()}方法替换它
*/
@Deprecated
public void save(){
// do something
}
让代码远离 bug
禁止使用构造方法 BigDecimal(double)
BigDecimal value = new BigDecimal(0.1D); // 0.100000000000000005551115...
BigDecimal value = BigDecimal.valueOf(0.1D);; // 0.1
返回空数组和空集合而不是 null
public static Result[] getResults() {
return null;
}
public static List<Result> getResultList() {
return null;
}
public static Map<String, Result> getResultMap() {
return null;
}
public static void main(String[] args) {
Result[] results = getResults();
if (results != null) {
for (Result result : results) {
...
}
}
List<Result> resultList = getResultList();
if (resultList != null) {
for (Result result : resultList) {
...
}
}
Map<String, Result> resultMap = getResultMap();
if (resultMap != null) {
for (Map.Entry<String, Result> resultEntry : resultMap) {
...
}
}
}
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();
}
public static void main(String[] args) {
Result[] results = getResults();
for (Result result : results) {
...
}
List<Result> resultList = getResultList();
for (Result result : resultList) {
...
}
Map<String, Result> resultMap = getResultMap();
for (Map.Entry<String, Result> resultEntry : resultMap) {
...
}
}
public void isFinished(OrderStatus status) {
return status.equals(OrderStatus.FINISHED); // 可能抛空指针异常
}
public void isFinished(OrderStatus status) {
return OrderStatus.FINISHED.equals(status);
}
public void isFinished(OrderStatus status) {
return Objects.equals(status, OrderStatus.FINISHED);
}public enum UserStatus {DISABLED(0, "禁用"),ENABLED(1, "启用");public int value;private String description;private UserStatus(int value, String description) {this.value = value;this.description = description;}public String getDescription() {return description;}public void setDescription(String description) {this.description = description;}}
正例：
public enum UserStatus {
DISABLED(0, "禁用"),
ENABLED(1, "启用");
private final int value;
private final String description;
private UserStatus(int value, String description) {
this.value = value;
this.description = description;
}
public int getValue() {
return value;
}
public String getDescription() {
return description;
}
}
"a.ab.abc".split("."); // 结果为[]
"a|ab|abc".split("|"); // 结果为["a", "|", "a", "b", "|", "a", "b", "c"]
"a.ab.abc".split("\\."); // 结果为["a", "ab", "abc"]
"a|ab|abc".split("\\|"); // 结果为["a", "ab", "abc"]
总结
这篇文章，可以说是从事 Java 开发的经验总结，分享出来以供大家参考。希望能帮大家避免踩坑，让代码更加高效优雅。
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